Regression in 2024-11-28: llvm-cov export errors with "truncated coverage data"

[alex]

Code

Beginning with today's nightly, processing coverage produced by rustc binaries fails. Here is an example CI run: https://github.com/pyca/cryptography/actions/runs/12077661037/job/33681030705?pr=12066

The relevant snippet:

nox > /home/runner/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/bin/llvm-cov export /home/runner/work/cryptography/cryptography/.nox/tests/lib/python3.12/site-packages/cryptography/hazmat/bindings/_rust.abi3.so -instr-profile=rust-cov.profdata '--ignore-filename-regex=[/\].cargo[/\]' '--ignore-filename-regex=[/\]rustc[/\]' '--ignore-filename-regex=[/\].rustup[/\]toolchains[/\]' '--ignore-filename-regex=[/\]target[/\]' --format=lcov
nox > Command /home/runner/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/bin/llvm-cov export /home/runner/work/cryptography/cryptography/.nox/tests/lib/python3.12/site-packages/cryptography/hazmat/bindings/_rust.abi3.so -instr-profile=rust-cov.profdata '--ignore-filename-regex=[/\].cargo[/\]' '--ignore-filename-regex=[/\]rustc[/\]' '--ignore-filename-regex=[/\].rustup[/\]toolchains[/\]' '--ignore-filename-regex=[/\]target[/\]' --format=lcov failed with exit code 1:
error: failed to load coverage: '/home/runner/work/cryptography/cryptography/.nox/tests/lib/python3.12/site-packages/cryptography/hazmat/bindings/_rust.abi3.so': truncated coverage data
error: could not load coverage information

#133418 is the only change in the regression range with a commit message mentioning coverage. cc: @Zalathar

Happy Thanksgiving!

Version it worked on

rustc 1.85.0-nightly (6b6a867ae 2024-11-27)

Version with regression

rustc 1.85.0-nightly (a2545fd6f 2024-11-28)

[Zalathar]

Hmm, my guess is that 619a272 is making it possible for all of a CGU's functions to be discarded at a late stage, so parts of the metadata end up being empty in a way that the llvm-cov isn't happy with.

Finding and testing a fix is probably going to be non-trivial (especially without a small repro), so for now it's probably wisest to revert.

[alex]

Regrettably, I don't have a reproducer smaller than "our entire large project with a bunch of deps" :-(