`char::EscapeDebug` displays a char it's already yielded

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Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
78/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
rust
Domain
backend

Research direction

The issue points to library/core/src/escape.rs around the EscapeDebug display logic. Read how next() updates the iterator state and how Display writes printable characters, then add a regression test for consuming the character before formatting the exhausted iterator. Done when the exhausted iterator formats as an empty string while existing escaping behavior remains covered.

Written by the indexing model from the issue text.

Description

A-fmt A-iterators A-str C-bug I-prioritize regression-from-stable-to-stable T-libs
Code

I tried this code:

fn main() {
    let mut e = 'a'.escape_debug();
    assert_eq!(e.next(), Some('a'));
    assert_eq!(e.len(), 0);
    println!("{:?}", e.to_string());
}

I expected to see this happen: "" (the iterator is exhausted).

Instead, this happened: "a"

scope:

  • It only affects chars that char::escape_debug prints as-is (e.g. 'a', 'é'). Escaped characters like \n work correctly
  • str::escape_debug works correctly. For example, calling next() on "a".escape_debug() correctly exhausts the iterator

cause:

  • I think this came from #138237
  • Before, EscabeDebug::next called clear() after yielding a printable char
  • Now, the char is stored in EscapeIterInner, and next() only advances alive.start. alive.end still marks the char as present, so Display writes it again.

how i found this:

  • I found this while writing Kani proofs for core::escape as part of verify-rust-std

proposed fix:

-  return f.write_char(c);
+  return if self.alive.is_empty() { Ok(()) } else { f.write_char(c) };

https://github.com/rust-lang/rust/blob/41089d07a0a2afc16adfe28a760d94d73d7298a4/library/core/src/escape.rs#L346-L357

Version it worked on

It most recently worked on: 1.88.0

Version with regression

rustc --version --verbose: 1.89.0

rustc 1.89.0 (29483883e 2025-08-04)
binary: rustc
commit-hash: 29483883eed69d5fb4db01964cdf2af4d86e9cb2
commit-date: 2025-08-04
host: aarch64-apple-darwin
release: 1.89.0
LLVM version: 20.1.7

@rustbot modify labels: +regression-from-stable-to-stable -regression-untriaged

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