Squashed 'subprojects/thiserror/' content from commit 247eab5

git-subtree-dir: subprojects/thiserror
git-subtree-split: 247eab5d79e27ad28859afdf8bc600a4242829b7

impl/Cargo.toml

@@ -0,0 +1,28 @@
+[package]
+name = "thiserror-impl"
+version = "2.0.17"
+authors = ["David Tolnay <dtolnay@gmail.com>"]
+description = "Implementation detail of the `thiserror` crate"
+edition = "2021"
+license = "MIT OR Apache-2.0"
+repository = "https://github.com/dtolnay/thiserror"
+rust-version = "1.68"
+
+[lib]
+proc-macro = true
+
+[dependencies]
+proc-macro2 = "1.0.74"
+quote = "1.0.35"
+syn = "2.0.87"
+
+[package.metadata.docs.rs]
+targets = ["x86_64-unknown-linux-gnu"]
+rustdoc-args = [
+    "--generate-link-to-definition",
+    "--generate-macro-expansion",
+    "--extern-html-root-url=core=https://doc.rust-lang.org",
+    "--extern-html-root-url=alloc=https://doc.rust-lang.org",
+    "--extern-html-root-url=std=https://doc.rust-lang.org",
+    "--extern-html-root-url=proc_macro=https://doc.rust-lang.org",
+]

impl/LICENSE-APACHE

@@ -0,0 +1 @@
+../LICENSE-APACHE

impl/LICENSE-MIT

@@ -0,0 +1 @@
+../LICENSE-MIT

impl/src/ast.rs

@@ -0,0 +1,185 @@
+use crate::attr::{self, Attrs};
+use crate::generics::ParamsInScope;
+use crate::unraw::{IdentUnraw, MemberUnraw};
+use proc_macro2::Span;
+use std::fmt::{self, Display};
+use syn::{
+    Data, DataEnum, DataStruct, DeriveInput, Error, Fields, Generics, Ident, Index, Result, Type,
+};
+
+pub enum Input<'a> {
+    Struct(Struct<'a>),
+    Enum(Enum<'a>),
+}
+
+pub struct Struct<'a> {
+    pub attrs: Attrs<'a>,
+    pub ident: Ident,
+    pub generics: &'a Generics,
+    pub fields: Vec<Field<'a>>,
+}
+
+pub struct Enum<'a> {
+    pub attrs: Attrs<'a>,
+    pub ident: Ident,
+    pub generics: &'a Generics,
+    pub variants: Vec<Variant<'a>>,
+}
+
+pub struct Variant<'a> {
+    pub original: &'a syn::Variant,
+    pub attrs: Attrs<'a>,
+    pub ident: Ident,
+    pub fields: Vec<Field<'a>>,
+}
+
+pub struct Field<'a> {
+    pub original: &'a syn::Field,
+    pub attrs: Attrs<'a>,
+    pub member: MemberUnraw,
+    pub ty: &'a Type,
+    pub contains_generic: bool,
+}
+
+#[derive(Copy, Clone)]
+pub enum ContainerKind {
+    Struct,
+    TupleStruct,
+    UnitStruct,
+    StructVariant,
+    TupleVariant,
+    UnitVariant,
+}
+
+impl<'a> Input<'a> {
+    pub fn from_syn(node: &'a DeriveInput) -> Result<Self> {
+        match &node.data {
+            Data::Struct(data) => Struct::from_syn(node, data).map(Input::Struct),
+            Data::Enum(data) => Enum::from_syn(node, data).map(Input::Enum),
+            Data::Union(_) => Err(Error::new_spanned(
+                node,
+                "union as errors are not supported",
+            )),
+        }
+    }
+}
+
+impl<'a> Struct<'a> {
+    fn from_syn(node: &'a DeriveInput, data: &'a DataStruct) -> Result<Self> {
+        let mut attrs = attr::get(&node.attrs)?;
+        let scope = ParamsInScope::new(&node.generics);
+        let fields = Field::multiple_from_syn(&data.fields, &scope)?;
+        if let Some(display) = &mut attrs.display {
+            let container = ContainerKind::from_struct(data);
+            display.expand_shorthand(&fields, container)?;
+        }
+        Ok(Struct {
+            attrs,
+            ident: node.ident.clone(),
+            generics: &node.generics,
+            fields,
+        })
+    }
+}
+
+impl<'a> Enum<'a> {
+    fn from_syn(node: &'a DeriveInput, data: &'a DataEnum) -> Result<Self> {
+        let attrs = attr::get(&node.attrs)?;
+        let scope = ParamsInScope::new(&node.generics);
+        let variants = data
+            .variants
+            .iter()
+            .map(|node| {
+                let mut variant = Variant::from_syn(node, &scope)?;
+                if variant.attrs.display.is_none()
+                    && variant.attrs.transparent.is_none()
+                    && variant.attrs.fmt.is_none()
+                {
+                    variant.attrs.display.clone_from(&attrs.display);
+                    variant.attrs.transparent = attrs.transparent;
+                    variant.attrs.fmt.clone_from(&attrs.fmt);
+                }
+                if let Some(display) = &mut variant.attrs.display {
+                    let container = ContainerKind::from_variant(node);
+                    display.expand_shorthand(&variant.fields, container)?;
+                }
+                Ok(variant)
+            })
+            .collect::<Result<_>>()?;
+        Ok(Enum {
+            attrs,
+            ident: node.ident.clone(),
+            generics: &node.generics,
+            variants,
+        })
+    }
+}
+
+impl<'a> Variant<'a> {
+    fn from_syn(node: &'a syn::Variant, scope: &ParamsInScope<'a>) -> Result<Self> {
+        let attrs = attr::get(&node.attrs)?;
+        Ok(Variant {
+            original: node,
+            attrs,
+            ident: node.ident.clone(),
+            fields: Field::multiple_from_syn(&node.fields, scope)?,
+        })
+    }
+}
+
+impl<'a> Field<'a> {
+    fn multiple_from_syn(fields: &'a Fields, scope: &ParamsInScope<'a>) -> Result<Vec<Self>> {
+        fields
+            .iter()
+            .enumerate()
+            .map(|(i, field)| Field::from_syn(i, field, scope))
+            .collect()
+    }
+
+    fn from_syn(i: usize, node: &'a syn::Field, scope: &ParamsInScope<'a>) -> Result<Self> {
+        Ok(Field {
+            original: node,
+            attrs: attr::get(&node.attrs)?,
+            member: match &node.ident {
+                Some(name) => MemberUnraw::Named(IdentUnraw::new(name.clone())),
+                None => MemberUnraw::Unnamed(Index {
+                    index: i as u32,
+                    span: Span::call_site(),
+                }),
+            },
+            ty: &node.ty,
+            contains_generic: scope.intersects(&node.ty),
+        })
+    }
+}
+
+impl ContainerKind {
+    fn from_struct(node: &DataStruct) -> Self {
+        match node.fields {
+            Fields::Named(_) => ContainerKind::Struct,
+            Fields::Unnamed(_) => ContainerKind::TupleStruct,
+            Fields::Unit => ContainerKind::UnitStruct,
+        }
+    }
+
+    fn from_variant(node: &syn::Variant) -> Self {
+        match node.fields {
+            Fields::Named(_) => ContainerKind::StructVariant,
+            Fields::Unnamed(_) => ContainerKind::TupleVariant,
+            Fields::Unit => ContainerKind::UnitVariant,
+        }
+    }
+}
+
+impl Display for ContainerKind {
+    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        formatter.write_str(match self {
+            ContainerKind::Struct => "struct",
+            ContainerKind::TupleStruct => "tuple struct",
+            ContainerKind::UnitStruct => "unit struct",
+            ContainerKind::StructVariant => "struct variant",
+            ContainerKind::TupleVariant => "tuple variant",
+            ContainerKind::UnitVariant => "unit variant",
+        })
+    }
+}

