thread 'rustc' panicked at 'Stack should be empty: final_buf=FrameData ...'

[dwrensha]

On nightly, I'm seeing an internal compiler error on the following input (found by fuzz-rustc):

fn m(){#[c=[r
e
e

rustc output

$ rustc bug.rs
error: this file contains an unclosed delimiter
 --> bug.rs:3:2
  |
1 | fn m(){#[c=[r
  |       - -  - unclosed delimiter
  |       | |
  |       | unclosed delimiter
  |       unclosed delimiter
2 | e
3 | e
  |  ^

error: expected `;`, found `e`
 --> bug.rs:1:14
  |
1 | fn m(){#[c=[r
  |              ^ help: add `;` here
2 | e
  | - unexpected token

error: expected `;`, found `e`
 --> bug.rs:2:2
  |
2 | e
  |  ^ help: add `;` here
3 | e
  | - unexpected token

error: expected one of `!`, `.`, `::`, `;`, `?`, `{`, `}`, or an operator, found `]`
 --> bug.rs:3:2
  |
1 | fn m(){#[c=[r
  |       - unclosed delimiter
2 | e
3 | e
  |  ^
  |  |
  |  expected one of 8 possible tokens
  |  help: `}` may belong here

error[E0658]: attributes on expressions are experimental
 --> bug.rs:1:8
  |
1 | fn m(){#[c=[r
  |        ^^^^^^
  |
  = note: see issue #15701 <https://github.com/rust-lang/rust/issues/15701> for more information
  = help: add `#![feature(stmt_expr_attributes)]` to the crate attributes to enable

error: unexpected token: `[r]`
 --> bug.rs:1:12
  |
1 | fn m(){#[c=[r
  |            ^^

error: cannot find attribute `c` in this scope
 --> bug.rs:1:10
  |
1 | fn m(){#[c=[r
  |          ^

error[E0425]: cannot find value `e` in this scope
 --> bug.rs:2:1
  |
2 | e
  | ^ not found in this scope

error[E0425]: cannot find value `r` in this scope
 --> bug.rs:1:13
  |
1 | fn m(){#[c=[r
  |             ^ not found in this scope

thread 'rustc' panicked at 'Stack should be empty: final_buf=FrameData { open: bug.rs:1:12: 1:13 (#0), open_delim: Bracket, inner: [(Token(Token { kind: Ident("r", false), span: bug.rs:1:13: 1:14 (#0) }), Alone)] } stack=[FrameData { open: no-location (#0), open_delim: NoDelim, inner: [] }]', compiler/rustc_parse/src/parser/attr_wrapper.rs:501:5
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/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.56.0-nightly (25b764849 2021-08-04) running on x86_64-unknown-linux-gnu

query stack during panic:
end of query stack
error: aborting due to 9 previous errors

Some errors have detailed explanations: E0425, E0658.
For more information about an error, try `rustc --explain E0425`.

[dwrensha]

searched nightlies: from nightly-2021-02-10 to nightly-2021-08-04
regressed nightly: nightly-2021-07-28
searched commits: from 08095fc to 2faabf5
regressed commit: 8bebfe5

bisected with cargo-bisect-rustc v0.6.0

Host triple: x86_64-unknown-linux-gnu
Reproduce with:

cargo bisect-rustc --start=2021-2-10 --end=2021-08-04 --regress ice 

[dwrensha]

It looks to me like the regression was introduced in this pull request: #87436
cc @ebobrow

[apiraino]

Assigning priority as discussed in the Zulip thread of the Prioritization Working Group.

@rustbot label -I-prioritize +P-low