Field attributes lost in proc macro input

[dtolnay]

If we have an attribute macro whose input contains attributes on fields of a struct-expression, those attributes all seem to disappear before the macro is invoked.

The following repro compiles successfully if #[repro::repro] is removed, but fails with "error[E0062]: field `field` specified more than once" if the macro is present even though it does nothing.

struct C {
    field: u8,
}

#[repro::repro]
const C: C = C {
    #[cfg(debug_assertions)]
    field: 0,
    #[cfg(not(debug_assertions))]
    field: 1,
};

fn main() {}

extern crate proc_macro;
use proc_macro::TokenStream;

#[proc_macro_attribute]
pub fn repro(_args: TokenStream, input: TokenStream) -> TokenStream {
    println!("{:#?}", input);
    input.into_iter().collect()
}

Here is a script that reproduces the behavior as of rustc 1.42.0-nightly (212b2c7 2020-01-30)

#!/bin/bash

cargo new repro

echo >>repro/Cargo.toml '
[lib]
proc-macro = true
'

echo >repro/src/lib.rs '
extern crate proc_macro;
use proc_macro::TokenStream;

#[proc_macro_attribute]
pub fn repro(_args: TokenStream, input: TokenStream) -> TokenStream {
    println!("{:#?}", input);
    input.into_iter().collect()
}
'

echo >repro/src/main.rs '
struct C {
    field: u8,
}
#[repro::repro]
const C: C = C {
    #[cfg(debug_assertions)]
    field: 0,
    #[cfg(not(debug_assertions))]
    field: 1,
};
fn main() {}
'

cargo +nightly check --manifest-path repro/Cargo.toml

Here is the input token stream as received by the proc macro

TokenStream [
    Ident {
        ident: "const",
        span: #0 bytes(0..0),
    },
    Ident {
        ident: "C",
        span: #0 bytes(0..0),
    },
    Punct {
        ch: ':',
        spacing: Alone,
        span: #0 bytes(0..0),
    },
    Ident {
        ident: "C",
        span: #0 bytes(0..0),
    },
    Punct {
        ch: '=',
        spacing: Alone,
        span: #0 bytes(0..0),
    },
    Ident {
        ident: "C",
        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),
    },
]

cc dtolnay/async-trait#63

[Centril]

I have a fix in #68737. The bug was in pretty printing (usually the case in these types of bugs).