impl/src/attr.rs

@@ -0,0 +1,358 @@
+use proc_macro2::{Delimiter, Group, Literal, Punct, Spacing, Span, TokenStream, TokenTree};
+use quote::{format_ident, quote, quote_spanned, ToTokens};
+use std::collections::BTreeSet as Set;
+use syn::parse::discouraged::Speculative;
+use syn::parse::{End, ParseStream};
+use syn::{
+    braced, bracketed, parenthesized, token, Attribute, Error, ExprPath, Ident, Index, LitFloat,
+    LitInt, LitStr, Meta, Result, Token,
+};
+
+pub struct Attrs<'a> {
+    pub display: Option<Display<'a>>,
+    pub source: Option<Source<'a>>,
+    pub backtrace: Option<&'a Attribute>,
+    pub from: Option<From<'a>>,
+    pub transparent: Option<Transparent<'a>>,
+    pub fmt: Option<Fmt<'a>>,
+}
+
+#[derive(Clone)]
+pub struct Display<'a> {
+    pub original: &'a Attribute,
+    pub fmt: LitStr,
+    pub args: TokenStream,
+    pub requires_fmt_machinery: bool,
+    pub has_bonus_display: bool,
+    pub infinite_recursive: bool,
+    pub implied_bounds: Set<(usize, Trait)>,
+    pub bindings: Vec<(Ident, TokenStream)>,
+}
+
+#[derive(Copy, Clone)]
+pub struct Source<'a> {
+    pub original: &'a Attribute,
+    pub span: Span,
+}
+
+#[derive(Copy, Clone)]
+pub struct From<'a> {
+    pub original: &'a Attribute,
+    pub span: Span,
+}
+
+#[derive(Copy, Clone)]
+pub struct Transparent<'a> {
+    pub original: &'a Attribute,
+    pub span: Span,
+}
+
+#[derive(Clone)]
+pub struct Fmt<'a> {
+    pub original: &'a Attribute,
+    pub path: ExprPath,
+}
+
+#[derive(Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Debug)]
+pub enum Trait {
+    Debug,
+    Display,
+    Octal,
+    LowerHex,
+    UpperHex,
+    Pointer,
+    Binary,
+    LowerExp,
+    UpperExp,
+}
+
+pub fn get(input: &[Attribute]) -> Result<Attrs> {
+    let mut attrs = Attrs {
+        display: None,
+        source: None,
+        backtrace: None,
+        from: None,
+        transparent: None,
+        fmt: None,
+    };
+
+    for attr in input {
+        if attr.path().is_ident("error") {
+            parse_error_attribute(&mut attrs, attr)?;
+        } else if attr.path().is_ident("source") {
+            attr.meta.require_path_only()?;
+            if attrs.source.is_some() {
+                return Err(Error::new_spanned(attr, "duplicate #[source] attribute"));
+            }
+            let span = (attr.pound_token.span)
+                .join(attr.bracket_token.span.join())
+                .unwrap_or(attr.path().get_ident().unwrap().span());
+            attrs.source = Some(Source {
+                original: attr,
+                span,
+            });
+        } else if attr.path().is_ident("backtrace") {
+            attr.meta.require_path_only()?;
+            if attrs.backtrace.is_some() {
+                return Err(Error::new_spanned(attr, "duplicate #[backtrace] attribute"));
+            }
+            attrs.backtrace = Some(attr);
+        } else if attr.path().is_ident("from") {
+            match attr.meta {
+                Meta::Path(_) => {}
+                Meta::List(_) | Meta::NameValue(_) => {
+                    // Assume this is meant for derive_more crate or something.
+                    continue;
+                }
+            }
+            if attrs.from.is_some() {
+                return Err(Error::new_spanned(attr, "duplicate #[from] attribute"));
+            }
+            let span = (attr.pound_token.span)
+                .join(attr.bracket_token.span.join())
+                .unwrap_or(attr.path().get_ident().unwrap().span());
+            attrs.from = Some(From {
+                original: attr,
+                span,
+            });
+        }
+    }
+
+    Ok(attrs)
+}
+
+fn parse_error_attribute<'a>(attrs: &mut Attrs<'a>, attr: &'a Attribute) -> Result<()> {
+    mod kw {
+        syn::custom_keyword!(transparent);
+        syn::custom_keyword!(fmt);
+    }
+
+    attr.parse_args_with(|input: ParseStream| {
+        let lookahead = input.lookahead1();
+        let fmt = if lookahead.peek(LitStr) {
+            input.parse::<LitStr>()?
+        } else if lookahead.peek(kw::transparent) {
+            let kw: kw::transparent = input.parse()?;
+            if attrs.transparent.is_some() {
+                return Err(Error::new_spanned(
+                    attr,
+                    "duplicate #[error(transparent)] attribute",
+                ));
+            }
+            attrs.transparent = Some(Transparent {
+                original: attr,
+                span: kw.span,
+            });
+            return Ok(());
+        } else if lookahead.peek(kw::fmt) {
+            input.parse::<kw::fmt>()?;
+            input.parse::<Token![=]>()?;
+            let path: ExprPath = input.parse()?;
+            if attrs.fmt.is_some() {
+                return Err(Error::new_spanned(
+                    attr,
+                    "duplicate #[error(fmt = ...)] attribute",
+                ));
+            }
+            attrs.fmt = Some(Fmt {
+                original: attr,
+                path,
+            });
+            return Ok(());
+        } else {
+            return Err(lookahead.error());
+        };
+
+        let args = if input.is_empty() || input.peek(Token![,]) && input.peek2(End) {
+            input.parse::<Option<Token![,]>>()?;
+            TokenStream::new()
+        } else {
+            parse_token_expr(input, false)?
+        };
+
+        let requires_fmt_machinery = !args.is_empty();
+
+        let display = Display {
+            original: attr,
+            fmt,
+            args,
+            requires_fmt_machinery,
+            has_bonus_display: false,
+            infinite_recursive: false,
+            implied_bounds: Set::new(),
+            bindings: Vec::new(),
+        };
+        if attrs.display.is_some() {
+            return Err(Error::new_spanned(
+                attr,
+                "only one #[error(...)] attribute is allowed",
+            ));
+        }
+        attrs.display = Some(display);
+        Ok(())
+    })
+}
+
+fn parse_token_expr(input: ParseStream, mut begin_expr: bool) -> Result<TokenStream> {
+    let mut tokens = Vec::new();
+    while !input.is_empty() {
+        if input.peek(token::Group) {
+            let group: TokenTree = input.parse()?;
+            tokens.push(group);
+            begin_expr = false;
+            continue;
+        }
+
+        if begin_expr && input.peek(Token![.]) {
+            if input.peek2(Ident) {
+                input.parse::<Token![.]>()?;
+                begin_expr = false;
+                continue;
+            } else if input.peek2(LitInt) {
+                input.parse::<Token![.]>()?;
+                let int: Index = input.parse()?;
+                tokens.push({
+                    let ident = format_ident!("_{}", int.index, span = int.span);
+                    TokenTree::Ident(ident)
+                });
+                begin_expr = false;
+                continue;
+            } else if input.peek2(LitFloat) {
+                let ahead = input.fork();
+                ahead.parse::<Token![.]>()?;
+                let float: LitFloat = ahead.parse()?;
+                let repr = float.to_string();
+                let mut indices = repr.split('.').map(syn::parse_str::<Index>);
+                if let (Some(Ok(first)), Some(Ok(second)), None) =
+                    (indices.next(), indices.next(), indices.next())
+                {
+                    input.advance_to(&ahead);
+                    tokens.push({
+                        let ident = format_ident!("_{}", first, span = float.span());
+                        TokenTree::Ident(ident)
+                    });
+                    tokens.push({
+                        let mut punct = Punct::new('.', Spacing::Alone);
+                        punct.set_span(float.span());
+                        TokenTree::Punct(punct)
+                    });
+                    tokens.push({
+                        let mut literal = Literal::u32_unsuffixed(second.index);
+                        literal.set_span(float.span());
+                        TokenTree::Literal(literal)
+                    });
+                    begin_expr = false;
+                    continue;
+                }
+            }
+        }
+
+        begin_expr = input.peek(Token![break])
+            || input.peek(Token![continue])
+            || input.peek(Token![if])
+            || input.peek(Token![in])
+            || input.peek(Token![match])
+            || input.peek(Token![mut])
+            || input.peek(Token![return])
+            || input.peek(Token![while])
+            || input.peek(Token![+])
+            || input.peek(Token![&])
+            || input.peek(Token![!])
+            || input.peek(Token![^])
+            || input.peek(Token![,])
+            || input.peek(Token![/])
+            || input.peek(Token![=])
+            || input.peek(Token![>])
+            || input.peek(Token![<])
+            || input.peek(Token![|])
+            || input.peek(Token![%])
+            || input.peek(Token![;])
+            || input.peek(Token![*])
+            || input.peek(Token![-]);
+
+        let token: TokenTree = if input.peek(token::Paren) {
+            let content;
+            let delimiter = parenthesized!(content in input);
+            let nested = parse_token_expr(&content, true)?;
+            let mut group = Group::new(Delimiter::Parenthesis, nested);
+            group.set_span(delimiter.span.join());
+            TokenTree::Group(group)
+        } else if input.peek(token::Brace) {
+            let content;
+            let delimiter = braced!(content in input);
+            let nested = parse_token_expr(&content, true)?;
+            let mut group = Group::new(Delimiter::Brace, nested);
+            group.set_span(delimiter.span.join());
+            TokenTree::Group(group)
+        } else if input.peek(token::Bracket) {
+            let content;
+            let delimiter = bracketed!(content in input);
+            let nested = parse_token_expr(&content, true)?;
+            let mut group = Group::new(Delimiter::Bracket, nested);
+            group.set_span(delimiter.span.join());
+            TokenTree::Group(group)
+        } else {
+            input.parse()?
+        };
+        tokens.push(token);
+    }
+    Ok(TokenStream::from_iter(tokens))
+}
+
+impl ToTokens for Display<'_> {
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        if self.infinite_recursive {
+            let span = self.fmt.span();
+            tokens.extend(quote_spanned! {span=>
+                #[warn(unconditional_recursion)]
+                fn _fmt() { _fmt() }
+            });
+        }
+
+        let fmt = &self.fmt;
+        let args = &self.args;
+
+        // Currently `write!(f, "text")` produces less efficient code than
+        // `f.write_str("text")`. We recognize the case when the format string
+        // has no braces and no interpolated values, and generate simpler code.
+        let write = if self.requires_fmt_machinery {
+            quote! {
+                ::core::write!(__formatter, #fmt #args)
+            }
+        } else {
+            quote! {
+                __formatter.write_str(#fmt)
+            }
+        };
+
+        tokens.extend(if self.bindings.is_empty() {
+            write
+        } else {
+            let locals = self.bindings.iter().map(|(local, _value)| local);
+            let values = self.bindings.iter().map(|(_local, value)| value);
+            quote! {
+                match (#(#values,)*) {
+                    (#(#locals,)*) => #write
+                }
+            }
+        });
+    }
+}
+
+impl ToTokens for Trait {
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        let trait_name = match self {
+            Trait::Debug => "Debug",
+            Trait::Display => "Display",
+            Trait::Octal => "Octal",
+            Trait::LowerHex => "LowerHex",
+            Trait::UpperHex => "UpperHex",
+            Trait::Pointer => "Pointer",
+            Trait::Binary => "Binary",
+            Trait::LowerExp => "LowerExp",
+            Trait::UpperExp => "UpperExp",
+        };
+        let ident = Ident::new(trait_name, Span::call_site());
+        tokens.extend(quote!(::core::fmt::#ident));
+    }
+}

