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

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

tests/compiletest.rs

@@ -0,0 +1,7 @@
+#[rustversion::attr(not(nightly), ignore = "requires nightly")]
+#[cfg_attr(miri, ignore = "incompatible with miri")]
+#[test]
+fn ui() {
+    let t = trybuild::TestCases::new();
+    t.compile_fail("tests/ui/*.rs");
+}

tests/no-std/Cargo.toml

@@ -0,0 +1,12 @@
+[package]
+name = "thiserror_no_std_test"
+version = "0.0.0"
+authors = ["David Tolnay <dtolnay@gmail.com>"]
+edition = "2021"
+publish = false
+
+[lib]
+path = "test.rs"
+
+[dependencies]
+thiserror = { path = "../..", default-features = false }

tests/no-std/test.rs

@@ -0,0 +1,58 @@
+#![no_std]
+
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+pub enum Error {
+    #[error("Error::E")]
+    E(#[from] SourceError),
+}
+
+#[derive(Error, Debug)]
+#[error("SourceError {field}")]
+pub struct SourceError {
+    pub field: i32,
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::{Error, SourceError};
+    use core::error::Error as _;
+    use core::fmt::{self, Write};
+    use core::mem;
+
+    struct Buf<'a>(&'a mut [u8]);
+
+    impl Write for Buf<'_> {
+        fn write_str(&mut self, s: &str) -> fmt::Result {
+            if s.len() <= self.0.len() {
+                let (out, rest) = mem::take(&mut self.0).split_at_mut(s.len());
+                out.copy_from_slice(s.as_bytes());
+                self.0 = rest;
+                Ok(())
+            } else {
+                Err(fmt::Error)
+            }
+        }
+    }
+
+    #[test]
+    fn test() {
+        let source = SourceError { field: -1 };
+        let error = Error::from(source);
+
+        let source = error
+            .source()
+            .unwrap()
+            .downcast_ref::<SourceError>()
+            .unwrap();
+
+        let mut msg = [b'~'; 17];
+        write!(Buf(&mut msg), "{error}").unwrap();
+        assert_eq!(msg, *b"Error::E~~~~~~~~~");
+
+        let mut msg = [b'~'; 17];
+        write!(Buf(&mut msg), "{source}").unwrap();
+        assert_eq!(msg, *b"SourceError -1~~~");
+    }
+}

tests/test_backtrace.rs

@@ -0,0 +1,289 @@
+#![cfg(feature = "std")]
+#![cfg_attr(thiserror_nightly_testing, feature(error_generic_member_access))]
+
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+#[error("...")]
+pub struct Inner;
+
+#[cfg(thiserror_nightly_testing)]
+#[derive(Error, Debug)]
+#[error("...")]
+pub struct InnerBacktrace {
+    backtrace: std::backtrace::Backtrace,
+}
+
+#[cfg(thiserror_nightly_testing)]
+pub mod structs {
+    use super::{Inner, InnerBacktrace};
+    use std::backtrace::Backtrace;
+    use std::error::{self, Error};
+    use std::sync::Arc;
+    use thiserror::Error;
+
+    mod not_backtrace {
+        #[derive(Debug)]
+        pub struct Backtrace;
+    }
+
+    #[derive(Error, Debug)]
+    #[error("...")]
+    pub struct PlainBacktrace {
+        backtrace: Backtrace,
+    }
+
+    #[derive(Error, Debug)]
+    #[error("...")]
+    pub struct ExplicitBacktrace {
+        #[backtrace]
+        backtrace: Backtrace,
+    }
+
+    #[derive(Error, Debug)]
+    #[error("...")]
+    pub struct NotBacktrace {
+        backtrace: crate::structs::not_backtrace::r#Backtrace,
+    }
+
+    #[derive(Error, Debug)]
+    #[error("...")]
+    pub struct OptBacktrace {
+        #[backtrace]
+        backtrace: Option<Backtrace>,
+    }
+
+    #[derive(Error, Debug)]
+    #[error("...")]
+    pub struct ArcBacktrace {
+        #[backtrace]
+        backtrace: Arc<Backtrace>,
+    }
+
+    #[derive(Error, Debug)]
+    #[error("...")]
+    pub struct BacktraceFrom {
+        #[from]
+        source: Inner,
+        #[backtrace]
+        backtrace: Backtrace,
+    }
+
+    #[derive(Error, Debug)]
+    #[error("...")]
+    pub struct CombinedBacktraceFrom {
+        #[from]
+        #[backtrace]
+        source: InnerBacktrace,
+    }
+
+    #[derive(Error, Debug)]
+    #[error("...")]
+    pub struct OptBacktraceFrom {
+        #[from]
+        source: Inner,
+        #[backtrace]
+        backtrace: Option<Backtrace>,
+    }
+
+    #[derive(Error, Debug)]
+    #[error("...")]
+    pub struct ArcBacktraceFrom {
+        #[from]
+        source: Inner,
+        #[backtrace]
+        backtrace: Arc<Backtrace>,
+    }
+
+    #[derive(Error, Debug)]
+    #[error("...")]
+    pub struct AnyhowBacktrace {
+        #[backtrace]
+        source: anyhow::Error,
+    }
+
+    #[derive(Error, Debug)]
+    #[error("...")]
+    pub struct BoxDynErrorBacktrace {
+        #[backtrace]
+        source: Box<dyn Error>,
+    }
+
+    #[test]
+    fn test_backtrace() {
+        let error = PlainBacktrace {
+            backtrace: Backtrace::capture(),
+        };
+        assert!(error::request_ref::<Backtrace>(&error).is_some());
+
+        let error = ExplicitBacktrace {
+            backtrace: Backtrace::capture(),
+        };
+        assert!(error::request_ref::<Backtrace>(&error).is_some());
+
+        let error = OptBacktrace {
+            backtrace: Some(Backtrace::capture()),
+        };
+        assert!(error::request_ref::<Backtrace>(&error).is_some());
+
+        let error = ArcBacktrace {
+            backtrace: Arc::new(Backtrace::capture()),
+        };
+        assert!(error::request_ref::<Backtrace>(&error).is_some());
+
+        let error = BacktraceFrom::from(Inner);
+        assert!(error::request_ref::<Backtrace>(&error).is_some());
+
+        let error = CombinedBacktraceFrom::from(InnerBacktrace {
+            backtrace: Backtrace::capture(),
+        });
+        assert!(error::request_ref::<Backtrace>(&error).is_some());
+
+        let error = OptBacktraceFrom::from(Inner);
+        assert!(error::request_ref::<Backtrace>(&error).is_some());
+
+        let error = ArcBacktraceFrom::from(Inner);
+        assert!(error::request_ref::<Backtrace>(&error).is_some());
+
+        let error = AnyhowBacktrace {
+            source: anyhow::Error::msg("..."),
+        };
+        assert!(error::request_ref::<Backtrace>(&error).is_some());
+
+        let error = BoxDynErrorBacktrace {
+            source: Box::new(PlainBacktrace {
+                backtrace: Backtrace::capture(),
+            }),
+        };
+        assert!(error::request_ref::<Backtrace>(&error).is_some());
+    }
+}
+
+#[cfg(thiserror_nightly_testing)]
+pub mod enums {
+    use super::{Inner, InnerBacktrace};
+    use std::backtrace::Backtrace;
+    use std::error;
+    use std::sync::Arc;
+    use thiserror::Error;
+
+    #[derive(Error, Debug)]
+    pub enum PlainBacktrace {
+        #[error("...")]
+        Test { backtrace: Backtrace },
+    }
+
+    #[derive(Error, Debug)]
+    pub enum ExplicitBacktrace {
+        #[error("...")]
+        Test {
+            #[backtrace]
+            backtrace: Backtrace,
+        },
+    }
+
+    #[derive(Error, Debug)]
+    pub enum OptBacktrace {
+        #[error("...")]
+        Test {
+            #[backtrace]
+            backtrace: Option<Backtrace>,
+        },
+    }
+
+    #[derive(Error, Debug)]
+    pub enum ArcBacktrace {
+        #[error("...")]
+        Test {
+            #[backtrace]
+            backtrace: Arc<Backtrace>,
+        },
+    }
+
+    #[derive(Error, Debug)]
+    pub enum BacktraceFrom {
+        #[error("...")]
+        Test {
+            #[from]
+            source: Inner,
+            #[backtrace]
+            backtrace: Backtrace,
+        },
+    }
+
+    #[derive(Error, Debug)]
+    pub enum CombinedBacktraceFrom {
+        #[error("...")]
+        Test {
+            #[from]
+            #[backtrace]
+            source: InnerBacktrace,
+        },
+    }
+
+    #[derive(Error, Debug)]
+    pub enum OptBacktraceFrom {
+        #[error("...")]
+        Test {
+            #[from]
+            source: Inner,
+            #[backtrace]
+            backtrace: Option<Backtrace>,
+        },
+    }
+
+    #[derive(Error, Debug)]
+    pub enum ArcBacktraceFrom {
+        #[error("...")]
+        Test {
+            #[from]
+            source: Inner,
+            #[backtrace]
+            backtrace: Arc<Backtrace>,
+        },
+    }
+
+    #[test]
+    fn test_backtrace() {
+        let error = PlainBacktrace::Test {
+            backtrace: Backtrace::capture(),
+        };
+        assert!(error::request_ref::<Backtrace>(&error).is_some());
+
+        let error = ExplicitBacktrace::Test {
+            backtrace: Backtrace::capture(),
+        };
+        assert!(error::request_ref::<Backtrace>(&error).is_some());
+
+        let error = OptBacktrace::Test {
+            backtrace: Some(Backtrace::capture()),
+        };
+        assert!(error::request_ref::<Backtrace>(&error).is_some());
+
+        let error = ArcBacktrace::Test {
+            backtrace: Arc::new(Backtrace::capture()),
+        };
+        assert!(error::request_ref::<Backtrace>(&error).is_some());
+
+        let error = BacktraceFrom::from(Inner);
+        assert!(error::request_ref::<Backtrace>(&error).is_some());
+
+        let error = CombinedBacktraceFrom::from(InnerBacktrace {
+            backtrace: Backtrace::capture(),
+        });
+        assert!(error::request_ref::<Backtrace>(&error).is_some());
+
+        let error = OptBacktraceFrom::from(Inner);
+        assert!(error::request_ref::<Backtrace>(&error).is_some());
+
+        let error = ArcBacktraceFrom::from(Inner);
+        assert!(error::request_ref::<Backtrace>(&error).is_some());
+    }
+}
+
+#[test]
+#[cfg_attr(
+    not(thiserror_nightly_testing),
+    ignore = "requires `--cfg=thiserror_nightly_testing`"
+)]
+fn test_backtrace() {}

