code coverage reporting for `let-else` interacts poorly with rustfmt

[briansmith]

I tried this code:

 let (m_original, []) = slice::as_chunks::<_, 8>(m.limbs()) else {
         return Err(LimbSliceError::len_mismatch(LenMismatchError::new(8)));
}

In my test suite, the positive if case is covered, but the else is never covered.

I expected to see this happen:

Code coverage tools should clearly report that let (m_original, []) = slice::as_chunks::<_, 8>(m.limbs()) is fully covered and should clearly report that the entire else case is not covered.

Instead, this happened:

Code coverage reports that let (m_original, []) = slice::as_chunks::<_, 8>(m.limbs()) else { is partially covered and that the rest of the else is uncovered:

This is really unfortunate as it leads to confusion about whether part of the positive case is uncovered.

Meta

rustc +nightly --version --verbose:

rustc 1.90.0-nightly (706f244db 2025-06-23)
binary: rustc
commit-hash: 706f244db581212cabf2e619e0113d70999b2bbe
commit-date: 2025-06-23
host: aarch64-apple-darwin
release: 1.90.0-nightly
LLVM version: 20.1.7