impl/src/expand.rs

@@ -0,0 +1,584 @@
+use crate::ast::{Enum, Field, Input, Struct};
+use crate::attr::Trait;
+use crate::fallback;
+use crate::generics::InferredBounds;
+use crate::private;
+use crate::unraw::MemberUnraw;
+use proc_macro2::{Ident, Span, TokenStream};
+use quote::{format_ident, quote, quote_spanned, ToTokens};
+use std::collections::BTreeSet as Set;
+use syn::{DeriveInput, GenericArgument, PathArguments, Result, Token, Type};
+
+pub fn derive(input: &DeriveInput) -> TokenStream {
+    match try_expand(input) {
+        Ok(expanded) => expanded,
+        // If there are invalid attributes in the input, expand to an Error impl
+        // anyway to minimize spurious secondary errors in other code that uses
+        // this type as an Error.
+        Err(error) => fallback::expand(input, error),
+    }
+}
+
+fn try_expand(input: &DeriveInput) -> Result<TokenStream> {
+    let input = Input::from_syn(input)?;
+    input.validate()?;
+    Ok(match input {
+        Input::Struct(input) => impl_struct(input),
+        Input::Enum(input) => impl_enum(input),
+    })
+}
+
+fn impl_struct(input: Struct) -> TokenStream {
+    let ty = call_site_ident(&input.ident);
+    let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
+    let mut error_inferred_bounds = InferredBounds::new();
+
+    let source_body = if let Some(transparent_attr) = &input.attrs.transparent {
+        let only_field = &input.fields[0];
+        if only_field.contains_generic {
+            error_inferred_bounds.insert(only_field.ty, quote!(::thiserror::#private::Error));
+        }
+        let member = &only_field.member;
+        Some(quote_spanned! {transparent_attr.span=>
+            ::thiserror::#private::Error::source(self.#member.as_dyn_error())
+        })
+    } else if let Some(source_field) = input.source_field() {
+        let source = &source_field.member;
+        if source_field.contains_generic {
+            let ty = unoptional_type(source_field.ty);
+            error_inferred_bounds.insert(ty, quote!(::thiserror::#private::Error + 'static));
+        }
+        let asref = if type_is_option(source_field.ty) {
+            Some(quote_spanned!(source.span()=> .as_ref()?))
+        } else {
+            None
+        };
+        let dyn_error = quote_spanned! {source_field.source_span()=>
+            self.#source #asref.as_dyn_error()
+        };
+        Some(quote! {
+            ::core::option::Option::Some(#dyn_error)
+        })
+    } else {
+        None
+    };
+    let source_method = source_body.map(|body| {
+        quote! {
+            fn source(&self) -> ::core::option::Option<&(dyn ::thiserror::#private::Error + 'static)> {
+                use ::thiserror::#private::AsDynError as _;
+                #body
+            }
+        }
+    });
+
+    let provide_method = input.backtrace_field().map(|backtrace_field| {
+        let request = quote!(request);
+        let backtrace = &backtrace_field.member;
+        let body = if let Some(source_field) = input.source_field() {
+            let source = &source_field.member;
+            let source_provide = if type_is_option(source_field.ty) {
+                quote_spanned! {source.span()=>
+                    if let ::core::option::Option::Some(source) = &self.#source {
+                        source.thiserror_provide(#request);
+                    }
+                }
+            } else {
+                quote_spanned! {source.span()=>
+                    self.#source.thiserror_provide(#request);
+                }
+            };
+            let self_provide = if source == backtrace {
+                None
+            } else if type_is_option(backtrace_field.ty) {
+                Some(quote! {
+                    if let ::core::option::Option::Some(backtrace) = &self.#backtrace {
+                        #request.provide_ref::<::thiserror::#private::Backtrace>(backtrace);
+                    }
+                })
+            } else {
+                Some(quote! {
+                    #request.provide_ref::<::thiserror::#private::Backtrace>(&self.#backtrace);
+                })
+            };
+            quote! {
+                use ::thiserror::#private::ThiserrorProvide as _;
+                #source_provide
+                #self_provide
+            }
+        } else if type_is_option(backtrace_field.ty) {
+            quote! {
+                if let ::core::option::Option::Some(backtrace) = &self.#backtrace {
+                    #request.provide_ref::<::thiserror::#private::Backtrace>(backtrace);
+                }
+            }
+        } else {
+            quote! {
+                #request.provide_ref::<::thiserror::#private::Backtrace>(&self.#backtrace);
+            }
+        };
+        quote! {
+            fn provide<'_request>(&'_request self, #request: &mut ::core::error::Request<'_request>) {
+                #body
+            }
+        }
+    });
+
+    let mut display_implied_bounds = Set::new();
+    let display_body = if input.attrs.transparent.is_some() {
+        let only_field = &input.fields[0].member;
+        display_implied_bounds.insert((0, Trait::Display));
+        Some(quote! {
+            ::core::fmt::Display::fmt(&self.#only_field, __formatter)
+        })
+    } else if let Some(display) = &input.attrs.display {
+        display_implied_bounds.clone_from(&display.implied_bounds);
+        let use_as_display = use_as_display(display.has_bonus_display);
+        let pat = fields_pat(&input.fields);
+        Some(quote! {
+            #use_as_display
+            #[allow(unused_variables, deprecated)]
+            let Self #pat = self;
+            #display
+        })
+    } else {
+        None
+    };
+    let display_impl = display_body.map(|body| {
+        let mut display_inferred_bounds = InferredBounds::new();
+        for (field, bound) in display_implied_bounds {
+            let field = &input.fields[field];
+            if field.contains_generic {
+                display_inferred_bounds.insert(field.ty, bound);
+            }
+        }
+        let display_where_clause = display_inferred_bounds.augment_where_clause(input.generics);
+        quote! {
+            #[allow(unused_qualifications)]
+            #[automatically_derived]
+            impl #impl_generics ::core::fmt::Display for #ty #ty_generics #display_where_clause {
+                #[allow(clippy::used_underscore_binding)]
+                fn fmt(&self, __formatter: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
+                    #body
+                }
+            }
+        }
+    });
+
+    let from_impl = input.from_field().map(|from_field| {
+        let span = from_field.attrs.from.unwrap().span;
+        let backtrace_field = input.distinct_backtrace_field();
+        let from = unoptional_type(from_field.ty);
+        let source_var = Ident::new("source", span);
+        let body = from_initializer(from_field, backtrace_field, &source_var);
+        let from_function = quote! {
+            fn from(#source_var: #from) -> Self {
+                #ty #body
+            }
+        };
+        let from_impl = quote_spanned! {span=>
+            #[automatically_derived]
+            impl #impl_generics ::core::convert::From<#from> for #ty #ty_generics #where_clause {
+                #from_function
+            }
+        };
+        Some(quote! {
+            #[allow(
+                deprecated,
+                unused_qualifications,
+                clippy::elidable_lifetime_names,
+                clippy::needless_lifetimes,
+            )]
+            #from_impl
+        })
+    });
+
+    if input.generics.type_params().next().is_some() {
+        let self_token = <Token![Self]>::default();
+        error_inferred_bounds.insert(self_token, Trait::Debug);
+        error_inferred_bounds.insert(self_token, Trait::Display);
+    }
+    let error_where_clause = error_inferred_bounds.augment_where_clause(input.generics);
+
+    quote! {
+        #[allow(unused_qualifications)]
+        #[automatically_derived]
+        impl #impl_generics ::thiserror::#private::Error for #ty #ty_generics #error_where_clause {
+            #source_method
+            #provide_method
+        }
+        #display_impl
+        #from_impl
+    }
+}
+
+fn impl_enum(input: Enum) -> TokenStream {
+    let ty = call_site_ident(&input.ident);
+    let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
+    let mut error_inferred_bounds = InferredBounds::new();
+
+    let source_method = if input.has_source() {
+        let arms = input.variants.iter().map(|variant| {
+            let ident = &variant.ident;
+            if let Some(transparent_attr) = &variant.attrs.transparent {
+                let only_field = &variant.fields[0];
+                if only_field.contains_generic {
+                    error_inferred_bounds.insert(only_field.ty, quote!(::thiserror::#private::Error));
+                }
+                let member = &only_field.member;
+                let source = quote_spanned! {transparent_attr.span=>
+                    ::thiserror::#private::Error::source(transparent.as_dyn_error())
+                };
+                quote! {
+                    #ty::#ident {#member: transparent} => #source,
+                }
+            } else if let Some(source_field) = variant.source_field() {
+                let source = &source_field.member;
+                if source_field.contains_generic {
+                    let ty = unoptional_type(source_field.ty);
+                    error_inferred_bounds.insert(ty, quote!(::thiserror::#private::Error + 'static));
+                }
+                let asref = if type_is_option(source_field.ty) {
+                    Some(quote_spanned!(source.span()=> .as_ref()?))
+                } else {
+                    None
+                };
+                let varsource = quote!(source);
+                let dyn_error = quote_spanned! {source_field.source_span()=>
+                    #varsource #asref.as_dyn_error()
+                };
+                quote! {
+                    #ty::#ident {#source: #varsource, ..} => ::core::option::Option::Some(#dyn_error),
+                }
+            } else {
+                quote! {
+                    #ty::#ident {..} => ::core::option::Option::None,
+                }
+            }
+        });
+        Some(quote! {
+            fn source(&self) -> ::core::option::Option<&(dyn ::thiserror::#private::Error + 'static)> {
+                use ::thiserror::#private::AsDynError as _;
+                #[allow(deprecated)]
+                match self {
+                    #(#arms)*
+                }
+            }
+        })
+    } else {
+        None
+    };
+
+    let provide_method = if input.has_backtrace() {
+        let request = quote!(request);
+        let arms = input.variants.iter().map(|variant| {
+            let ident = &variant.ident;
+            match (variant.backtrace_field(), variant.source_field()) {
+                (Some(backtrace_field), Some(source_field))
+                    if backtrace_field.attrs.backtrace.is_none() =>
+                {
+                    let backtrace = &backtrace_field.member;
+                    let source = &source_field.member;
+                    let varsource = quote!(source);
+                    let source_provide = if type_is_option(source_field.ty) {
+                        quote_spanned! {source.span()=>