tests/test_display.rs

@@ -0,0 +1,478 @@
+#![allow(
+    clippy::elidable_lifetime_names,
+    clippy::needless_lifetimes,
+    clippy::needless_raw_string_hashes,
+    clippy::trivially_copy_pass_by_ref,
+    clippy::uninlined_format_args
+)]
+
+use core::fmt::{self, Display};
+use thiserror::Error;
+
+fn assert<T: Display>(expected: &str, value: T) {
+    assert_eq!(expected, value.to_string());
+}
+
+#[test]
+fn test_braced() {
+    #[derive(Error, Debug)]
+    #[error("braced error: {msg}")]
+    struct Error {
+        msg: String,
+    }
+
+    let msg = "T".to_owned();
+    assert("braced error: T", Error { msg });
+}
+
+#[test]
+fn test_braced_unused() {
+    #[derive(Error, Debug)]
+    #[error("braced error")]
+    struct Error {
+        extra: usize,
+    }
+
+    assert("braced error", Error { extra: 0 });
+}
+
+#[test]
+fn test_tuple() {
+    #[derive(Error, Debug)]
+    #[error("tuple error: {0}")]
+    struct Error(usize);
+
+    assert("tuple error: 0", Error(0));
+}
+
+#[test]
+fn test_unit() {
+    #[derive(Error, Debug)]
+    #[error("unit error")]
+    struct Error;
+
+    assert("unit error", Error);
+}
+
+#[test]
+fn test_enum() {
+    #[derive(Error, Debug)]
+    enum Error {
+        #[error("braced error: {id}")]
+        Braced { id: usize },
+        #[error("tuple error: {0}")]
+        Tuple(usize),
+        #[error("unit error")]
+        Unit,
+    }
+
+    assert("braced error: 0", Error::Braced { id: 0 });
+    assert("tuple error: 0", Error::Tuple(0));
+    assert("unit error", Error::Unit);
+}
+
+#[test]
+fn test_constants() {
+    #[derive(Error, Debug)]
+    #[error("{MSG}: {id:?} (code {CODE:?})")]
+    struct Error {
+        id: &'static str,
+    }
+
+    const MSG: &str = "failed to do";
+    const CODE: usize = 9;
+
+    assert("failed to do: \"\" (code 9)", Error { id: "" });
+}
+
+#[test]
+fn test_inherit() {
+    #[derive(Error, Debug)]
+    #[error("{0}")]
+    enum Error {
+        Some(&'static str),
+        #[error("other error")]
+        Other(&'static str),
+    }
+
+    assert("some error", Error::Some("some error"));
+    assert("other error", Error::Other("..."));
+}
+
+#[test]
+fn test_brace_escape() {
+    #[derive(Error, Debug)]
+    #[error("fn main() {{}}")]
+    struct Error;
+
+    assert("fn main() {}", Error);
+}
+
+#[test]
+fn test_expr() {
+    #[derive(Error, Debug)]
+    #[error("1 + 1 = {}", 1 + 1)]
+    struct Error;
+    assert("1 + 1 = 2", Error);
+}
+
+#[test]
+fn test_nested() {
+    #[derive(Error, Debug)]
+    #[error("!bool = {}", not(.0))]
+    struct Error(bool);
+
+    #[allow(clippy::trivially_copy_pass_by_ref)]
+    fn not(bool: &bool) -> bool {
+        !*bool
+    }
+
+    assert("!bool = false", Error(true));
+}
+
+#[test]
+fn test_match() {
+    #[derive(Error, Debug)]
+    #[error("{intro}: {0}", intro = match .1 {
+        Some(n) => format!("error occurred with {}", n),
+        None => "there was an empty error".to_owned(),
+    })]
+    struct Error(String, Option<usize>);
+
+    assert(
+        "error occurred with 1: ...",
+        Error("...".to_owned(), Some(1)),
+    );
+    assert(
+        "there was an empty error: ...",
+        Error("...".to_owned(), None),
+    );
+}
+
+#[test]
+fn test_nested_display() {
+    // Same behavior as the one in `test_match`, but without String allocations.
+    #[derive(Error, Debug)]
+    #[error("{}", {
+        struct Msg<'a>(&'a String, &'a Option<usize>);
+        impl<'a> Display for Msg<'a> {
+            fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+                match self.1 {
+                    Some(n) => write!(formatter, "error occurred with {}", n),
+                    None => write!(formatter, "there was an empty error"),
+                }?;
+                write!(formatter, ": {}", self.0)
+            }
+        }
+        Msg(.0, .1)
+    })]
+    struct Error(String, Option<usize>);
+
+    assert(
+        "error occurred with 1: ...",
+        Error("...".to_owned(), Some(1)),
+    );
+    assert(
+        "there was an empty error: ...",
+        Error("...".to_owned(), None),
+    );
+}
+
+#[test]
+fn test_void() {
+    #[allow(clippy::empty_enums)]
+    #[derive(Error, Debug)]
+    #[error("...")]
+    pub enum Error {}
+
+    let _: Error;
+}
+
+#[test]
+fn test_mixed() {
+    #[derive(Error, Debug)]
+    #[error("a={a} :: b={} :: c={c} :: d={d}", 1, c = 2, d = 3)]
+    struct Error {
+        a: usize,
+        d: usize,
+    }
+
+    assert("a=0 :: b=1 :: c=2 :: d=3", Error { a: 0, d: 0 });
+}
+
+#[test]
+fn test_ints() {
+    #[derive(Error, Debug)]
+    enum Error {
+        #[error("error {0}")]
+        Tuple(usize, usize),
+        #[error("error {0}", '?')]
+        Struct { v: usize },
+    }
+
+    assert("error 9", Error::Tuple(9, 0));
+    assert("error ?", Error::Struct { v: 0 });
+}
+
+#[test]
+fn test_trailing_comma() {
+    #[derive(Error, Debug)]
+    #[error(
+        "error {0}",
+    )]
+    #[rustfmt::skip]
+    struct Error(char);
+
+    assert("error ?", Error('?'));
+}
+
+#[test]
+fn test_field() {
+    #[derive(Debug)]
+    struct Inner {
+        data: usize,
+    }
+
+    #[derive(Error, Debug)]
+    #[error("{}", .0.data)]
+    struct Error(Inner);
+
+    assert("0", Error(Inner { data: 0 }));
+}
+
+#[test]
+fn test_nested_tuple_field() {
+    #[derive(Debug)]
+    struct Inner(usize);
+
+    #[derive(Error, Debug)]
+    #[error("{}", .0.0)]
+    struct Error(Inner);
+
+    assert("0", Error(Inner(0)));
+}
+
+#[test]
+fn test_pointer() {
+    #[derive(Error, Debug)]
+    #[error("{field:p}")]
+    pub struct Struct {
+        field: Box<i32>,
+    }
+
+    let s = Struct {
+        field: Box::new(-1),
+    };
+    assert_eq!(s.to_string(), format!("{:p}", s.field));
+}
+
+#[test]
+fn test_macro_rules_variant_from_call_site() {
+    // Regression test for https://github.com/dtolnay/thiserror/issues/86
+
+    macro_rules! decl_error {
+        ($variant:ident($value:ident)) => {
+            #[derive(Error, Debug)]
+            pub enum Error0 {
+                #[error("{0:?}")]
+                $variant($value),
+            }
+
+            #[derive(Error, Debug)]
+            #[error("{0:?}")]
+            pub enum Error1 {
+                $variant($value),
+            }
+        };
+    }
+
+    decl_error!(Repro(u8));
+
+    assert("0", Error0::Repro(0));
+    assert("0", Error1::Repro(0));
+}
+
+#[test]
+fn test_macro_rules_message_from_call_site() {
+    // Regression test for https://github.com/dtolnay/thiserror/issues/398
+
+    macro_rules! decl_error {
+        ($($errors:tt)*) => {
+            #[derive(Error, Debug)]
+            pub enum Error {
+                $($errors)*
+            }
+        };
+    }
+
+    decl_error! {
+        #[error("{0}")]
+        Unnamed(u8),
+        #[error("{x}")]
+        Named { x: u8 },
+    }
+
+    assert("0", Error::Unnamed(0));
+    assert("0", Error::Named { x: 0 });
+}
+
+#[test]
+fn test_raw() {
+    #[derive(Error, Debug)]
+    #[error("braced raw error: {fn}")]
+    struct Error {
+        r#fn: &'static str,
+    }
+
+    assert("braced raw error: T", Error { r#fn: "T" });
+}
+
+#[test]
+fn test_raw_enum() {
+    #[derive(Error, Debug)]
+    enum Error {
+        #[error("braced raw error: {fn}")]
+        Braced { r#fn: &'static str },
+    }
+
+    assert("braced raw error: T", Error::Braced { r#fn: "T" });
+}
+
+#[test]
+fn test_keyword() {
+    #[derive(Error, Debug)]
+    #[error("error: {type}", type = 1)]
+    struct Error;
+
+    assert("error: 1", Error);
+}
+
+#[test]
+fn test_self() {
+    #[derive(Error, Debug)]
+    #[error("error: {self:?}")]
+    struct Error;
+
+    assert("error: Error", Error);
+}
+
+#[test]
+fn test_str_special_chars() {
+    #[derive(Error, Debug)]
+    pub enum Error {
+        #[error("brace left {{")]
+        BraceLeft,
+        #[error("brace left 2 \x7B\x7B")]
+        BraceLeft2,
+        #[error("brace left 3 \u{7B}\u{7B}")]
+        BraceLeft3,
+        #[error("brace right }}")]
+        BraceRight,
+        #[error("brace right 2 \x7D\x7D")]
+        BraceRight2,
+        #[error("brace right 3 \u{7D}\u{7D}")]
+        BraceRight3,
+        #[error(
+            "new_\
+line"
+        )]
+        NewLine,
+        #[error("escape24 \u{78}")]
+        Escape24,
+    }
+
+    assert("brace left {", Error::BraceLeft);
+    assert("brace left 2 {", Error::BraceLeft2);
+    assert("brace left 3 {", Error::BraceLeft3);
+    assert("brace right }", Error::BraceRight);
+    assert("brace right 2 }", Error::BraceRight2);
+    assert("brace right 3 }", Error::BraceRight3);
+    assert("new_line", Error::NewLine);
+    assert("escape24 x", Error::Escape24);
+}
+
+#[test]
+fn test_raw_str() {
+    #[derive(Error, Debug)]
+    pub enum Error {
+        #[error(r#"raw brace left {{"#)]
+        BraceLeft,
+        #[error(r#"raw brace left 2 \x7B"#)]
+        BraceLeft2,
+        #[error(r#"raw brace right }}"#)]
+        BraceRight,
+        #[error(r#"raw brace right 2 \x7D"#)]
+        BraceRight2,
+    }
+
+    assert(r#"raw brace left {"#, Error::BraceLeft);
+    assert(r#"raw brace left 2 \x7B"#, Error::BraceLeft2);
+    assert(r#"raw brace right }"#, Error::BraceRight);
+    assert(r#"raw brace right 2 \x7D"#, Error::BraceRight2);
+}
+
+mod util {
+    use core::fmt::{self, Octal};
+
+    pub fn octal<T: Octal>(value: &T, formatter: &mut fmt::Formatter) -> fmt::Result {
+        write!(formatter, "0o{:o}", value)
+    }
+}
+
+#[test]
+fn test_fmt_path() {
+    fn unit(formatter: &mut fmt::Formatter) -> fmt::Result {
+        formatter.write_str("unit=")
+    }
+
+    fn pair(k: &i32, v: &i32, formatter: &mut fmt::Formatter) -> fmt::Result {
+        write!(formatter, "pair={k}:{v}")
+    }
+
+    #[derive(Error, Debug)]
+    pub enum Error {
+        #[error(fmt = unit)]
+        Unit,
+        #[error(fmt = pair)]
+        Tuple(i32, i32),
+        #[error(fmt = pair)]
+        Entry { k: i32, v: i32 },
+        #[error(fmt = crate::util::octal)]
+        I16(i16),
+        #[error(fmt = crate::util::octal::<i32>)]
+        I32 { n: i32 },
+        #[error(fmt = core::fmt::Octal::fmt)]
+        I64(i64),
+        #[error("...{0}")]
+        Other(bool),
+    }
+
+    assert("unit=", Error::Unit);
+    assert("pair=10:0", Error::Tuple(10, 0));
+    assert("pair=10:0", Error::Entry { k: 10, v: 0 });
+    assert("0o777", Error::I16(0o777));
+    assert("0o777", Error::I32 { n: 0o777 });
+    assert("777", Error::I64(0o777));
+    assert("...false", Error::Other(false));
+}
+
+#[test]
+fn test_fmt_path_inherited() {
+    #[derive(Error, Debug)]
+    #[error(fmt = crate::util::octal)]
+    pub enum Error {
+        I16(i16),
+        I32 {
+            n: i32,
+        },
+        #[error(fmt = core::fmt::Octal::fmt)]
+        I64(i64),
+        #[error("...{0}")]
+        Other(bool),
+    }
+
+    assert("0o777", Error::I16(0o777));
+    assert("0o777", Error::I32 { n: 0o777 });
+    assert("777", Error::I64(0o777));
+    assert("...false", Error::Other(false));
+}

