Attribute drops (some) attributes inside trait function implementations

[SergioBenitez]

When attributes appear inside of a function in a trait implementation decorated with #[async_trait], those attributes can be dropped depending on what they're decorating. In particular, attributes decorating fields are dropped entirely.

Consider the following example:

#[async_trait::async_trait]
trait Foo: Sized {
    fn g(&self);
}

struct K { field: usize }

#[async_trait::async_trait]
impl Foo for K {
    fn g(&self) {
        let _ = K {
            #[cfg(debug_assertions)]
            field: 0,
            #[cfg(not(debug_assertions))]
            field: 1,
        };
    }
}

This impl expand to:

impl Foo for K {
    fn g(&self) {
        let _ = K { field: 0, field: 1, };
    }
}

Which results in the (incorrect) compile-time error:

error[E0062]: field `field` specified more than once

error: aborting due to previous error

Note that span information appears to be lost as well.

[dtolnay]

This is a compiler bug. Based on adding the following to the macro,

eprintln!("{}", input);
eprintln!("{:#?}", input);

rustc is passing in:

impl Foo for K {
    fn g(&self) { let _ = K{field: 0, field: 1,}; }
}
TokenStream [
    Ident {
        ident: "impl",
        span: #0 bytes(0..0),
    },
    Ident {
        ident: "Foo",
        span: #0 bytes(0..0),
    },
    Ident {
        ident: "for",
        span: #0 bytes(0..0),
    },
    Ident {
        ident: "K",
        span: #0 bytes(0..0),
    },
    Group {
        delimiter: Brace,
        stream: TokenStream [
            Ident {
                ident: "fn",
                span: #0 bytes(0..0),
            },
            Ident {
                ident: "g",
                span: #0 bytes(0..0),
            },
            Group {
                delimiter: Parenthesis,
                stream: TokenStream [
                    Punct {
                        ch: '&',
                        spacing: Alone,
                        span: #0 bytes(0..0),
                    },
                    Ident {
                        ident: "self",
                        span: #0 bytes(0..0),
                    },
                ],
                span: #0 bytes(0..0),
            },
            Group {
                delimiter: Brace,
                stream: TokenStream [
                    Ident {
                        ident: "let",
                        span: #0 bytes(0..0),
                    },
                    Ident {
                        ident: "_",
                        span: #0 bytes(0..0),
                    },
                    Punct {
                        ch: '=',
                        spacing: Alone,
                        span: #0 bytes(0..0),
                    },
                    Ident {
                        ident: "K",
                        span: #0 bytes(0..0),
                    },
                    Group {
                        delimiter: Brace,
                        stream: TokenStream [
                            Ident {
                                ident: "field",
                                span: #0 bytes(0..0),
                            },
                            Punct {
                                ch: ':',
                                spacing: Alone,
                                span: #0 bytes(0..0),
                            },
                            Literal { lit: Lit { kind: Integer, symbol: "0", suffix: None }, span: Span { lo: BytePos(0), hi: BytePos(0), ctxt: #0 } },
                            Punct {
                                ch: ',',
                                spacing: Alone,
                                span: #0 bytes(0..0),
                            },
                            Ident {
                                ident: "field",
                                span: #0 bytes(0..0),
                            },
                            Punct {
                                ch: ':',
                                spacing: Alone,
                                span: #0 bytes(0..0),
                            },
                            Literal { lit: Lit { kind: Integer, symbol: "1", suffix: None }, span: Span { lo: BytePos(0), hi: BytePos(0), ctxt: #0 } },
                            Punct {
                                ch: ',',
                                spacing: Alone,
                                span: #0 bytes(0..0),
                            },
                        ],
                        span: #0 bytes(0..0),
                    },
                    Punct {
                        ch: ';',
                        spacing: Alone,
                        span: #0 bytes(0..0),
                    },
                ],
                span: #0 bytes(0..0),
            },
        ],
        span: #0 bytes(0..0),
    },
]

[dtolnay]

Fixed in rust-lang/rust#68737.