+                            if let ::core::option::Option::Some(source) = #varsource {
+                                source.thiserror_provide(#request);
+                            }
+                        }
+                    } else {
+                        quote_spanned! {source.span()=>
+                            #varsource.thiserror_provide(#request);
+                        }
+                    };
+                    let self_provide = if type_is_option(backtrace_field.ty) {
+                        quote! {
+                            if let ::core::option::Option::Some(backtrace) = backtrace {
+                                #request.provide_ref::<::thiserror::#private::Backtrace>(backtrace);
+                            }
+                        }
+                    } else {
+                        quote! {
+                            #request.provide_ref::<::thiserror::#private::Backtrace>(backtrace);
+                        }
+                    };
+                    quote! {
+                        #ty::#ident {
+                            #backtrace: backtrace,
+                            #source: #varsource,
+                            ..
+                        } => {
+                            use ::thiserror::#private::ThiserrorProvide as _;
+                            #source_provide
+                            #self_provide
+                        }
+                    }
+                }
+                (Some(backtrace_field), Some(source_field))
+                    if backtrace_field.member == source_field.member =>
+                {
+                    let backtrace = &backtrace_field.member;
+                    let varsource = quote!(source);
+                    let source_provide = if type_is_option(source_field.ty) {
+                        quote_spanned! {backtrace.span()=>
+                            if let ::core::option::Option::Some(source) = #varsource {
+                                source.thiserror_provide(#request);
+                            }
+                        }
+                    } else {
+                        quote_spanned! {backtrace.span()=>
+                            #varsource.thiserror_provide(#request);
+                        }
+                    };
+                    quote! {
+                        #ty::#ident {#backtrace: #varsource, ..} => {
+                            use ::thiserror::#private::ThiserrorProvide as _;
+                            #source_provide
+                        }
+                    }
+                }
+                (Some(backtrace_field), _) => {
+                    let backtrace = &backtrace_field.member;
+                    let body = if type_is_option(backtrace_field.ty) {
+                        quote! {
+                            if let ::core::option::Option::Some(backtrace) = backtrace {
+                                #request.provide_ref::<::thiserror::#private::Backtrace>(backtrace);
+                            }
+                        }
+                    } else {
+                        quote! {
+                            #request.provide_ref::<::thiserror::#private::Backtrace>(backtrace);
+                        }
+                    };
+                    quote! {
+                        #ty::#ident {#backtrace: backtrace, ..} => {
+                            #body
+                        }
+                    }
+                }
+                (None, _) => quote! {
+                    #ty::#ident {..} => {}
+                },
+            }
+        });
+        Some(quote! {
+            fn provide<'_request>(&'_request self, #request: &mut ::core::error::Request<'_request>) {
+                #[allow(deprecated)]
+                match self {
+                    #(#arms)*
+                }
+            }
+        })
+    } else {
+        None
+    };
+
+    let display_impl = if input.has_display() {
+        let mut display_inferred_bounds = InferredBounds::new();
+        let has_bonus_display = input.variants.iter().any(|v| {
+            v.attrs
+                .display
+                .as_ref()
+                .map_or(false, |display| display.has_bonus_display)
+        });
+        let use_as_display = use_as_display(has_bonus_display);
+        let void_deref = if input.variants.is_empty() {
+            Some(quote!(*))
+        } else {
+            None
+        };
+        let arms = input.variants.iter().map(|variant| {
+            let mut display_implied_bounds = Set::new();
+            let display = if let Some(display) = &variant.attrs.display {
+                display_implied_bounds.clone_from(&display.implied_bounds);
+                display.to_token_stream()
+            } else if let Some(fmt) = &variant.attrs.fmt {
+                let fmt_path = &fmt.path;
+                let vars = variant.fields.iter().map(|field| match &field.member {
+                    MemberUnraw::Named(ident) => ident.to_local(),
+                    MemberUnraw::Unnamed(index) => format_ident!("_{}", index),
+                });
+                quote!(#fmt_path(#(#vars,)* __formatter))
+            } else {
+                let only_field = match &variant.fields[0].member {
+                    MemberUnraw::Named(ident) => ident.to_local(),
+                    MemberUnraw::Unnamed(index) => format_ident!("_{}", index),
+                };
+                display_implied_bounds.insert((0, Trait::Display));
+                quote!(::core::fmt::Display::fmt(#only_field, __formatter))
+            };
+            for (field, bound) in display_implied_bounds {
+                let field = &variant.fields[field];
+                if field.contains_generic {
+                    display_inferred_bounds.insert(field.ty, bound);
+                }
+            }
+            let ident = &variant.ident;
+            let pat = fields_pat(&variant.fields);
+            quote! {
+                #ty::#ident #pat => #display
+            }
+        });
+        let arms = arms.collect::<Vec<_>>();
+        let display_where_clause = display_inferred_bounds.augment_where_clause(input.generics);
+        Some(quote! {
+            #[allow(unused_qualifications)]
+            #[automatically_derived]
+            impl #impl_generics ::core::fmt::Display for #ty #ty_generics #display_where_clause {
+                fn fmt(&self, __formatter: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
+                    #use_as_display
+                    #[allow(unused_variables, deprecated, clippy::used_underscore_binding)]
+                    match #void_deref self {
+                        #(#arms,)*
+                    }
+                }
+            }
+        })
+    } else {
+        None
+    };
+
+    let from_impls = input.variants.iter().filter_map(|variant| {
+        let from_field = variant.from_field()?;
+        let span = from_field.attrs.from.unwrap().span;
+        let backtrace_field = variant.distinct_backtrace_field();
+        let variant = &variant.ident;
+        let from = unoptional_type(from_field.ty);
+        let source_var = Ident::new("source", span);
+        let body = from_initializer(from_field, backtrace_field, &source_var);
+        let from_function = quote! {
+            fn from(#source_var: #from) -> Self {
+                #ty::#variant #body
+            }
+        };
+        let from_impl = quote_spanned! {span=>
+            #[automatically_derived]
+            impl #impl_generics ::core::convert::From<#from> for #ty #ty_generics #where_clause {
+                #from_function
+            }
+        };
+        Some(quote! {
+            #[allow(
+                deprecated,
+                unused_qualifications,
+                clippy::elidable_lifetime_names,
+                clippy::needless_lifetimes,
+            )]
+            #from_impl
+        })
+    });
+
+    if input.generics.type_params().next().is_some() {
+        let self_token = <Token![Self]>::default();
+        error_inferred_bounds.insert(self_token, Trait::Debug);
+        error_inferred_bounds.insert(self_token, Trait::Display);
+    }
+    let error_where_clause = error_inferred_bounds.augment_where_clause(input.generics);
+
+    quote! {
+        #[allow(unused_qualifications)]
+        #[automatically_derived]
+        impl #impl_generics ::thiserror::#private::Error for #ty #ty_generics #error_where_clause {
+            #source_method
+            #provide_method
+        }
+        #display_impl
+        #(#from_impls)*
+    }
+}
+
+// Create an ident with which we can expand `impl Trait for #ident {}` on a
+// deprecated type without triggering deprecation warning on the generated impl.
+pub(crate) fn call_site_ident(ident: &Ident) -> Ident {
+    let mut ident = ident.clone();
+    ident.set_span(ident.span().resolved_at(Span::call_site()));
+    ident
+}
+
+fn fields_pat(fields: &[Field]) -> TokenStream {
+    let mut members = fields.iter().map(|field| &field.member).peekable();
+    match members.peek() {
+        Some(MemberUnraw::Named(_)) => quote!({ #(#members),* }),
+        Some(MemberUnraw::Unnamed(_)) => {
+            let vars = members.map(|member| match member {
+                MemberUnraw::Unnamed(index) => format_ident!("_{}", index),
+                MemberUnraw::Named(_) => unreachable!(),
+            });
+            quote!((#(#vars),*))
+        }
+        None => quote!({}),
+    }
+}
+
+fn use_as_display(needs_as_display: bool) -> Option<TokenStream> {
+    if needs_as_display {
+        Some(quote! {
+            use ::thiserror::#private::AsDisplay as _;
+        })
+    } else {
+        None
+    }
+}
+
+fn from_initializer(
+    from_field: &Field,
+    backtrace_field: Option<&Field>,
+    source_var: &Ident,
+) -> TokenStream {
+    let from_member = &from_field.member;
+    let some_source = if type_is_option(from_field.ty) {
+        quote!(::core::option::Option::Some(#source_var))
+    } else {
+        quote!(#source_var)
+    };
+    let backtrace = backtrace_field.map(|backtrace_field| {
+        let backtrace_member = &backtrace_field.member;
+        if type_is_option(backtrace_field.ty) {
+            quote! {
+                #backtrace_member: ::core::option::Option::Some(::thiserror::#private::Backtrace::capture()),
+            }
+        } else {
+            quote! {
+                #backtrace_member: ::core::convert::From::from(::thiserror::#private::Backtrace::capture()),
+            }
+        }
+    });
+    quote!({
+        #from_member: #some_source,
+        #backtrace
+    })
+}
+
+fn type_is_option(ty: &Type) -> bool {
+    type_parameter_of_option(ty).is_some()
+}
+
+fn unoptional_type(ty: &Type) -> TokenStream {
+    let unoptional = type_parameter_of_option(ty).unwrap_or(ty);
+    quote!(#unoptional)
+}
+
+fn type_parameter_of_option(ty: &Type) -> Option<&Type> {
+    let path = match ty {
+        Type::Path(ty) => &ty.path,
+        _ => return None,
+    };
+
+    let last = path.segments.last().unwrap();
+    if last.ident != "Option" {
+        return None;
+    }
+
+    let bracketed = match &last.arguments {
+        PathArguments::AngleBracketed(bracketed) => bracketed,
+        _ => return None,
+    };
+
+    if bracketed.args.len() != 1 {
+        return None;
+    }
+
+    match &bracketed.args[0] {
+        GenericArgument::Type(arg) => Some(arg),
+        _ => None,
+    }
+}