tests/test_error.rs

@@ -0,0 +1,56 @@
+#![allow(dead_code)]
+
+use core::fmt::{self, Display};
+use std::io;
+use thiserror::Error;
+
+macro_rules! unimplemented_display {
+    ($ty:ty) => {
+        impl Display for $ty {
+            fn fmt(&self, _formatter: &mut fmt::Formatter) -> fmt::Result {
+                unimplemented!()
+            }
+        }
+    };
+}
+
+#[derive(Error, Debug)]
+struct BracedError {
+    msg: String,
+    pos: usize,
+}
+
+#[derive(Error, Debug)]
+struct TupleError(String, usize);
+
+#[derive(Error, Debug)]
+struct UnitError;
+
+#[derive(Error, Debug)]
+struct WithSource {
+    #[source]
+    cause: io::Error,
+}
+
+#[derive(Error, Debug)]
+struct WithAnyhow {
+    #[source]
+    cause: anyhow::Error,
+}
+
+#[derive(Error, Debug)]
+enum EnumError {
+    Braced {
+        #[source]
+        cause: io::Error,
+    },
+    Tuple(#[source] io::Error),
+    Unit,
+}
+
+unimplemented_display!(BracedError);
+unimplemented_display!(TupleError);
+unimplemented_display!(UnitError);
+unimplemented_display!(WithSource);
+unimplemented_display!(WithAnyhow);
+unimplemented_display!(EnumError);

tests/test_expr.rs

@@ -0,0 +1,118 @@
+#![allow(clippy::iter_cloned_collect, clippy::uninlined_format_args)]
+
+use core::fmt::Display;
+#[cfg(feature = "std")]
+use std::path::PathBuf;
+use thiserror::Error;
+
+// Some of the elaborate cases from the rcc codebase, which is a C compiler in
+// Rust. https://github.com/jyn514/rcc/blob/0.8.0/src/data/error.rs
+#[derive(Error, Debug)]
+pub enum CompilerError {
+    #[error("cannot shift {} by {maximum} or more bits (got {current})", if *.is_left { "left" } else { "right" })]
+    TooManyShiftBits {
+        is_left: bool,
+        maximum: u64,
+        current: u64,
+    },
+
+    #[error("#error {}", (.0).iter().copied().collect::<Vec<_>>().join(" "))]
+    User(Vec<&'static str>),
+
+    #[error("overflow while parsing {}integer literal",
+        if let Some(signed) = .is_signed {
+            if *signed { "signed "} else { "unsigned "}
+        } else {
+            ""
+        }
+    )]
+    IntegerOverflow { is_signed: Option<bool> },
+
+    #[error("overflow while parsing {}integer literal", match .is_signed {
+        Some(true) => "signed ",
+        Some(false) => "unsigned ",
+        None => "",
+    })]
+    IntegerOverflow2 { is_signed: Option<bool> },
+}
+
+// Examples drawn from Rustup.
+#[derive(Error, Debug)]
+pub enum RustupError {
+    #[error(
+        "toolchain '{name}' does not contain component {component}{}",
+        .suggestion
+            .as_ref()
+            .map_or_else(String::new, |s| format!("; did you mean '{}'?", s)),
+    )]
+    UnknownComponent {
+        name: String,
+        component: String,
+        suggestion: Option<String>,
+    },
+}
+
+#[track_caller]
+fn assert<T: Display>(expected: &str, value: T) {
+    assert_eq!(expected, value.to_string());
+}
+
+#[test]
+fn test_rcc() {
+    assert(
+        "cannot shift left by 32 or more bits (got 50)",
+        CompilerError::TooManyShiftBits {
+            is_left: true,
+            maximum: 32,
+            current: 50,
+        },
+    );
+
+    assert("#error A B C", CompilerError::User(vec!["A", "B", "C"]));
+
+    assert(
+        "overflow while parsing signed integer literal",
+        CompilerError::IntegerOverflow {
+            is_signed: Some(true),
+        },
+    );
+}
+
+#[test]
+fn test_rustup() {
+    assert(
+        "toolchain 'nightly' does not contain component clipy; did you mean 'clippy'?",
+        RustupError::UnknownComponent {
+            name: "nightly".to_owned(),
+            component: "clipy".to_owned(),
+            suggestion: Some("clippy".to_owned()),
+        },
+    );
+}
+
+// Regression test for https://github.com/dtolnay/thiserror/issues/335
+#[cfg(feature = "std")]
+#[test]
+#[allow(non_snake_case)]
+fn test_assoc_type_equality_constraint() {
+    pub trait Trait<T>: Display {
+        type A;
+    }
+
+    impl<T> Trait<T> for i32 {
+        type A = i32;
+    }
+
+    #[derive(Error, Debug)]
+    #[error("{A} {b}", b = &0 as &dyn Trait<i32, A = i32>)]
+    pub struct Error {
+        pub A: PathBuf,
+    }
+
+    assert(
+        "... 0",
+        Error {
+            A: PathBuf::from("..."),
+        },
+    );
+}

tests/test_from.rs

@@ -0,0 +1,64 @@
+#![allow(clippy::extra_unused_type_parameters)]
+
+use std::io;
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+#[error("...")]
+pub struct ErrorStruct {
+    #[from]
+    source: io::Error,
+}
+
+#[derive(Error, Debug)]
+#[error("...")]
+pub struct ErrorStructOptional {
+    #[from]
+    source: Option<io::Error>,
+}
+
+#[derive(Error, Debug)]
+#[error("...")]
+pub struct ErrorTuple(#[from] io::Error);
+
+#[derive(Error, Debug)]
+#[error("...")]
+pub struct ErrorTupleOptional(#[from] Option<io::Error>);
+
+#[derive(Error, Debug)]
+#[error("...")]
+pub enum ErrorEnum {
+    Test {
+        #[from]
+        source: io::Error,
+    },
+}
+
+#[derive(Error, Debug)]
+#[error("...")]
+pub enum ErrorEnumOptional {
+    Test {
+        #[from]
+        source: Option<io::Error>,
+    },
+}
+
+#[derive(Error, Debug)]
+#[error("...")]
+pub enum Many {
+    Any(#[from] anyhow::Error),
+    Io(#[from] io::Error),
+}
+
+fn assert_impl<T: From<io::Error>>() {}
+
+#[test]
+fn test_from() {
+    assert_impl::<ErrorStruct>();
+    assert_impl::<ErrorStructOptional>();
+    assert_impl::<ErrorTuple>();
+    assert_impl::<ErrorTupleOptional>();
+    assert_impl::<ErrorEnum>();
+    assert_impl::<ErrorEnumOptional>();
+    assert_impl::<Many>();
+}

