Internal Compiler Error 'assertion failed: bpos.to_u32() >= mbc.pos.to_u32() + mbc.bytes as u32'

[dwrensha]

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

#[i=i::<ښܖ<

$ rustc bug.rs
error: this file contains an unclosed delimiter
 --> bug.rs:1:13
  |
1 | #[i=i::<ښܖ<
  |  -          ^
  |  |
  |  unclosed delimiter

thread 'rustc' panicked at 'assertion failed: bpos.to_u32() >= mbc.pos.to_u32() + mbc.bytes as u32', compiler/rustc_span/src/lib.rs:1505:17
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 1.53.0-nightly (a836d9b64 2021-04-10) running on x86_64-unknown-linux-gnu

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

[dwrensha]

Possibly related to #80224.

[dwrensha]

This is a regression, introduced in #78837.

cc @petrochenkov

searched nightlies: from nightly-2020-10-11 to nightly-2021-04-03
regressed nightly: nightly-2020-12-11
searched commits: from f0f6877 to
d32c320
regressed commit: 58d2bad
bisected with cargo-bisect-rustc v0.6.0