impl/src/fallback.rs

@@ -0,0 +1,33 @@
+use crate::expand::call_site_ident;
+use crate::private;
+use proc_macro2::TokenStream;
+use quote::quote;
+use syn::DeriveInput;
+
+pub(crate) fn expand(input: &DeriveInput, error: syn::Error) -> TokenStream {
+    let ty = call_site_ident(&input.ident);
+    let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
+
+    let error = error.to_compile_error();
+
+    quote! {
+        #error
+
+        #[allow(unused_qualifications)]
+        #[automatically_derived]
+        impl #impl_generics ::thiserror::#private::Error for #ty #ty_generics #where_clause
+        where
+            // Work around trivial bounds being unstable.
+            // https://github.com/rust-lang/rust/issues/48214
+            for<'workaround> #ty #ty_generics: ::core::fmt::Debug,
+        {}
+
+        #[allow(unused_qualifications)]
+        #[automatically_derived]
+        impl #impl_generics ::core::fmt::Display for #ty #ty_generics #where_clause {
+            fn fmt(&self, __formatter: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
+                ::core::unreachable!()
+            }
+        }
+    }
+}

impl/src/fmt.rs

@@ -0,0 +1,323 @@
+use crate::ast::{ContainerKind, Field};
+use crate::attr::{Display, Trait};
+use crate::private;
+use crate::scan_expr::scan_expr;
+use crate::unraw::{IdentUnraw, MemberUnraw};
+use proc_macro2::{Delimiter, TokenStream, TokenTree};
+use quote::{format_ident, quote, quote_spanned, ToTokens as _};
+use std::collections::{BTreeSet, HashMap};
+use std::iter;
+use syn::ext::IdentExt;
+use syn::parse::discouraged::Speculative;
+use syn::parse::{Error, ParseStream, Parser, Result};
+use syn::{Expr, Ident, Index, LitStr, Token};
+
+impl Display<'_> {
+    pub fn expand_shorthand(&mut self, fields: &[Field], container: ContainerKind) -> Result<()> {
+        let raw_args = self.args.clone();
+        let FmtArguments {
+            named: user_named_args,
+            first_unnamed,
+        } = explicit_named_args.parse2(raw_args).unwrap();
+
+        let mut member_index = HashMap::new();
+        let mut extra_positional_arguments_allowed = true;
+        for (i, field) in fields.iter().enumerate() {
+            member_index.insert(&field.member, i);
+            extra_positional_arguments_allowed &= matches!(&field.member, MemberUnraw::Named(_));
+        }
+
+        let span = self.fmt.span();
+        let fmt = self.fmt.value();
+        let mut read = fmt.as_str();
+        let mut out = String::new();
+        let mut has_bonus_display = false;
+        let mut infinite_recursive = false;
+        let mut implied_bounds = BTreeSet::new();
+        let mut bindings = Vec::new();
+        let mut macro_named_args = BTreeSet::new();
+
+        self.requires_fmt_machinery = self.requires_fmt_machinery || fmt.contains('}');
+
+        while let Some(brace) = read.find('{') {
+            self.requires_fmt_machinery = true;
+            out += &read[..brace + 1];
+            read = &read[brace + 1..];
+            if read.starts_with('{') {
+                out.push('{');
+                read = &read[1..];
+                continue;
+            }
+            let next = match read.chars().next() {
+                Some(next) => next,
+                None => return Ok(()),
+            };
+            let member = match next {
+                '0'..='9' => {
+                    let int = take_int(&mut read);
+                    if !extra_positional_arguments_allowed {
+                        if let Some(first_unnamed) = &first_unnamed {
+                            let msg = format!("ambiguous reference to positional arguments by number in a {container}; change this to a named argument");
+                            return Err(Error::new_spanned(first_unnamed, msg));
+                        }
+                    }
+                    match int.parse::<u32>() {
+                        Ok(index) => MemberUnraw::Unnamed(Index { index, span }),
+                        Err(_) => return Ok(()),
+                    }
+                }
+                'a'..='z' | 'A'..='Z' | '_' => {
+                    if read.starts_with("r#") {
+                        continue;
+                    }
+                    let repr = take_ident(&mut read);
+                    if repr == "_" {
+                        // Invalid. Let rustc produce the diagnostic.
+                        out += repr;
+                        continue;
+                    }
+                    let ident = IdentUnraw::new(Ident::new(repr, span));
+                    if user_named_args.contains(&ident) {
+                        // Refers to a named argument written by the user, not to field.
+                        out += repr;
+                        continue;
+                    }
+                    MemberUnraw::Named(ident)
+                }
+                _ => continue,
+            };
+            let end_spec = match read.find('}') {
+                Some(end_spec) => end_spec,
+                None => return Ok(()),
+            };
+            let mut bonus_display = false;
+            let bound = match read[..end_spec].chars().next_back() {
+                Some('?') => Trait::Debug,
+                Some('o') => Trait::Octal,
+                Some('x') => Trait::LowerHex,
+                Some('X') => Trait::UpperHex,
+                Some('p') => Trait::Pointer,
+                Some('b') => Trait::Binary,
+                Some('e') => Trait::LowerExp,
+                Some('E') => Trait::UpperExp,
+                Some(_) => Trait::Display,
+                None => {
+                    bonus_display = true;
+                    has_bonus_display = true;
+                    Trait::Display
+                }
+            };
+            infinite_recursive |= member == *"self" && bound == Trait::Display;
+            let field = match member_index.get(&member) {
+                Some(&field) => field,
+                None => {
+                    out += &member.to_string();
+                    continue;
+                }
+            };
+            implied_bounds.insert((field, bound));
+            let formatvar_prefix = if bonus_display {
+                "__display"
+            } else if bound == Trait::Pointer {
+                "__pointer"
+            } else {
+                "__field"
+            };
+            let mut formatvar = IdentUnraw::new(match &member {
+                MemberUnraw::Unnamed(index) => format_ident!("{}{}", formatvar_prefix, index),
+                MemberUnraw::Named(ident) => {
+                    format_ident!("{}_{}", formatvar_prefix, ident.to_string())
+                }
+            });
+            while user_named_args.contains(&formatvar) {
+                formatvar = IdentUnraw::new(format_ident!("_{}", formatvar.to_string()));
+            }
+            formatvar.set_span(span);
+            out += &formatvar.to_string();
+            if !macro_named_args.insert(formatvar.clone()) {
+                // Already added to bindings by a previous use.
+                continue;
+            }
+            let mut binding_value = match &member {
+                MemberUnraw::Unnamed(index) => format_ident!("_{}", index),
+                MemberUnraw::Named(ident) => ident.to_local(),
+            };
+            binding_value.set_span(span.resolved_at(fields[field].member.span()));
+            let wrapped_binding_value = if bonus_display {
+                quote_spanned!(span=> #binding_value.as_display())
+            } else if bound == Trait::Pointer {
+                quote!(::thiserror::#private::Var(#binding_value))
+            } else {
+                binding_value.into_token_stream()
+            };
+            bindings.push((formatvar.to_local(), wrapped_binding_value));
+        }
+
+        out += read;
+        self.fmt = LitStr::new(&out, self.fmt.span());
+        self.has_bonus_display = has_bonus_display;
+        self.infinite_recursive = infinite_recursive;
+        self.implied_bounds = implied_bounds;
+        self.bindings = bindings;
+        Ok(())
+    }
+}
+
+struct FmtArguments {
+    named: BTreeSet<IdentUnraw>,
+    first_unnamed: Option<TokenStream>,
+}
+
+#[allow(clippy::unnecessary_wraps)]
+fn explicit_named_args(input: ParseStream) -> Result<FmtArguments> {
+    let ahead = input.fork();
+    if let Ok(set) = try_explicit_named_args(&ahead) {
+        input.advance_to(&ahead);
+        return Ok(set);
+    }
+
+    let ahead = input.fork();
+    if let Ok(set) = fallback_explicit_named_args(&ahead) {
+        input.advance_to(&ahead);
+        return Ok(set);
+    }
+
+    input.parse::<TokenStream>().unwrap();
+    Ok(FmtArguments {
+        named: BTreeSet::new(),
+        first_unnamed: None,
+    })
+}
+
+fn try_explicit_named_args(input: ParseStream) -> Result<FmtArguments> {
+    let mut syn_full = None;
+    let mut args = FmtArguments {
+        named: BTreeSet::new(),
+        first_unnamed: None,
+    };
+
+    while !input.is_empty() {
+        input.parse::<Token![,]>()?;
+        if input.is_empty() {
+            break;
+        }
+
+        let mut begin_unnamed = None;
+        if input.peek(Ident::peek_any) && input.peek2(Token![=]) && !input.peek2(Token![==]) {
+            let ident: IdentUnraw = input.parse()?;
+            input.parse::<Token![=]>()?;
+            args.named.insert(ident);
+        } else {
+            begin_unnamed = Some(input.fork());
+        }
+
+        let ahead = input.fork();
+        if *syn_full.get_or_insert_with(is_syn_full) && ahead.parse::<Expr>().is_ok() {
+            input.advance_to(&ahead);
+        } else {
+            scan_expr(input)?;
+        }
+
+        if let Some(begin_unnamed) = begin_unnamed {
+            if args.first_unnamed.is_none() {
+                args.first_unnamed = Some(between(&begin_unnamed, input));
+            }
+        }
+    }
+
+    Ok(args)
+}
+
+fn fallback_explicit_named_args(input: ParseStream) -> Result<FmtArguments> {
+    let mut args = FmtArguments {
+        named: BTreeSet::new(),
+        first_unnamed: None,
+    };
+
+    while !input.is_empty() {
+        if input.peek(Token![,])
+            && input.peek2(Ident::peek_any)
+            && input.peek3(Token![=])
+            && !input.peek3(Token![==])
+        {
+            input.parse::<Token![,]>()?;
+            let ident: IdentUnraw = input.parse()?;
+            input.parse::<Token![=]>()?;
+            args.named.insert(ident);
+        } else {
+            input.parse::<TokenTree>()?;
+        }
+    }
+
+    Ok(args)
+}
+
+fn is_syn_full() -> bool {
+    // Expr::Block contains syn::Block which contains Vec<syn::Stmt>. In the
+    // current version of Syn, syn::Stmt is exhaustive and could only plausibly
+    // represent `trait Trait {}` in Stmt::Item which contains syn::Item. Most
+    // of the point of syn's non-"full" mode is to avoid compiling Item and the
+    // entire expansive syntax tree it comprises. So the following expression
+    // being parsed to Expr::Block is a reliable indication that "full" is
+    // enabled.
+    let test = quote!({
+        trait Trait {}
+    });
+    match syn::parse2(test) {
+        Ok(Expr::Verbatim(_)) | Err(_) => false,
+        Ok(Expr::Block(_)) => true,
+        Ok(_) => unreachable!(),
+    }
+}
+
+fn take_int<'a>(read: &mut &'a str) -> &'a str {
+    let mut int_len = 0;
+    for ch in read.chars() {
+        match ch {
+            '0'..='9' => int_len += 1,
+            _ => break,
+        }
+    }
+    let (int, rest) = read.split_at(int_len);
+    *read = rest;
+    int
+}
+
+fn take_ident<'a>(read: &mut &'a str) -> &'a str {
+    let mut ident_len = 0;
+    for ch in read.chars() {
+        match ch {
+            'a'..='z' | 'A'..='Z' | '0'..='9' | '_' => ident_len += 1,
+            _ => break,
+        }
+    }
+    let (ident, rest) = read.split_at(ident_len);
+    *read = rest;
+    ident
+}
+
+fn between<'a>(begin: ParseStream<'a>, end: ParseStream<'a>) -> TokenStream {
+    let end = end.cursor();
+    let mut cursor = begin.cursor();
+    let mut tokens = TokenStream::new();
+
+    while cursor < end {
+        let (tt, next) = cursor.token_tree().unwrap();
+
+        if end < next {
+            if let Some((inside, _span, _after)) = cursor.group(Delimiter::None) {
+                cursor = inside;
+                continue;
+            }
+            if tokens.is_empty() {
+                tokens.extend(iter::once(tt));
+            }
+            break;
+        }
+
+        tokens.extend(iter::once(tt));
+        cursor = next;
+    }
+
+    tokens
+}