tests/test_generics.rs

@@ -0,0 +1,205 @@
+#![allow(clippy::needless_late_init, clippy::uninlined_format_args)]
+
+use core::fmt::{self, Debug, Display};
+use core::str::FromStr;
+use thiserror::Error;
+
+pub struct NoFormat;
+
+#[derive(Debug)]
+pub struct DebugOnly;
+
+pub struct DisplayOnly;
+
+impl Display for DisplayOnly {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.write_str("display only")
+    }
+}
+
+#[derive(Debug)]
+pub struct DebugAndDisplay;
+
+impl Display for DebugAndDisplay {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.write_str("debug and display")
+    }
+}
+
+// Should expand to:
+//
+//     impl<E> Display for EnumDebugField<E>
+//     where
+//         E: Debug;
+//
+//     impl<E> Error for EnumDebugField<E>
+//     where
+//         Self: Debug + Display;
+//
+#[derive(Error, Debug)]
+pub enum EnumDebugGeneric<E> {
+    #[error("{0:?}")]
+    FatalError(E),
+}
+
+// Should expand to:
+//
+//     impl<E> Display for EnumFromGeneric<E>;
+//
+//     impl<E> Error for EnumFromGeneric<E>
+//     where
+//         EnumDebugGeneric<E>: Error + 'static,
+//         Self: Debug + Display;
+//
+#[derive(Error, Debug)]
+pub enum EnumFromGeneric<E> {
+    #[error("enum from generic")]
+    Source(#[from] EnumDebugGeneric<E>),
+}
+
+// Should expand to:
+//
+//     impl<HasDisplay, HasDebug, HasNeither> Display
+//         for EnumCompound<HasDisplay, HasDebug, HasNeither>
+//     where
+//         HasDisplay: Display,
+//         HasDebug: Debug;
+//
+//     impl<HasDisplay, HasDebug, HasNeither> Error
+//         for EnumCompound<HasDisplay, HasDebug, HasNeither>
+//     where
+//         Self: Debug + Display;
+//
+#[derive(Error)]
+pub enum EnumCompound<HasDisplay, HasDebug, HasNeither> {
+    #[error("{0} {1:?}")]
+    DisplayDebug(HasDisplay, HasDebug),
+    #[error("{0}")]
+    Display(HasDisplay, HasNeither),
+    #[error("{1:?}")]
+    Debug(HasNeither, HasDebug),
+}
+
+impl<HasDisplay, HasDebug, HasNeither> Debug for EnumCompound<HasDisplay, HasDebug, HasNeither> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.write_str("EnumCompound")
+    }
+}
+
+#[test]
+fn test_display_enum_compound() {
+    let mut instance: EnumCompound<DisplayOnly, DebugOnly, NoFormat>;
+
+    instance = EnumCompound::DisplayDebug(DisplayOnly, DebugOnly);
+    assert_eq!(format!("{}", instance), "display only DebugOnly");
+
+    instance = EnumCompound::Display(DisplayOnly, NoFormat);
+    assert_eq!(format!("{}", instance), "display only");
+
+    instance = EnumCompound::Debug(NoFormat, DebugOnly);
+    assert_eq!(format!("{}", instance), "DebugOnly");
+}
+
+// Should expand to:
+//
+//     impl<E> Display for EnumTransparentGeneric<E>
+//     where
+//         E: Display;
+//
+//     impl<E> Error for EnumTransparentGeneric<E>
+//     where
+//         E: Error,
+//         Self: Debug + Display;
+//
+#[derive(Error, Debug)]
+pub enum EnumTransparentGeneric<E> {
+    #[error(transparent)]
+    Other(E),
+}
+
+// Should expand to:
+//
+//     impl<E> Display for StructDebugGeneric<E>
+//     where
+//         E: Debug;
+//
+//     impl<E> Error for StructDebugGeneric<E>
+//     where
+//         Self: Debug + Display;
+//
+#[derive(Error, Debug)]
+#[error("{underlying:?}")]
+pub struct StructDebugGeneric<E> {
+    pub underlying: E,
+}
+
+// Should expand to:
+//
+//     impl<E> Error for StructFromGeneric<E>
+//     where
+//         StructDebugGeneric<E>: Error + 'static,
+//         Self: Debug + Display;
+//
+#[derive(Error, Debug)]
+pub struct StructFromGeneric<E> {
+    #[from]
+    pub source: StructDebugGeneric<E>,
+}
+
+// Should expand to:
+//
+//     impl<E> Display for StructTransparentGeneric<E>
+//     where
+//         E: Display;
+//
+//     impl<E> Error for StructTransparentGeneric<E>
+//     where
+//         E: Error,
+//         Self: Debug + Display;
+//
+#[derive(Error, Debug)]
+#[error(transparent)]
+pub struct StructTransparentGeneric<E>(pub E);
+
+// Should expand to:
+//
+//     impl<T: FromStr> Display for AssociatedTypeError<T>
+//     where
+//         T::Err: Display;
+//
+//     impl<T: FromStr> Error for AssociatedTypeError<T>
+//     where
+//         Self: Debug + Display;
+//
+#[derive(Error, Debug)]
+pub enum AssociatedTypeError<T: FromStr> {
+    #[error("couldn't parse matrix")]
+    Other,
+    #[error("couldn't parse entry: {0}")]
+    EntryParseError(T::Err),
+}
+
+// Regression test for https://github.com/dtolnay/thiserror/issues/345
+#[test]
+fn test_no_bound_on_named_fmt() {
+    #[derive(Error, Debug)]
+    #[error("{thing}", thing = "...")]
+    struct Error<T> {
+        thing: T,
+    }
+
+    let error = Error { thing: DebugOnly };
+    assert_eq!(error.to_string(), "...");
+}
+
+#[test]
+fn test_multiple_bound() {
+    #[derive(Error, Debug)]
+    #[error("0x{thing:x} 0x{thing:X}")]
+    pub struct Error<T> {
+        thing: T,
+    }
+
+    let error = Error { thing: 0xFFi32 };
+    assert_eq!(error.to_string(), "0xff 0xFF");
+}

tests/test_lints.rs

@@ -0,0 +1,98 @@
+#![allow(clippy::mixed_attributes_style)]
+
+use thiserror::Error;
+
+pub use std::error::Error;
+
+#[test]
+fn test_allow_attributes() {
+    #![deny(clippy::allow_attributes)]
+
+    #[derive(Error, Debug)]
+    #[error("...")]
+    pub struct MyError(#[from] anyhow::Error);
+
+    let _: MyError;
+}
+
+#[test]
+fn test_unused_qualifications() {
+    #![deny(unused_qualifications)]
+
+    // Expansion of derive(Error) macro can't know whether something like
+    // std::error::Error is already imported in the caller's scope so it must
+    // suppress unused_qualifications.
+
+    #[derive(Error, Debug)]
+    #[error("...")]
+    pub struct MyError;
+
+    let _: MyError;
+}
+
+#[test]
+fn test_needless_lifetimes() {
+    #![allow(dead_code)]
+    #![deny(clippy::elidable_lifetime_names, clippy::needless_lifetimes)]
+
+    #[derive(Error, Debug)]
+    #[error("...")]
+    pub enum MyError<'a> {
+        A(#[from] std::io::Error),
+        B(&'a ()),
+    }
+
+    let _: MyError;
+}
+
+#[test]
+fn test_deprecated() {
+    #![deny(deprecated)]
+
+    #[derive(Error, Debug)]
+    #[deprecated]
+    #[error("...")]
+    pub struct DeprecatedStruct;
+
+    #[derive(Error, Debug)]
+    #[error("{message} {}", .message)]
+    pub struct DeprecatedStructField {
+        #[deprecated]
+        message: String,
+    }
+
+    #[derive(Error, Debug)]
+    #[deprecated]
+    pub enum DeprecatedEnum {
+        #[error("...")]
+        Variant,
+    }
+
+    #[derive(Error, Debug)]
+    pub enum DeprecatedVariant {
+        #[deprecated]
+        #[error("...")]
+        Variant,
+    }
+
+    #[derive(Error, Debug)]
+    pub enum DeprecatedFrom {
+        #[error(transparent)]
+        Variant(
+            #[from]
+            #[allow(deprecated)]
+            DeprecatedStruct,
+        ),
+    }
+
+    #[allow(deprecated)]
+    let _: DeprecatedStruct;
+    #[allow(deprecated)]
+    let _: DeprecatedStructField;
+    #[allow(deprecated)]
+    let _ = DeprecatedEnum::Variant;
+    #[allow(deprecated)]
+    let _ = DeprecatedVariant::Variant;
+    #[allow(deprecated)]
+    let _ = DeprecatedFrom::Variant(DeprecatedStruct);
+}

tests/test_option.rs

@@ -0,0 +1,109 @@
+#![cfg(feature = "std")]
+#![cfg_attr(thiserror_nightly_testing, feature(error_generic_member_access))]
+
+#[cfg(thiserror_nightly_testing)]
+pub mod structs {
+    use std::backtrace::Backtrace;
+    use thiserror::Error;
+
+    #[derive(Error, Debug)]
+    #[error("...")]
+    pub struct OptSourceNoBacktrace {
+        #[source]
+        pub source: Option<anyhow::Error>,
+    }
+
+    #[derive(Error, Debug)]
+    #[error("...")]
+    pub struct OptSourceAlwaysBacktrace {
+        #[source]
+        pub source: Option<anyhow::Error>,
+        pub backtrace: Backtrace,
+    }
+
+    #[derive(Error, Debug)]
+    #[error("...")]
+    pub struct NoSourceOptBacktrace {
+        #[backtrace]
+        pub backtrace: Option<Backtrace>,
+    }
+
+    #[derive(Error, Debug)]
+    #[error("...")]
+    pub struct AlwaysSourceOptBacktrace {
+        pub source: anyhow::Error,
+        #[backtrace]
+        pub backtrace: Option<Backtrace>,
+    }
+
+    #[derive(Error, Debug)]
+    #[error("...")]
+    pub struct OptSourceOptBacktrace {
+        #[source]
+        pub source: Option<anyhow::Error>,
+        #[backtrace]
+        pub backtrace: Option<Backtrace>,
+    }
+}
+
+#[cfg(thiserror_nightly_testing)]
+pub mod enums {
+    use std::backtrace::Backtrace;
+    use thiserror::Error;
+
+    #[derive(Error, Debug)]
+    pub enum OptSourceNoBacktrace {
+        #[error("...")]
+        Test {
+            #[source]
+            source: Option<anyhow::Error>,
+        },
+    }
+
+    #[derive(Error, Debug)]
+    pub enum OptSourceAlwaysBacktrace {
+        #[error("...")]
+        Test {
+            #[source]
+            source: Option<anyhow::Error>,
+            backtrace: Backtrace,
+        },
+    }
+
+    #[derive(Error, Debug)]
+    pub enum NoSourceOptBacktrace {
+        #[error("...")]
+        Test {
+            #[backtrace]
+            backtrace: Option<Backtrace>,
+        },
+    }
+
+    #[derive(Error, Debug)]
+    pub enum AlwaysSourceOptBacktrace {
+        #[error("...")]
+        Test {
+            source: anyhow::Error,
+            #[backtrace]
+            backtrace: Option<Backtrace>,
+        },
+    }
+
+    #[derive(Error, Debug)]
+    pub enum OptSourceOptBacktrace {
+        #[error("...")]
+        Test {
+            #[source]
+            source: Option<anyhow::Error>,
+            #[backtrace]
+            backtrace: Option<Backtrace>,
+        },
+    }
+}
+
+#[test]
+#[cfg_attr(
+    not(thiserror_nightly_testing),
+    ignore = "requires `--cfg=thiserror_nightly_testing`"
+)]
+fn test_option() {}

