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

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

Start by reproducing the provided let-else example with the referenced nightly rustc and rustfmt, then inspect how coverage is reported for the positive pattern and else block. Done means the positive case is clearly reported as fully covered while the entire else case is clearly reported as uncovered.