impl/src/generics.rs

@@ -0,0 +1,83 @@
+use proc_macro2::TokenStream;
+use quote::ToTokens;
+use std::collections::btree_map::Entry;
+use std::collections::{BTreeMap as Map, BTreeSet as Set};
+use syn::punctuated::Punctuated;
+use syn::{parse_quote, GenericArgument, Generics, Ident, PathArguments, Token, Type, WhereClause};
+
+pub struct ParamsInScope<'a> {
+    names: Set<&'a Ident>,
+}
+
+impl<'a> ParamsInScope<'a> {
+    pub fn new(generics: &'a Generics) -> Self {
+        ParamsInScope {
+            names: generics.type_params().map(|param| &param.ident).collect(),
+        }
+    }
+
+    pub fn intersects(&self, ty: &Type) -> bool {
+        let mut found = false;
+        crawl(self, ty, &mut found);
+        found
+    }
+}
+
+fn crawl(in_scope: &ParamsInScope, ty: &Type, found: &mut bool) {
+    if let Type::Path(ty) = ty {
+        if let Some(qself) = &ty.qself {
+            crawl(in_scope, &qself.ty, found);
+        } else {
+            let front = ty.path.segments.first().unwrap();
+            if front.arguments.is_none() && in_scope.names.contains(&front.ident) {
+                *found = true;
+            }
+        }
+        for segment in &ty.path.segments {
+            if let PathArguments::AngleBracketed(arguments) = &segment.arguments {
+                for arg in &arguments.args {
+                    if let GenericArgument::Type(ty) = arg {
+                        crawl(in_scope, ty, found);
+                    }
+                }
+            }
+        }
+    }
+}
+
+pub struct InferredBounds {
+    bounds: Map<String, (Set<String>, Punctuated<TokenStream, Token![+]>)>,
+    order: Vec<TokenStream>,
+}
+
+impl InferredBounds {
+    pub fn new() -> Self {
+        InferredBounds {
+            bounds: Map::new(),
+            order: Vec::new(),
+        }
+    }
+
+    pub fn insert(&mut self, ty: impl ToTokens, bound: impl ToTokens) {
+        let ty = ty.to_token_stream();
+        let bound = bound.to_token_stream();
+        let entry = self.bounds.entry(ty.to_string());
+        if let Entry::Vacant(_) = entry {
+            self.order.push(ty);
+        }
+        let (set, tokens) = entry.or_default();
+        if set.insert(bound.to_string()) {
+            tokens.push(bound);
+        }
+    }
+
+    pub fn augment_where_clause(&self, generics: &Generics) -> WhereClause {
+        let mut generics = generics.clone();
+        let where_clause = generics.make_where_clause();
+        for ty in &self.order {
+            let (_set, bounds) = &self.bounds[&ty.to_string()];
+            where_clause.predicates.push(parse_quote!(#ty: #bounds));
+        }
+        generics.where_clause.unwrap()
+    }
+}

impl/src/lib.rs

@@ -0,0 +1,55 @@
+#![allow(
+    clippy::blocks_in_conditions,
+    clippy::cast_lossless,
+    clippy::cast_possible_truncation,
+    clippy::enum_glob_use,
+    clippy::expl_impl_clone_on_copy, // https://github.com/rust-lang/rust-clippy/issues/15842
+    clippy::manual_find,
+    clippy::manual_let_else,
+    clippy::manual_map,
+    clippy::map_unwrap_or,
+    clippy::module_name_repetitions,
+    clippy::needless_pass_by_value,
+    clippy::range_plus_one,
+    clippy::single_match_else,
+    clippy::struct_field_names,
+    clippy::too_many_lines,
+    clippy::wrong_self_convention
+)]
+#![allow(unknown_lints, mismatched_lifetime_syntaxes)]
+
+extern crate proc_macro;
+
+mod ast;
+mod attr;
+mod expand;
+mod fallback;
+mod fmt;
+mod generics;
+mod prop;
+mod scan_expr;
+mod unraw;
+mod valid;
+
+use proc_macro::TokenStream;
+use proc_macro2::{Ident, Span};
+use quote::{ToTokens, TokenStreamExt as _};
+use syn::{parse_macro_input, DeriveInput};
+
+#[proc_macro_derive(Error, attributes(backtrace, error, from, source))]
+pub fn derive_error(input: TokenStream) -> TokenStream {
+    let input = parse_macro_input!(input as DeriveInput);
+    expand::derive(&input).into()
+}
+
+#[allow(non_camel_case_types)]
+struct private;
+
+impl ToTokens for private {
+    fn to_tokens(&self, tokens: &mut proc_macro2::TokenStream) {
+        tokens.append(Ident::new(
+            concat!("__private", env!("CARGO_PKG_VERSION_PATCH")),
+            Span::call_site(),
+        ));
+    }
+}

impl/src/prop.rs

@@ -0,0 +1,148 @@
+use crate::ast::{Enum, Field, Struct, Variant};
+use crate::unraw::MemberUnraw;
+use proc_macro2::Span;
+use syn::Type;
+
+impl Struct<'_> {
+    pub(crate) fn from_field(&self) -> Option<&Field> {
+        from_field(&self.fields)
+    }
+
+    pub(crate) fn source_field(&self) -> Option<&Field> {
+        source_field(&self.fields)
+    }
+
+    pub(crate) fn backtrace_field(&self) -> Option<&Field> {
+        backtrace_field(&self.fields)
+    }
+
+    pub(crate) fn distinct_backtrace_field(&self) -> Option<&Field> {
+        let backtrace_field = self.backtrace_field()?;
+        distinct_backtrace_field(backtrace_field, self.from_field())
+    }
+}
+
+impl Enum<'_> {
+    pub(crate) fn has_source(&self) -> bool {
+        self.variants
+            .iter()
+            .any(|variant| variant.source_field().is_some() || variant.attrs.transparent.is_some())
+    }
+
+    pub(crate) fn has_backtrace(&self) -> bool {
+        self.variants
+            .iter()
+            .any(|variant| variant.backtrace_field().is_some())
+    }
+
+    pub(crate) fn has_display(&self) -> bool {
+        self.attrs.display.is_some()
+            || self.attrs.transparent.is_some()
+            || self.attrs.fmt.is_some()
+            || self
+                .variants
+                .iter()
+                .any(|variant| variant.attrs.display.is_some() || variant.attrs.fmt.is_some())
+            || self
+                .variants
+                .iter()
+                .all(|variant| variant.attrs.transparent.is_some())
+    }
+}
+
+impl Variant<'_> {
+    pub(crate) fn from_field(&self) -> Option<&Field> {
+        from_field(&self.fields)
+    }
+
+    pub(crate) fn source_field(&self) -> Option<&Field> {
+        source_field(&self.fields)
+    }
+
+    pub(crate) fn backtrace_field(&self) -> Option<&Field> {
+        backtrace_field(&self.fields)
+    }
+
+    pub(crate) fn distinct_backtrace_field(&self) -> Option<&Field> {
+        let backtrace_field = self.backtrace_field()?;
+        distinct_backtrace_field(backtrace_field, self.from_field())
+    }
+}
+
+impl Field<'_> {
+    pub(crate) fn is_backtrace(&self) -> bool {
+        type_is_backtrace(self.ty)
+    }
+
+    pub(crate) fn source_span(&self) -> Span {
+        if let Some(source_attr) = &self.attrs.source {
+            source_attr.span
+        } else if let Some(from_attr) = &self.attrs.from {
+            from_attr.span
+        } else {
+            self.member.span()
+        }
+    }
+}
+
+fn from_field<'a, 'b>(fields: &'a [Field<'b>]) -> Option<&'a Field<'b>> {
+    for field in fields {
+        if field.attrs.from.is_some() {
+            return Some(field);
+        }
+    }
+    None
+}
+
+fn source_field<'a, 'b>(fields: &'a [Field<'b>]) -> Option<&'a Field<'b>> {
+    for field in fields {
+        if field.attrs.from.is_some() || field.attrs.source.is_some() {
+            return Some(field);
+        }
+    }
+    for field in fields {
+        match &field.member {
+            MemberUnraw::Named(ident) if ident == "source" => return Some(field),
+            _ => {}
+        }
+    }
+    None
+}
+
+fn backtrace_field<'a, 'b>(fields: &'a [Field<'b>]) -> Option<&'a Field<'b>> {
+    for field in fields {
+        if field.attrs.backtrace.is_some() {
+            return Some(field);
+        }
+    }
+    for field in fields {
+        if field.is_backtrace() {
+            return Some(field);
+        }
+    }
+    None
+}
+
+// The #[backtrace] field, if it is not the same as the #[from] field.
+fn distinct_backtrace_field<'a, 'b>(
+    backtrace_field: &'a Field<'b>,
+    from_field: Option<&Field>,
+) -> Option<&'a Field<'b>> {
+    if from_field.map_or(false, |from_field| {
+        from_field.member == backtrace_field.member
+    }) {
+        None
+    } else {
+        Some(backtrace_field)
+    }
+}
+
+fn type_is_backtrace(ty: &Type) -> bool {
+    let path = match ty {
+        Type::Path(ty) => &ty.path,
+        _ => return false,
+    };
+
+    let last = path.segments.last().unwrap();
+    last.ident == "Backtrace" && last.arguments.is_empty()
+}