tests/test_path.rs

@@ -0,0 +1,54 @@
+#![cfg(feature = "std")]
+
+use core::fmt::Display;
+use ref_cast::RefCast;
+use std::path::{Path, PathBuf};
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+#[error("failed to read '{file}'")]
+struct StructPathBuf {
+    file: PathBuf,
+}
+
+#[derive(Error, Debug, RefCast)]
+#[repr(C)]
+#[error("failed to read '{file}'")]
+struct StructPath {
+    file: Path,
+}
+
+#[derive(Error, Debug)]
+enum EnumPathBuf {
+    #[error("failed to read '{0}'")]
+    Read(PathBuf),
+}
+
+#[derive(Error, Debug)]
+#[error("{tail}")]
+pub struct UnsizedError {
+    pub head: i32,
+    pub tail: str,
+}
+
+#[derive(Error, Debug)]
+pub enum BothError {
+    #[error("display:{0} debug:{0:?}")]
+    DisplayDebug(PathBuf),
+    #[error("debug:{0:?} display:{0}")]
+    DebugDisplay(PathBuf),
+}
+
+fn assert<T: Display>(expected: &str, value: T) {
+    assert_eq!(expected, value.to_string());
+}
+
+#[test]
+fn test_display() {
+    let path = Path::new("/thiserror");
+    let file = path.to_owned();
+    assert("failed to read '/thiserror'", StructPathBuf { file });
+    let file = path.to_owned();
+    assert("failed to read '/thiserror'", EnumPathBuf::Read(file));
+    assert("failed to read '/thiserror'", StructPath::ref_cast(path));
+}

tests/test_source.rs

@@ -0,0 +1,82 @@
+use std::error::Error as StdError;
+use std::io;
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+#[error("implicit source")]
+pub struct ImplicitSource {
+    source: io::Error,
+}
+
+#[derive(Error, Debug)]
+#[error("explicit source")]
+pub struct ExplicitSource {
+    source: String,
+    #[source]
+    io: io::Error,
+}
+
+#[derive(Error, Debug)]
+#[error("boxed source")]
+pub struct BoxedSource {
+    #[source]
+    source: Box<dyn StdError + Send + 'static>,
+}
+
+#[test]
+fn test_implicit_source() {
+    let io = io::Error::new(io::ErrorKind::Other, "oh no!");
+    let error = ImplicitSource { source: io };
+    error.source().unwrap().downcast_ref::<io::Error>().unwrap();
+}
+
+#[test]
+fn test_explicit_source() {
+    let io = io::Error::new(io::ErrorKind::Other, "oh no!");
+    let error = ExplicitSource {
+        source: String::new(),
+        io,
+    };
+    error.source().unwrap().downcast_ref::<io::Error>().unwrap();
+}
+
+#[test]
+fn test_boxed_source() {
+    let source = Box::new(io::Error::new(io::ErrorKind::Other, "oh no!"));
+    let error = BoxedSource { source };
+    error.source().unwrap().downcast_ref::<io::Error>().unwrap();
+}
+
+macro_rules! error_from_macro {
+    ($($variants:tt)*) => {
+        #[derive(Error)]
+        #[derive(Debug)]
+        pub enum MacroSource {
+            $($variants)*
+        }
+    }
+}
+
+// Test that we generate impls with the proper hygiene
+#[rustfmt::skip]
+error_from_macro! {
+    #[error("Something")]
+    Variant(#[from] io::Error)
+}
+
+#[test]
+fn test_not_source() {
+    #[derive(Error, Debug)]
+    #[error("{source} ==> {destination}")]
+    pub struct NotSource {
+        r#source: char,
+        destination: char,
+    }
+
+    let error = NotSource {
+        source: 'S',
+        destination: 'D',
+    };
+    assert_eq!(error.to_string(), "S ==> D");
+    assert!(error.source().is_none());
+}

tests/test_transparent.rs

@@ -0,0 +1,96 @@
+use anyhow::anyhow;
+use std::error::Error as _;
+use std::io;
+use thiserror::Error;
+
+#[test]
+fn test_transparent_struct() {
+    #[derive(Error, Debug)]
+    #[error(transparent)]
+    struct Error(ErrorKind);
+
+    #[derive(Error, Debug)]
+    enum ErrorKind {
+        #[error("E0")]
+        E0,
+        #[error("E1")]
+        E1(#[from] io::Error),
+    }
+
+    let error = Error(ErrorKind::E0);
+    assert_eq!("E0", error.to_string());
+    assert!(error.source().is_none());
+
+    let io = io::Error::new(io::ErrorKind::Other, "oh no!");
+    let error = Error(ErrorKind::from(io));
+    assert_eq!("E1", error.to_string());
+    error.source().unwrap().downcast_ref::<io::Error>().unwrap();
+}
+
+#[test]
+fn test_transparent_enum() {
+    #[derive(Error, Debug)]
+    enum Error {
+        #[error("this failed")]
+        This,
+        #[error(transparent)]
+        Other(anyhow::Error),
+    }
+
+    let error = Error::This;
+    assert_eq!("this failed", error.to_string());
+
+    let error = Error::Other(anyhow!("inner").context("outer"));
+    assert_eq!("outer", error.to_string());
+    assert_eq!("inner", error.source().unwrap().to_string());
+}
+
+#[test]
+fn test_transparent_enum_with_default_message() {
+    #[derive(Error, Debug)]
+    #[error("this failed: {0}_{1}")]
+    enum Error {
+        This(i32, i32),
+        #[error(transparent)]
+        Other(anyhow::Error),
+    }
+
+    let error = Error::This(-1, -1);
+    assert_eq!("this failed: -1_-1", error.to_string());
+
+    let error = Error::Other(anyhow!("inner").context("outer"));
+    assert_eq!("outer", error.to_string());
+    assert_eq!("inner", error.source().unwrap().to_string());
+}
+
+#[test]
+fn test_anyhow() {
+    #[derive(Error, Debug)]
+    #[error(transparent)]
+    struct Any(#[from] anyhow::Error);
+
+    let error = Any::from(anyhow!("inner").context("outer"));
+    assert_eq!("outer", error.to_string());
+    assert_eq!("inner", error.source().unwrap().to_string());
+}
+
+#[test]
+fn test_non_static() {
+    #[derive(Error, Debug)]
+    #[error(transparent)]
+    struct Error<'a> {
+        inner: ErrorKind<'a>,
+    }
+
+    #[derive(Error, Debug)]
+    enum ErrorKind<'a> {
+        #[error("unexpected token: {:?}", token)]
+        Unexpected { token: &'a str },
+    }
+
+    let error = Error {
+        inner: ErrorKind::Unexpected { token: "error" },
+    };
+    assert_eq!("unexpected token: \"error\"", error.to_string());
+    assert!(error.source().is_none());
+}

tests/ui/bad-field-attr.rs

@@ -0,0 +1,7 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+#[error(transparent)]
+pub struct Error(#[error(transparent)] std::io::Error);
+
+fn main() {}

tests/ui/bad-field-attr.stderr

@@ -0,0 +1,5 @@
+error: #[error(transparent)] needs to go outside the enum or struct, not on an individual field
+ --> tests/ui/bad-field-attr.rs:5:18
+  |
+5 | pub struct Error(#[error(transparent)] std::io::Error);
+  |                  ^^^^^^^^^^^^^^^^^^^^^

tests/ui/concat-display.rs

@@ -0,0 +1,15 @@
+use thiserror::Error;
+
+macro_rules! error_type {
+    ($name:ident, $what:expr) => {
+        // Use #[error("invalid {}", $what)] instead.
+
+        #[derive(Error, Debug)]
+        #[error(concat!("invalid ", $what))]
+        pub struct $name;
+    };
+}
+
+error_type!(Error, "foo");
+
+fn main() {}

tests/ui/concat-display.stderr

@@ -0,0 +1,10 @@
+error: expected one of: string literal, `transparent`, `fmt`
+  --> tests/ui/concat-display.rs:8:17
+   |
+ 8 |         #[error(concat!("invalid ", $what))]
+   |                 ^^^^^^
+...
+13 | error_type!(Error, "foo");
+   | ------------------------- in this macro invocation
+   |
+   = note: this error originates in the macro `error_type` (in Nightly builds, run with -Z macro-backtrace for more info)

tests/ui/display-underscore.rs

@@ -0,0 +1,7 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+#[error("{_}")]
+pub struct Error;
+
+fn main() {}

tests/ui/display-underscore.stderr

@@ -0,0 +1,7 @@
+error: invalid format string: invalid argument name `_`
+ --> tests/ui/display-underscore.rs:4:11
+  |
+4 | #[error("{_}")]
+  |           ^ invalid argument name in format string
+  |
+  = note: argument name cannot be a single underscore

tests/ui/duplicate-enum-source.rs

@@ -0,0 +1,13 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+pub enum ErrorEnum {
+    Confusing {
+        #[source]
+        a: std::io::Error,
+        #[source]
+        b: anyhow::Error,
+    },
+}
+
+fn main() {}

