"Don't know the field number in this context"

[dtolnay]

src/lib.rs:

struct S(#[attr] u8);

Stable, and nightly-2019-09-09:

error[E0658]: the attribute `attr` is currently unknown to the compiler and may have meaning added to it in the future
 --> src/main.rs:1:10
  |
1 | struct S(#[attr] u8);
  |          ^^^^^^^
  |
  = note: for more information, see https://github.com/rust-lang/rust/issues/29642
  = help: add `#![feature(custom_attribute)]` to the crate attributes to enable

nightly-2019-09-10:

thread 'rustc' panicked at 'don't know the field number in this context', src/librustc/hir/map/def_collector.rs:346:36
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.

error: internal compiler error: unexpected panic

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.39.0-nightly (0b36e9dea 2019-09-09) running on x86_64-unknown-linux-gnu

note: compiler flags: -C debuginfo=2 -C incremental --crate-type bin

note: some of the compiler flags provided by cargo are hidden

2b8116d...0b36e9d

Mentioning @c410-f3r @petrochenkov because this is likely from #63468.

[petrochenkov]

Yeah, this is some known shortcut taken in #63468, sorry.
Didn't expect people to hit it that fast.
Will fix ~this weekend.

[dtolnay]

Thanks. The realistic usage where this appears is if you forget to import a derive macro, which happens commonly.

//use serde::Deserialize;

#[derive(Deserialize)]
struct Outer<'a>(#[serde(borrow)] Inner<'a>);

[Centril]

Un-nominating since Vadim self-assigned.

[pnkfelix]

triage: P-high.

[petrochenkov]

Fixed in #64457.