impl/src/scan_expr.rs

@@ -0,0 +1,264 @@
+use self::{Action::*, Input::*};
+use proc_macro2::{Delimiter, Ident, Spacing, TokenTree};
+use syn::parse::{ParseStream, Result};
+use syn::{AngleBracketedGenericArguments, BinOp, Expr, ExprPath, Lifetime, Lit, Token, Type};
+
+enum Input {
+    Keyword(&'static str),
+    Punct(&'static str),
+    ConsumeAny,
+    ConsumeBinOp,
+    ConsumeBrace,
+    ConsumeDelimiter,
+    ConsumeIdent,
+    ConsumeLifetime,
+    ConsumeLiteral,
+    ConsumeNestedBrace,
+    ExpectPath,
+    ExpectTurbofish,
+    ExpectType,
+    CanBeginExpr,
+    Otherwise,
+    Empty,
+}
+
+enum Action {
+    SetState(&'static [(Input, Action)]),
+    IncDepth,
+    DecDepth,
+    Finish,
+}
+
+static INIT: [(Input, Action); 28] = [
+    (ConsumeDelimiter, SetState(&POSTFIX)),
+    (Keyword("async"), SetState(&ASYNC)),
+    (Keyword("break"), SetState(&BREAK_LABEL)),
+    (Keyword("const"), SetState(&CONST)),
+    (Keyword("continue"), SetState(&CONTINUE)),
+    (Keyword("for"), SetState(&FOR)),
+    (Keyword("if"), IncDepth),
+    (Keyword("let"), SetState(&PATTERN)),
+    (Keyword("loop"), SetState(&BLOCK)),
+    (Keyword("match"), IncDepth),
+    (Keyword("move"), SetState(&CLOSURE)),
+    (Keyword("return"), SetState(&RETURN)),
+    (Keyword("static"), SetState(&CLOSURE)),
+    (Keyword("unsafe"), SetState(&BLOCK)),
+    (Keyword("while"), IncDepth),
+    (Keyword("yield"), SetState(&RETURN)),
+    (Keyword("_"), SetState(&POSTFIX)),
+    (Punct("!"), SetState(&INIT)),
+    (Punct("#"), SetState(&[(ConsumeDelimiter, SetState(&INIT))])),
+    (Punct("&"), SetState(&REFERENCE)),
+    (Punct("*"), SetState(&INIT)),
+    (Punct("-"), SetState(&INIT)),
+    (Punct("..="), SetState(&INIT)),
+    (Punct(".."), SetState(&RANGE)),
+    (Punct("|"), SetState(&CLOSURE_ARGS)),
+    (ConsumeLifetime, SetState(&[(Punct(":"), SetState(&INIT))])),
+    (ConsumeLiteral, SetState(&POSTFIX)),
+    (ExpectPath, SetState(&PATH)),
+];
+
+static POSTFIX: [(Input, Action); 10] = [
+    (Keyword("as"), SetState(&[(ExpectType, SetState(&POSTFIX))])),
+    (Punct("..="), SetState(&INIT)),
+    (Punct(".."), SetState(&RANGE)),
+    (Punct("."), SetState(&DOT)),
+    (Punct("?"), SetState(&POSTFIX)),
+    (ConsumeBinOp, SetState(&INIT)),
+    (Punct("="), SetState(&INIT)),
+    (ConsumeNestedBrace, SetState(&IF_THEN)),
+    (ConsumeDelimiter, SetState(&POSTFIX)),
+    (Empty, Finish),
+];
+
+static ASYNC: [(Input, Action); 3] = [
+    (Keyword("move"), SetState(&ASYNC)),
+    (Punct("|"), SetState(&CLOSURE_ARGS)),
+    (ConsumeBrace, SetState(&POSTFIX)),
+];
+
+static BLOCK: [(Input, Action); 1] = [(ConsumeBrace, SetState(&POSTFIX))];
+
+static BREAK_LABEL: [(Input, Action); 2] = [
+    (ConsumeLifetime, SetState(&BREAK_VALUE)),
+    (Otherwise, SetState(&BREAK_VALUE)),
+];
+
+static BREAK_VALUE: [(Input, Action); 3] = [
+    (ConsumeNestedBrace, SetState(&IF_THEN)),
+    (CanBeginExpr, SetState(&INIT)),
+    (Otherwise, SetState(&POSTFIX)),
+];
+
+static CLOSURE: [(Input, Action); 6] = [
+    (Keyword("async"), SetState(&CLOSURE)),
+    (Keyword("move"), SetState(&CLOSURE)),
+    (Punct(","), SetState(&CLOSURE)),
+    (Punct(">"), SetState(&CLOSURE)),
+    (Punct("|"), SetState(&CLOSURE_ARGS)),
+    (ConsumeLifetime, SetState(&CLOSURE)),
+];
+
+static CLOSURE_ARGS: [(Input, Action); 2] = [
+    (Punct("|"), SetState(&CLOSURE_RET)),
+    (ConsumeAny, SetState(&CLOSURE_ARGS)),
+];
+
+static CLOSURE_RET: [(Input, Action); 2] = [
+    (Punct("->"), SetState(&[(ExpectType, SetState(&BLOCK))])),
+    (Otherwise, SetState(&INIT)),
+];
+
+static CONST: [(Input, Action); 2] = [
+    (Punct("|"), SetState(&CLOSURE_ARGS)),
+    (ConsumeBrace, SetState(&POSTFIX)),
+];
+
+static CONTINUE: [(Input, Action); 2] = [
+    (ConsumeLifetime, SetState(&POSTFIX)),
+    (Otherwise, SetState(&POSTFIX)),
+];
+
+static DOT: [(Input, Action); 3] = [
+    (Keyword("await"), SetState(&POSTFIX)),
+    (ConsumeIdent, SetState(&METHOD)),
+    (ConsumeLiteral, SetState(&POSTFIX)),
+];
+
+static FOR: [(Input, Action); 2] = [
+    (Punct("<"), SetState(&CLOSURE)),
+    (Otherwise, SetState(&PATTERN)),
+];
+
+static IF_ELSE: [(Input, Action); 2] = [(Keyword("if"), SetState(&INIT)), (ConsumeBrace, DecDepth)];
+static IF_THEN: [(Input, Action); 2] =
+    [(Keyword("else"), SetState(&IF_ELSE)), (Otherwise, DecDepth)];
+
+static METHOD: [(Input, Action); 1] = [(ExpectTurbofish, SetState(&POSTFIX))];
+
+static PATH: [(Input, Action); 4] = [
+    (Punct("!="), SetState(&INIT)),
+    (Punct("!"), SetState(&INIT)),
+    (ConsumeNestedBrace, SetState(&IF_THEN)),
+    (Otherwise, SetState(&POSTFIX)),
+];
+
+static PATTERN: [(Input, Action); 15] = [
+    (ConsumeDelimiter, SetState(&PATTERN)),
+    (Keyword("box"), SetState(&PATTERN)),
+    (Keyword("in"), IncDepth),
+    (Keyword("mut"), SetState(&PATTERN)),
+    (Keyword("ref"), SetState(&PATTERN)),
+    (Keyword("_"), SetState(&PATTERN)),
+    (Punct("!"), SetState(&PATTERN)),
+    (Punct("&"), SetState(&PATTERN)),
+    (Punct("..="), SetState(&PATTERN)),
+    (Punct(".."), SetState(&PATTERN)),
+    (Punct("="), SetState(&INIT)),
+    (Punct("@"), SetState(&PATTERN)),
+    (Punct("|"), SetState(&PATTERN)),
+    (ConsumeLiteral, SetState(&PATTERN)),
+    (ExpectPath, SetState(&PATTERN)),
+];
+
+static RANGE: [(Input, Action); 6] = [
+    (Punct("..="), SetState(&INIT)),
+    (Punct(".."), SetState(&RANGE)),
+    (Punct("."), SetState(&DOT)),
+    (ConsumeNestedBrace, SetState(&IF_THEN)),
+    (Empty, Finish),
+    (Otherwise, SetState(&INIT)),
+];
+
+static RAW: [(Input, Action); 3] = [
+    (Keyword("const"), SetState(&INIT)),
+    (Keyword("mut"), SetState(&INIT)),
+    (Otherwise, SetState(&POSTFIX)),
+];
+
+static REFERENCE: [(Input, Action); 3] = [
+    (Keyword("mut"), SetState(&INIT)),
+    (Keyword("raw"), SetState(&RAW)),
+    (Otherwise, SetState(&INIT)),
+];
+
+static RETURN: [(Input, Action); 2] = [
+    (CanBeginExpr, SetState(&INIT)),
+    (Otherwise, SetState(&POSTFIX)),
+];
+
+pub(crate) fn scan_expr(input: ParseStream) -> Result<()> {
+    let mut state = INIT.as_slice();
+    let mut depth = 0usize;
+    'table: loop {
+        for rule in state {
+            if match rule.0 {
+                Input::Keyword(expected) => input.step(|cursor| match cursor.ident() {
+                    Some((ident, rest)) if ident == expected => Ok((true, rest)),
+                    _ => Ok((false, *cursor)),
+                })?,
+                Input::Punct(expected) => input.step(|cursor| {
+                    let begin = *cursor;
+                    let mut cursor = begin;
+                    for (i, ch) in expected.chars().enumerate() {
+                        match cursor.punct() {
+                            Some((punct, _)) if punct.as_char() != ch => break,
+                            Some((_, rest)) if i == expected.len() - 1 => {
+                                return Ok((true, rest));
+                            }
+                            Some((punct, rest)) if punct.spacing() == Spacing::Joint => {
+                                cursor = rest;
+                            }
+                            _ => break,
+                        }
+                    }
+                    Ok((false, begin))
+                })?,
+                Input::ConsumeAny => input.parse::<Option<TokenTree>>()?.is_some(),
+                Input::ConsumeBinOp => input.parse::<BinOp>().is_ok(),
+                Input::ConsumeBrace | Input::ConsumeNestedBrace => {
+                    (matches!(rule.0, Input::ConsumeBrace) || depth > 0)
+                        && input.step(|cursor| match cursor.group(Delimiter::Brace) {
+                            Some((_inside, _span, rest)) => Ok((true, rest)),
+                            None => Ok((false, *cursor)),
+                        })?
+                }
+                Input::ConsumeDelimiter => input.step(|cursor| match cursor.any_group() {
+                    Some((_inside, _delimiter, _span, rest)) => Ok((true, rest)),
+                    None => Ok((false, *cursor)),
+                })?,
+                Input::ConsumeIdent => input.parse::<Option<Ident>>()?.is_some(),
+                Input::ConsumeLifetime => input.parse::<Option<Lifetime>>()?.is_some(),
+                Input::ConsumeLiteral => input.parse::<Option<Lit>>()?.is_some(),
+                Input::ExpectPath => {
+                    input.parse::<ExprPath>()?;
+                    true
+                }
+                Input::ExpectTurbofish => {
+                    if input.peek(Token![::]) {
+                        input.parse::<AngleBracketedGenericArguments>()?;
+                    }
+                    true
+                }
+                Input::ExpectType => {
+                    Type::without_plus(input)?;
+                    true
+                }
+                Input::CanBeginExpr => Expr::peek(input),
+                Input::Otherwise => true,
+                Input::Empty => input.is_empty() || input.peek(Token![,]),
+            } {
+                state = match rule.1 {
+                    Action::SetState(next) => next,
+                    Action::IncDepth => (depth += 1, &INIT).1,
+                    Action::DecDepth => (depth -= 1, &POSTFIX).1,
+                    Action::Finish => return if depth == 0 { Ok(()) } else { break },
+                };
+                continue 'table;
+            }
+        }
+        return Err(input.error("unsupported expression"));
+    }
+}