tests/ui/duplicate-enum-source.stderr

@@ -0,0 +1,5 @@
+error: duplicate #[source] attribute
+ --> tests/ui/duplicate-enum-source.rs:8:9
+  |
+8 |         #[source]
+  |         ^^^^^^^^^

tests/ui/duplicate-fmt.rs

@@ -0,0 +1,23 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+#[error("...")]
+#[error("...")]
+pub struct Error;
+
+#[derive(Error, Debug)]
+#[error(fmt = core::fmt::Octal::fmt)]
+#[error(fmt = core::fmt::LowerHex::fmt)]
+pub enum FmtFmt {}
+
+#[derive(Error, Debug)]
+#[error(fmt = core::fmt::Octal::fmt)]
+#[error(transparent)]
+pub enum FmtTransparent {}
+
+#[derive(Error, Debug)]
+#[error(fmt = core::fmt::Octal::fmt)]
+#[error("...")]
+pub enum FmtDisplay {}
+
+fn main() {}

tests/ui/duplicate-fmt.stderr

@@ -0,0 +1,23 @@
+error: only one #[error(...)] attribute is allowed
+ --> tests/ui/duplicate-fmt.rs:5:1
+  |
+5 | #[error("...")]
+  | ^^^^^^^^^^^^^^^
+
+error: duplicate #[error(fmt = ...)] attribute
+  --> tests/ui/duplicate-fmt.rs:10:1
+   |
+10 | #[error(fmt = core::fmt::LowerHex::fmt)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: cannot have both #[error(transparent)] and #[error(fmt = ...)]
+  --> tests/ui/duplicate-fmt.rs:14:1
+   |
+14 | #[error(fmt = core::fmt::Octal::fmt)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: cannot have both #[error(fmt = ...)] and a format arguments attribute
+  --> tests/ui/duplicate-fmt.rs:20:1
+   |
+20 | #[error("...")]
+   | ^^^^^^^^^^^^^^^

tests/ui/duplicate-struct-source.rs

@@ -0,0 +1,11 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+pub struct ErrorStruct {
+    #[source]
+    a: std::io::Error,
+    #[source]
+    b: anyhow::Error,
+}
+
+fn main() {}

tests/ui/duplicate-struct-source.stderr

@@ -0,0 +1,5 @@
+error: duplicate #[source] attribute
+ --> tests/ui/duplicate-struct-source.rs:7:5
+  |
+7 |     #[source]
+  |     ^^^^^^^^^

tests/ui/duplicate-transparent.rs

@@ -0,0 +1,8 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+#[error(transparent)]
+#[error(transparent)]
+pub struct Error(anyhow::Error);
+
+fn main() {}

tests/ui/duplicate-transparent.stderr

@@ -0,0 +1,5 @@
+error: duplicate #[error(transparent)] attribute
+ --> tests/ui/duplicate-transparent.rs:5:1
+  |
+5 | #[error(transparent)]
+  | ^^^^^^^^^^^^^^^^^^^^^

tests/ui/expression-fallback.rs

@@ -0,0 +1,7 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+#[error("".yellow)]
+pub struct ArgError;
+
+fn main() {}

tests/ui/expression-fallback.stderr

@@ -0,0 +1,24 @@
+error: expected `,`, found `.`
+ --> tests/ui/expression-fallback.rs:4:11
+  |
+4 | #[error("".yellow)]
+  |           ^ expected `,`
+
+error: argument never used
+ --> tests/ui/expression-fallback.rs:4:12
+  |
+4 | #[error("".yellow)]
+  |         -- ^^^^^^ argument never used
+  |         |
+  |         formatting specifier missing
+  |
+help: format specifiers use curly braces, consider adding a format specifier
+  |
+4 | #[error("{}".yellow)]
+  |          ++
+
+error[E0425]: cannot find value `yellow` in this scope
+ --> tests/ui/expression-fallback.rs:4:12
+  |
+4 | #[error("".yellow)]
+  |            ^^^^^^ not found in this scope

tests/ui/fallback-impl-with-display.rs

@@ -0,0 +1,14 @@
+use core::fmt::{self, Display};
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+#[error]
+pub struct MyError;
+
+impl Display for MyError {
+    fn fmt(&self, _formatter: &mut fmt::Formatter) -> fmt::Result {
+        unimplemented!()
+    }
+}
+
+fn main() {}

tests/ui/fallback-impl-with-display.stderr

@@ -0,0 +1,16 @@
+error: expected attribute arguments in parentheses: #[error(...)]
+ --> tests/ui/fallback-impl-with-display.rs:5:3
+  |
+5 | #[error]
+  |   ^^^^^
+
+error[E0119]: conflicting implementations of trait `std::fmt::Display` for type `MyError`
+ --> tests/ui/fallback-impl-with-display.rs:4:10
+  |
+4 | #[derive(Error, Debug)]
+  |          ^^^^^ conflicting implementation for `MyError`
+...
+8 | impl Display for MyError {
+  | ------------------------ first implementation here
+  |
+  = note: this error originates in the derive macro `Error` (in Nightly builds, run with -Z macro-backtrace for more info)

tests/ui/from-backtrace-backtrace.rs

@@ -0,0 +1,15 @@
+// https://github.com/dtolnay/thiserror/issues/163
+
+use std::backtrace::Backtrace;
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+#[error("...")]
+pub struct Error(
+    #[from]
+    #[backtrace]
+    std::io::Error,
+    Backtrace,
+);
+
+fn main() {}

tests/ui/from-backtrace-backtrace.stderr

@@ -0,0 +1,5 @@
+error: deriving From requires no fields other than source and backtrace
+ --> tests/ui/from-backtrace-backtrace.rs:9:5
+  |
+9 |     #[from]
+  |     ^^^^^^^

tests/ui/from-not-source.rs

@@ -0,0 +1,11 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+pub struct Error {
+    #[source]
+    source: std::io::Error,
+    #[from]
+    other: anyhow::Error,
+}
+
+fn main() {}

tests/ui/from-not-source.stderr

@@ -0,0 +1,5 @@
+error: #[from] is only supported on the source field, not any other field
+ --> tests/ui/from-not-source.rs:7:5
+  |
+7 |     #[from]
+  |     ^^^^^^^

tests/ui/invalid-input-impl-anyway.rs

@@ -0,0 +1,11 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+#[error]
+pub struct MyError;
+
+fn main() {
+    // No error on the following line. Thiserror emits an Error impl despite the
+    // bad attribute.
+    _ = &MyError as &dyn std::error::Error;
+}

tests/ui/invalid-input-impl-anyway.stderr

@@ -0,0 +1,5 @@
+error: expected attribute arguments in parentheses: #[error(...)]
+ --> tests/ui/invalid-input-impl-anyway.rs:4:3
+  |
+4 | #[error]
+  |   ^^^^^

tests/ui/lifetime.rs

@@ -0,0 +1,24 @@
+use core::fmt::Debug;
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+#[error("error")]
+struct Error<'a>(#[from] Inner<'a>);
+
+#[derive(Error, Debug)]
+#[error("{0}")]
+struct Inner<'a>(&'a str);
+
+#[derive(Error, Debug)]
+enum Enum<'a> {
+    #[error("error")]
+    Foo(#[from] Generic<&'a str>),
+}
+
+#[derive(Error, Debug)]
+#[error("{0:?}")]
+struct Generic<T: Debug>(T);
+
+fn main() -> Result<(), Error<'static>> {
+    Err(Error(Inner("some text")))
+}

tests/ui/lifetime.stderr

@@ -0,0 +1,11 @@
+error: non-static lifetimes are not allowed in the source of an error, because std::error::Error requires the source is dyn Error + 'static
+ --> tests/ui/lifetime.rs:6:26
+  |
+6 | struct Error<'a>(#[from] Inner<'a>);
+  |                          ^^^^^^^^^
+
+error: non-static lifetimes are not allowed in the source of an error, because std::error::Error requires the source is dyn Error + 'static
+  --> tests/ui/lifetime.rs:15:17
+   |
+15 |     Foo(#[from] Generic<&'a str>),
+   |                 ^^^^^^^^^^^^^^^^

tests/ui/missing-display.rs

@@ -0,0 +1,9 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+pub enum MyError {
+    First,
+    Second,
+}
+
+fn main() {}

tests/ui/missing-display.stderr

@@ -0,0 +1,19 @@
+error[E0277]: `MyError` doesn't implement `std::fmt::Display`
+ --> tests/ui/missing-display.rs:4:10
+  |
+3 | #[derive(Error, Debug)]
+  |          ----- in this derive macro expansion
+4 | pub enum MyError {
+  |          ^^^^^^^ unsatisfied trait bound
+  |
+help: the trait `std::fmt::Display` is not implemented for `MyError`
+ --> tests/ui/missing-display.rs:4:1
+  |
+4 | pub enum MyError {
+  | ^^^^^^^^^^^^^^^^
+note: required by a bound in `std::error::Error`
+ --> $RUST/core/src/error.rs
+  |
+  | pub trait Error: Debug + Display {
+  |                          ^^^^^^^ required by this bound in `Error`
+  = note: this error originates in the derive macro `Error` (in Nightly builds, run with -Z macro-backtrace for more info)

tests/ui/missing-fmt.rs

@@ -0,0 +1,10 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+pub enum Error {
+    #[error("...")]
+    A(usize),
+    B(usize),
+}
+
+fn main() {}

tests/ui/missing-fmt.stderr

@@ -0,0 +1,5 @@
+error: missing #[error("...")] display attribute
+ --> tests/ui/missing-fmt.rs:7:5
+  |
+7 |     B(usize),
+  |     ^^^^^^^^

tests/ui/no-display.rs

@@ -0,0 +1,18 @@
+use thiserror::Error;
+
+#[derive(Debug)]
+struct NoDisplay;
+
+#[derive(Error, Debug)]
+#[error("thread: {thread}")]
+pub struct Error {
+    thread: NoDisplay,
+}
+
+#[derive(Error, Debug)]
+#[error("thread: {thread:o}")]
+pub struct ErrorOctal {
+    thread: NoDisplay,
+}
+
+fn main() {}

tests/ui/no-display.stderr

@@ -0,0 +1,46 @@
+error[E0599]: the method `as_display` exists for reference `&NoDisplay`, but its trait bounds were not satisfied
+ --> tests/ui/no-display.rs:7:9
+  |
+4 | struct NoDisplay;
+  | ---------------- doesn't satisfy `NoDisplay: std::fmt::Display`
+...
+7 | #[error("thread: {thread}")]
+  |         ^^^^^^^^^^^^^^^^^^ method cannot be called on `&NoDisplay` due to unsatisfied trait bounds
+  |
+  = note: the following trait bounds were not satisfied:
+          `NoDisplay: std::fmt::Display`
+          which is required by `&NoDisplay: AsDisplay<'_>`
+note: the trait `std::fmt::Display` must be implemented
+ --> $RUST/core/src/fmt/mod.rs
+  |
+  | pub trait Display: PointeeSized {
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  = help: items from traits can only be used if the trait is implemented and in scope
+  = note: the following trait defines an item `as_display`, perhaps you need to implement it:
+          candidate #1: `AsDisplay`
+
+error[E0277]: the trait bound `NoDisplay: Octal` is not satisfied
+  --> tests/ui/no-display.rs:13:9
+   |
+12 | #[derive(Error, Debug)]
+   |          ----- in this derive macro expansion
+13 | #[error("thread: {thread:o}")]
+   |         ^^^^^^^^^^^^^^^^^^^^ unsatisfied trait bound
+   |
+help: the trait `Octal` is not implemented for `NoDisplay`
+  --> tests/ui/no-display.rs:4:1
+   |
+ 4 | struct NoDisplay;
+   | ^^^^^^^^^^^^^^^^
+   = help: the following other types implement trait `Octal`:
+             &T
+             &mut T
+             NonZero<T>
+             Saturating<T>
+             Wrapping<T>
+             i128
+             i16
+             i32
+           and $N others
+   = note: required for `&NoDisplay` to implement `Octal`
+   = note: this error originates in the macro `$crate::format_args` which comes from the expansion of the derive macro `Error` (in Nightly builds, run with -Z macro-backtrace for more info)

tests/ui/numbered-positional-tuple.rs

@@ -0,0 +1,7 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+#[error("invalid rdo_lookahead_frames {0} (expected < {})", i32::MAX)]
+pub struct Error(u32);
+
+fn main() {}

tests/ui/numbered-positional-tuple.stderr

@@ -0,0 +1,5 @@
+error: ambiguous reference to positional arguments by number in a tuple struct; change this to a named argument
+ --> tests/ui/numbered-positional-tuple.rs:4:61
+  |
+4 | #[error("invalid rdo_lookahead_frames {0} (expected < {})", i32::MAX)]
+  |                                                             ^^^^^^^^

tests/ui/raw-identifier.rs

@@ -0,0 +1,12 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+#[error("error: {r#fn}")]
+pub struct Error {
+    r#fn: &'static str,
+}
+
+fn main() {
+    let r#fn = "...";
+    let _ = format!("error: {r#fn}");
+}

tests/ui/raw-identifier.stderr

@@ -0,0 +1,21 @@
+error: invalid format string: raw identifiers are not supported
+ --> tests/ui/raw-identifier.rs:4:18
+  |
+4 | #[error("error: {r#fn}")]
+  |                  --^^
+  |                  |
+  |                  raw identifier used here in format string
+  |                  help: remove the `r#`
+  |
+  = note: identifiers in format strings can be keywords and don't need to be prefixed with `r#`
+
+error: invalid format string: raw identifiers are not supported
+  --> tests/ui/raw-identifier.rs:11:30
+   |
+11 |     let _ = format!("error: {r#fn}");
+   |                              --^^
+   |                              |
+   |                              raw identifier used here in format string
+   |                              help: remove the `r#`
+   |
+   = note: identifiers in format strings can be keywords and don't need to be prefixed with `r#`

tests/ui/same-from-type.rs

@@ -0,0 +1,11 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+pub enum Error {
+    #[error("failed to open")]
+    OpenFile(#[from] std::io::Error),
+    #[error("failed to close")]
+    CloseFile(#[from] std::io::Error),
+}
+
+fn main() {}

tests/ui/same-from-type.stderr

@@ -0,0 +1,8 @@
+error[E0119]: conflicting implementations of trait `From<std::io::Error>` for type `Error`
+ --> tests/ui/same-from-type.rs:8:15
+  |
+6 |     OpenFile(#[from] std::io::Error),
+  |              ------- first implementation here
+7 |     #[error("failed to close")]
+8 |     CloseFile(#[from] std::io::Error),
+  |               ^^^^^^^ conflicting implementation for `Error`

tests/ui/source-enum-not-error.rs

@@ -0,0 +1,12 @@
+use thiserror::Error;
+
+#[derive(Debug)]
+pub struct NotError;
+
+#[derive(Error, Debug)]
+#[error("...")]
+pub enum ErrorEnum {
+    Broken { source: NotError },
+}
+
+fn main() {}

tests/ui/source-enum-not-error.stderr

@@ -0,0 +1,22 @@
+error[E0599]: the method `as_dyn_error` exists for reference `&NotError`, but its trait bounds were not satisfied
+ --> tests/ui/source-enum-not-error.rs:9:14
+  |
+4 | pub struct NotError;
+  | ------------------- doesn't satisfy `NotError: AsDynError<'_>` or `NotError: std::error::Error`
+...
+9 |     Broken { source: NotError },
+  |              ^^^^^^ method cannot be called on `&NotError` due to unsatisfied trait bounds
+  |
+  = note: the following trait bounds were not satisfied:
+          `NotError: std::error::Error`
+          which is required by `NotError: AsDynError<'_>`
+          `&NotError: std::error::Error`
+          which is required by `&NotError: AsDynError<'_>`
+note: the trait `std::error::Error` must be implemented
+ --> $RUST/core/src/error.rs
+  |
+  | pub trait Error: Debug + Display {
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  = help: items from traits can only be used if the trait is implemented and in scope
+  = note: the following trait defines an item `as_dyn_error`, perhaps you need to implement it:
+          candidate #1: `AsDynError`

tests/ui/source-enum-unnamed-field-not-error.rs

@@ -0,0 +1,12 @@
+use thiserror::Error;
+
+#[derive(Debug)]
+pub struct NotError;
+
+#[derive(Error, Debug)]
+#[error("...")]
+pub enum ErrorEnum {
+    Broken(#[source] NotError),
+}
+
+fn main() {}

tests/ui/source-enum-unnamed-field-not-error.stderr

@@ -0,0 +1,22 @@
+error[E0599]: the method `as_dyn_error` exists for reference `&NotError`, but its trait bounds were not satisfied
+ --> tests/ui/source-enum-unnamed-field-not-error.rs:9:12
+  |
+4 | pub struct NotError;
+  | ------------------- doesn't satisfy `NotError: AsDynError<'_>` or `NotError: std::error::Error`
+...
+9 |     Broken(#[source] NotError),
+  |            ^^^^^^^^^ method cannot be called on `&NotError` due to unsatisfied trait bounds
+  |
+  = note: the following trait bounds were not satisfied:
+          `NotError: std::error::Error`
+          which is required by `NotError: AsDynError<'_>`
+          `&NotError: std::error::Error`
+          which is required by `&NotError: AsDynError<'_>`
+note: the trait `std::error::Error` must be implemented
+ --> $RUST/core/src/error.rs
+  |
+  | pub trait Error: Debug + Display {
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  = help: items from traits can only be used if the trait is implemented and in scope
+  = note: the following trait defines an item `as_dyn_error`, perhaps you need to implement it:
+          candidate #1: `AsDynError`

tests/ui/source-struct-not-error.rs

@@ -0,0 +1,12 @@
+use thiserror::Error;
+
+#[derive(Debug)]
+struct NotError;
+
+#[derive(Error, Debug)]
+#[error("...")]
+pub struct ErrorStruct {
+    source: NotError,
+}
+
+fn main() {}

tests/ui/source-struct-not-error.stderr

@@ -0,0 +1,20 @@
+error[E0599]: the method `as_dyn_error` exists for struct `NotError`, but its trait bounds were not satisfied
+ --> tests/ui/source-struct-not-error.rs:9:5
+  |
+4 | struct NotError;
+  | --------------- method `as_dyn_error` not found for this struct because it doesn't satisfy `NotError: AsDynError<'_>` or `NotError: std::error::Error`
+...
+9 |     source: NotError,
+  |     ^^^^^^ method cannot be called on `NotError` due to unsatisfied trait bounds
+  |
+  = note: the following trait bounds were not satisfied:
+          `NotError: std::error::Error`
+          which is required by `NotError: AsDynError<'_>`
+note: the trait `std::error::Error` must be implemented
+ --> $RUST/core/src/error.rs
+  |
+  | pub trait Error: Debug + Display {
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  = help: items from traits can only be used if the trait is implemented and in scope
+  = note: the following trait defines an item `as_dyn_error`, perhaps you need to implement it:
+          candidate #1: `AsDynError`

tests/ui/source-struct-unnamed-field-not-error.rs

@@ -0,0 +1,10 @@
+use thiserror::Error;
+
+#[derive(Debug)]
+struct NotError;
+
+#[derive(Error, Debug)]
+#[error("...")]
+pub struct ErrorStruct(#[source] NotError);
+
+fn main() {}

tests/ui/source-struct-unnamed-field-not-error.stderr

@@ -0,0 +1,20 @@
+error[E0599]: the method `as_dyn_error` exists for struct `NotError`, but its trait bounds were not satisfied
+ --> tests/ui/source-struct-unnamed-field-not-error.rs:8:24
+  |
+4 | struct NotError;
+  | --------------- method `as_dyn_error` not found for this struct because it doesn't satisfy `NotError: AsDynError<'_>` or `NotError: std::error::Error`
+...
+8 | pub struct ErrorStruct(#[source] NotError);
+  |                        ^^^^^^^^^ method cannot be called on `NotError` due to unsatisfied trait bounds
+  |
+  = note: the following trait bounds were not satisfied:
+          `NotError: std::error::Error`
+          which is required by `NotError: AsDynError<'_>`
+note: the trait `std::error::Error` must be implemented
+ --> $RUST/core/src/error.rs
+  |
+  | pub trait Error: Debug + Display {
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  = help: items from traits can only be used if the trait is implemented and in scope
+  = note: the following trait defines an item `as_dyn_error`, perhaps you need to implement it:
+          candidate #1: `AsDynError`