impl/src/unraw.rs

@@ -0,0 +1,142 @@
+use proc_macro2::{Ident, Span, TokenStream};
+use quote::ToTokens;
+use std::cmp::Ordering;
+use std::fmt::{self, Display};
+use std::hash::{Hash, Hasher};
+use syn::ext::IdentExt as _;
+use syn::parse::{Parse, ParseStream, Result};
+use syn::Index;
+
+#[derive(Clone)]
+#[repr(transparent)]
+pub struct IdentUnraw(Ident);
+
+impl IdentUnraw {
+    pub fn new(ident: Ident) -> Self {
+        IdentUnraw(ident)
+    }
+
+    pub fn to_local(&self) -> Ident {
+        let unraw = self.0.unraw();
+        let repr = unraw.to_string();
+        if syn::parse_str::<Ident>(&repr).is_err() {
+            if let "_" | "super" | "self" | "Self" | "crate" = repr.as_str() {
+                // Some identifiers are never allowed to appear as raw, like r#self and r#_.
+            } else {
+                return Ident::new_raw(&repr, Span::call_site());
+            }
+        }
+        unraw
+    }
+
+    pub fn set_span(&mut self, span: Span) {
+        self.0.set_span(span);
+    }
+}
+
+impl Display for IdentUnraw {
+    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        Display::fmt(&self.0.unraw(), formatter)
+    }
+}
+
+impl Eq for IdentUnraw {}
+
+impl PartialEq for IdentUnraw {
+    fn eq(&self, other: &Self) -> bool {
+        PartialEq::eq(&self.0.unraw(), &other.0.unraw())
+    }
+}
+
+impl PartialEq<str> for IdentUnraw {
+    fn eq(&self, other: &str) -> bool {
+        self.0 == other
+    }
+}
+
+impl Ord for IdentUnraw {
+    fn cmp(&self, other: &Self) -> Ordering {
+        Ord::cmp(&self.0.unraw(), &other.0.unraw())
+    }
+}
+
+impl PartialOrd for IdentUnraw {
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+        Some(Self::cmp(self, other))
+    }
+}
+
+impl Parse for IdentUnraw {
+    fn parse(input: ParseStream) -> Result<Self> {
+        input.call(Ident::parse_any).map(IdentUnraw::new)
+    }
+}
+
+impl ToTokens for IdentUnraw {
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        self.0.unraw().to_tokens(tokens);
+    }
+}
+
+#[derive(Clone)]
+pub enum MemberUnraw {
+    Named(IdentUnraw),
+    Unnamed(Index),
+}
+
+impl MemberUnraw {
+    pub fn span(&self) -> Span {
+        match self {
+            MemberUnraw::Named(ident) => ident.0.span(),
+            MemberUnraw::Unnamed(index) => index.span,
+        }
+    }
+}
+
+impl Display for MemberUnraw {
+    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        match self {
+            MemberUnraw::Named(this) => Display::fmt(this, formatter),
+            MemberUnraw::Unnamed(this) => Display::fmt(&this.index, formatter),
+        }
+    }
+}
+
+impl Eq for MemberUnraw {}
+
+impl PartialEq for MemberUnraw {
+    fn eq(&self, other: &Self) -> bool {
+        match (self, other) {
+            (MemberUnraw::Named(this), MemberUnraw::Named(other)) => this == other,
+            (MemberUnraw::Unnamed(this), MemberUnraw::Unnamed(other)) => this == other,
+            _ => false,
+        }
+    }
+}
+
+impl PartialEq<str> for MemberUnraw {
+    fn eq(&self, other: &str) -> bool {
+        match self {
+            MemberUnraw::Named(this) => this == other,
+            MemberUnraw::Unnamed(_) => false,
+        }
+    }
+}
+
+impl Hash for MemberUnraw {
+    fn hash<H: Hasher>(&self, hasher: &mut H) {
+        match self {
+            MemberUnraw::Named(ident) => ident.0.unraw().hash(hasher),
+            MemberUnraw::Unnamed(index) => index.hash(hasher),
+        }
+    }
+}
+
+impl ToTokens for MemberUnraw {
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        match self {
+            MemberUnraw::Named(ident) => ident.to_local().to_tokens(tokens),
+            MemberUnraw::Unnamed(index) => index.to_tokens(tokens),
+        }
+    }
+}

impl/src/valid.rs

@@ -0,0 +1,248 @@
+use crate::ast::{Enum, Field, Input, Struct, Variant};
+use crate::attr::Attrs;
+use syn::{Error, GenericArgument, PathArguments, Result, Type};
+
+impl Input<'_> {
+    pub(crate) fn validate(&self) -> Result<()> {
+        match self {
+            Input::Struct(input) => input.validate(),
+            Input::Enum(input) => input.validate(),
+        }
+    }
+}
+
+impl Struct<'_> {
+    fn validate(&self) -> Result<()> {
+        check_non_field_attrs(&self.attrs)?;
+        if let Some(transparent) = self.attrs.transparent {
+            if self.fields.len() != 1 {
+                return Err(Error::new_spanned(
+                    transparent.original,
+                    "#[error(transparent)] requires exactly one field",
+                ));
+            }
+            if let Some(source) = self.fields.iter().find_map(|f| f.attrs.source) {
+                return Err(Error::new_spanned(
+                    source.original,
+                    "transparent error struct can't contain #[source]",
+                ));
+            }
+        }
+        if let Some(fmt) = &self.attrs.fmt {
+            return Err(Error::new_spanned(
+                fmt.original,
+                "#[error(fmt = ...)] is only supported in enums; for a struct, handwrite your own Display impl",
+            ));
+        }
+        check_field_attrs(&self.fields)?;
+        for field in &self.fields {
+            field.validate()?;
+        }
+        Ok(())
+    }
+}
+
+impl Enum<'_> {
+    fn validate(&self) -> Result<()> {
+        check_non_field_attrs(&self.attrs)?;
+        let has_display = self.has_display();
+        for variant in &self.variants {
+            variant.validate()?;
+            if has_display
+                && variant.attrs.display.is_none()
+                && variant.attrs.transparent.is_none()
+                && variant.attrs.fmt.is_none()
+            {
+                return Err(Error::new_spanned(
+                    variant.original,
+                    "missing #[error(\"...\")] display attribute",
+                ));
+            }
+        }
+        Ok(())
+    }
+}
+
+impl Variant<'_> {
+    fn validate(&self) -> Result<()> {
+        check_non_field_attrs(&self.attrs)?;
+        if self.attrs.transparent.is_some() {
+            if self.fields.len() != 1 {
+                return Err(Error::new_spanned(
+                    self.original,
+                    "#[error(transparent)] requires exactly one field",
+                ));
+            }
+            if let Some(source) = self.fields.iter().find_map(|f| f.attrs.source) {
+                return Err(Error::new_spanned(
+                    source.original,
+                    "transparent variant can't contain #[source]",
+                ));
+            }
+        }
+        check_field_attrs(&self.fields)?;
+        for field in &self.fields {
+            field.validate()?;
+        }
+        Ok(())
+    }
+}
+
+impl Field<'_> {
+    fn validate(&self) -> Result<()> {
+        if let Some(unexpected_display_attr) = if let Some(display) = &self.attrs.display {
+            Some(display.original)
+        } else if let Some(fmt) = &self.attrs.fmt {
+            Some(fmt.original)
+        } else {
+            None
+        } {
+            return Err(Error::new_spanned(
+                unexpected_display_attr,
+                "not expected here; the #[error(...)] attribute belongs on top of a struct or an enum variant",
+            ));
+        }
+        Ok(())
+    }
+}
+
+fn check_non_field_attrs(attrs: &Attrs) -> Result<()> {
+    if let Some(from) = &attrs.from {
+        return Err(Error::new_spanned(
+            from.original,
+            "not expected here; the #[from] attribute belongs on a specific field",
+        ));
+    }
+    if let Some(source) = &attrs.source {
+        return Err(Error::new_spanned(
+            source.original,
+            "not expected here; the #[source] attribute belongs on a specific field",
+        ));
+    }
+    if let Some(backtrace) = &attrs.backtrace {
+        return Err(Error::new_spanned(
+            backtrace,
+            "not expected here; the #[backtrace] attribute belongs on a specific field",
+        ));
+    }
+    if attrs.transparent.is_some() {
+        if let Some(display) = &attrs.display {
+            return Err(Error::new_spanned(
+                display.original,
+                "cannot have both #[error(transparent)] and a display attribute",
+            ));
+        }
+        if let Some(fmt) = &attrs.fmt {
+            return Err(Error::new_spanned(
+                fmt.original,
+                "cannot have both #[error(transparent)] and #[error(fmt = ...)]",
+            ));
+        }
+    } else if let (Some(display), Some(_)) = (&attrs.display, &attrs.fmt) {
+        return Err(Error::new_spanned(
+            display.original,
+            "cannot have both #[error(fmt = ...)] and a format arguments attribute",
+        ));
+    }
+
+    Ok(())
+}
+
+fn check_field_attrs(fields: &[Field]) -> Result<()> {
+    let mut from_field = None;
+    let mut source_field = None;
+    let mut backtrace_field = None;
+    let mut has_backtrace = false;
+    for field in fields {
+        if let Some(from) = field.attrs.from {
+            if from_field.is_some() {
+                return Err(Error::new_spanned(
+                    from.original,
+                    "duplicate #[from] attribute",
+                ));
+            }
+            from_field = Some(field);
+        }
+        if let Some(source) = field.attrs.source {
+            if source_field.is_some() {
+                return Err(Error::new_spanned(
+                    source.original,
+                    "duplicate #[source] attribute",
+                ));
+            }
+            source_field = Some(field);
+        }
+        if let Some(backtrace) = field.attrs.backtrace {
+            if backtrace_field.is_some() {
+                return Err(Error::new_spanned(
+                    backtrace,
+                    "duplicate #[backtrace] attribute",
+                ));
+            }
+            backtrace_field = Some(field);
+            has_backtrace = true;
+        }
+        if let Some(transparent) = field.attrs.transparent {
+            return Err(Error::new_spanned(
+                transparent.original,
+                "#[error(transparent)] needs to go outside the enum or struct, not on an individual field",
+            ));
+        }
+        has_backtrace |= field.is_backtrace();
+    }
+    if let (Some(from_field), Some(source_field)) = (from_field, source_field) {
+        if from_field.member != source_field.member {
+            return Err(Error::new_spanned(
+                from_field.attrs.from.unwrap().original,
+                "#[from] is only supported on the source field, not any other field",
+            ));
+        }
+    }
+    if let Some(from_field) = from_field {
+        let max_expected_fields = match backtrace_field {
+            Some(backtrace_field) => 1 + (from_field.member != backtrace_field.member) as usize,
+            None => 1 + has_backtrace as usize,
+        };
+        if fields.len() > max_expected_fields {
+            return Err(Error::new_spanned(
+                from_field.attrs.from.unwrap().original,
+                "deriving From requires no fields other than source and backtrace",
+            ));
+        }
+    }
+    if let Some(source_field) = source_field.or(from_field) {
+        if contains_non_static_lifetime(source_field.ty) {
+            return Err(Error::new_spanned(
+                &source_field.original.ty,
+                "non-static lifetimes are not allowed in the source of an error, because std::error::Error requires the source is dyn Error + 'static",
+            ));
+        }
+    }
+    Ok(())
+}
+
+fn contains_non_static_lifetime(ty: &Type) -> bool {
+    match ty {
+        Type::Path(ty) => {
+            let bracketed = match &ty.path.segments.last().unwrap().arguments {
+                PathArguments::AngleBracketed(bracketed) => bracketed,
+                _ => return false,
+            };
+            for arg in &bracketed.args {
+                match arg {
+                    GenericArgument::Type(ty) if contains_non_static_lifetime(ty) => return true,
+                    GenericArgument::Lifetime(lifetime) if lifetime.ident != "static" => {
+                        return true
+                    }
+                    _ => {}
+                }
+            }
+            false
+        }
+        Type::Reference(ty) => ty
+            .lifetime
+            .as_ref()
+            .map_or(false, |lifetime| lifetime.ident != "static"),
+        _ => false, // maybe implement later if there are common other cases
+    }
+}