tests/ui/struct-with-fmt.rs

@@ -0,0 +1,7 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+#[error(fmt = core::fmt::Octal::fmt)]
+pub struct Error(i32);
+
+fn main() {}

tests/ui/struct-with-fmt.stderr

@@ -0,0 +1,5 @@
+error: #[error(fmt = ...)] is only supported in enums; for a struct, handwrite your own Display impl
+ --> tests/ui/struct-with-fmt.rs:4:1
+  |
+4 | #[error(fmt = core::fmt::Octal::fmt)]
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

tests/ui/transparent-display.rs

@@ -0,0 +1,8 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+#[error(transparent)]
+#[error("...")]
+pub struct Error(anyhow::Error);
+
+fn main() {}

tests/ui/transparent-display.stderr

@@ -0,0 +1,5 @@
+error: cannot have both #[error(transparent)] and a display attribute
+ --> tests/ui/transparent-display.rs:5:1
+  |
+5 | #[error("...")]
+  | ^^^^^^^^^^^^^^^

tests/ui/transparent-enum-many.rs

@@ -0,0 +1,9 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+pub enum Error {
+    #[error(transparent)]
+    Other(anyhow::Error, String),
+}
+
+fn main() {}

tests/ui/transparent-enum-many.stderr

@@ -0,0 +1,6 @@
+error: #[error(transparent)] requires exactly one field
+ --> tests/ui/transparent-enum-many.rs:5:5
+  |
+5 | /     #[error(transparent)]
+6 | |     Other(anyhow::Error, String),
+  | |________________________________^

tests/ui/transparent-enum-not-error.rs

@@ -0,0 +1,9 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+pub enum Error {
+    #[error(transparent)]
+    Other { message: String },
+}
+
+fn main() {}

tests/ui/transparent-enum-not-error.stderr

@@ -0,0 +1,20 @@
+error[E0599]: the method `as_dyn_error` exists for reference `&String`, but its trait bounds were not satisfied
+ --> tests/ui/transparent-enum-not-error.rs:5:13
+  |
+5 |     #[error(transparent)]
+  |             ^^^^^^^^^^^ method cannot be called on `&String` due to unsatisfied trait bounds
+  |
+ ::: $RUST/alloc/src/string.rs
+  |
+  | pub struct String {
+  | ----------------- doesn't satisfy `String: AsDynError<'_>` or `String: std::error::Error`
+  |
+  = note: the following trait bounds were not satisfied:
+          `String: std::error::Error`
+          which is required by `String: AsDynError<'_>`
+          `&String: std::error::Error`
+          which is required by `&String: AsDynError<'_>`
+          `str: Sized`
+          which is required by `str: AsDynError<'_>`
+          `str: std::error::Error`
+          which is required by `str: AsDynError<'_>`

tests/ui/transparent-enum-source.rs

@@ -0,0 +1,9 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+pub enum Error {
+    #[error(transparent)]
+    Other(#[source] anyhow::Error),
+}
+
+fn main() {}

tests/ui/transparent-enum-source.stderr

@@ -0,0 +1,5 @@
+error: transparent variant can't contain #[source]
+ --> tests/ui/transparent-enum-source.rs:6:11
+  |
+6 |     Other(#[source] anyhow::Error),
+  |           ^^^^^^^^^

tests/ui/transparent-enum-unnamed-field-not-error.rs

@@ -0,0 +1,9 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+pub enum Error {
+    #[error(transparent)]
+    Other(String),
+}
+
+fn main() {}

tests/ui/transparent-enum-unnamed-field-not-error.stderr

@@ -0,0 +1,20 @@
+error[E0599]: the method `as_dyn_error` exists for reference `&String`, but its trait bounds were not satisfied
+ --> tests/ui/transparent-enum-unnamed-field-not-error.rs:5:13
+  |
+5 |     #[error(transparent)]
+  |             ^^^^^^^^^^^ method cannot be called on `&String` due to unsatisfied trait bounds
+  |
+ ::: $RUST/alloc/src/string.rs
+  |
+  | pub struct String {
+  | ----------------- doesn't satisfy `String: AsDynError<'_>` or `String: std::error::Error`
+  |
+  = note: the following trait bounds were not satisfied:
+          `String: std::error::Error`
+          which is required by `String: AsDynError<'_>`
+          `&String: std::error::Error`
+          which is required by `&String: AsDynError<'_>`
+          `str: Sized`
+          which is required by `str: AsDynError<'_>`
+          `str: std::error::Error`
+          which is required by `str: AsDynError<'_>`

tests/ui/transparent-struct-many.rs

@@ -0,0 +1,10 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+#[error(transparent)]
+pub struct Error {
+    inner: anyhow::Error,
+    what: String,
+}
+
+fn main() {}

tests/ui/transparent-struct-many.stderr

@@ -0,0 +1,5 @@
+error: #[error(transparent)] requires exactly one field
+ --> tests/ui/transparent-struct-many.rs:4:1
+  |
+4 | #[error(transparent)]
+  | ^^^^^^^^^^^^^^^^^^^^^

tests/ui/transparent-struct-not-error.rs

@@ -0,0 +1,9 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+#[error(transparent)]
+pub struct Error {
+    message: String,
+}
+
+fn main() {}

tests/ui/transparent-struct-not-error.stderr

@@ -0,0 +1,18 @@
+error[E0599]: the method `as_dyn_error` exists for struct `String`, but its trait bounds were not satisfied
+ --> tests/ui/transparent-struct-not-error.rs:4:9
+  |
+4 | #[error(transparent)]
+  |         ^^^^^^^^^^^ method cannot be called on `String` due to unsatisfied trait bounds
+  |
+ ::: $RUST/alloc/src/string.rs
+  |
+  | pub struct String {
+  | ----------------- doesn't satisfy `String: AsDynError<'_>` or `String: std::error::Error`
+  |
+  = note: the following trait bounds were not satisfied:
+          `String: std::error::Error`
+          which is required by `String: AsDynError<'_>`
+          `str: Sized`
+          which is required by `str: AsDynError<'_>`
+          `str: std::error::Error`
+          which is required by `str: AsDynError<'_>`

tests/ui/transparent-struct-source.rs

@@ -0,0 +1,7 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+#[error(transparent)]
+pub struct Error(#[source] anyhow::Error);
+
+fn main() {}

tests/ui/transparent-struct-source.stderr

@@ -0,0 +1,5 @@
+error: transparent error struct can't contain #[source]
+ --> tests/ui/transparent-struct-source.rs:5:18
+  |
+5 | pub struct Error(#[source] anyhow::Error);
+  |                  ^^^^^^^^^

tests/ui/transparent-struct-unnamed-field-not-error.rs

@@ -0,0 +1,7 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+#[error(transparent)]
+pub struct Error(String);
+
+fn main() {}

tests/ui/transparent-struct-unnamed-field-not-error.stderr

@@ -0,0 +1,18 @@
+error[E0599]: the method `as_dyn_error` exists for struct `String`, but its trait bounds were not satisfied
+ --> tests/ui/transparent-struct-unnamed-field-not-error.rs:4:9
+  |
+4 | #[error(transparent)]
+  |         ^^^^^^^^^^^ method cannot be called on `String` due to unsatisfied trait bounds
+  |
+ ::: $RUST/alloc/src/string.rs
+  |
+  | pub struct String {
+  | ----------------- doesn't satisfy `String: AsDynError<'_>` or `String: std::error::Error`
+  |
+  = note: the following trait bounds were not satisfied:
+          `String: std::error::Error`
+          which is required by `String: AsDynError<'_>`
+          `str: Sized`
+          which is required by `str: AsDynError<'_>`
+          `str: std::error::Error`
+          which is required by `str: AsDynError<'_>`

tests/ui/unconditional-recursion.rs

@@ -0,0 +1,9 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+#[error("{self}")]
+pub struct Error;
+
+fn main() {
+    __FAIL__;
+}

tests/ui/unconditional-recursion.stderr

@@ -0,0 +1,21 @@
+error[E0425]: cannot find value `__FAIL__` in this scope
+ --> tests/ui/unconditional-recursion.rs:8:5
+  |
+8 |     __FAIL__;
+  |     ^^^^^^^^ not found in this scope
+
+warning: function cannot return without recursing
+ --> tests/ui/unconditional-recursion.rs:4:9
+  |
+4 | #[error("{self}")]
+  |         ^^^^^^^^
+  |         |
+  |         cannot return without recursing
+  |         recursive call site
+  |
+  = help: a `loop` may express intention better if this is on purpose
+note: the lint level is defined here
+ --> tests/ui/unconditional-recursion.rs:4:9
+  |
+4 | #[error("{self}")]
+  |         ^^^^^^^^

tests/ui/unexpected-field-fmt.rs

@@ -0,0 +1,11 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+pub enum Error {
+    What {
+        #[error("...")]
+        io: std::io::Error,
+    },
+}
+
+fn main() {}

tests/ui/unexpected-field-fmt.stderr

@@ -0,0 +1,5 @@
+error: not expected here; the #[error(...)] attribute belongs on top of a struct or an enum variant
+ --> tests/ui/unexpected-field-fmt.rs:6:9
+  |
+6 |         #[error("...")]
+  |         ^^^^^^^^^^^^^^^

tests/ui/unexpected-struct-source.rs

@@ -0,0 +1,7 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+#[source]
+pub struct Error;
+
+fn main() {}

tests/ui/unexpected-struct-source.stderr

@@ -0,0 +1,5 @@
+error: not expected here; the #[source] attribute belongs on a specific field
+ --> tests/ui/unexpected-struct-source.rs:4:1
+  |
+4 | #[source]
+  | ^^^^^^^^^

tests/ui/union.rs

@@ -0,0 +1,9 @@
+use thiserror::Error;
+
+#[derive(Error)]
+pub union U {
+    msg: &'static str,
+    num: usize,
+}
+
+fn main() {}

tests/ui/union.stderr

@@ -0,0 +1,8 @@
+error: union as errors are not supported
+ --> tests/ui/union.rs:4:1
+  |
+4 | / pub union U {
+5 | |     msg: &'static str,
+6 | |     num: usize,
+7 | | }
+  | |_^