nightly regression: coverage contains bad paths

[alex]

Code

This issue concerns computing coverage. When computing coverage for https://github.com/pyca/cryptography, when I do an LCOV export (https://github.com/pyca/cryptography/blob/main/noxfile.py#L351-L376), I'm seeing a bad path in the output:

SF:/home/runner/work/cryptography/cryptography/library/std/src/sys/thread_local/native/mod.rs
FN:75,_RNCNKNvNtNtCs8lnbTqlVUHL_4pyo38internal5state12ATTACH_COUNT0s_0Csi2CMV9oxp0h_17cryptography_rust
FN:75,_RNCNKNvNtNtCs8lnbTqlVUHL_4pyo38internal5state12ATTACH_COUNT0s_0Cs7Va9g7vFn4f_17cryptography_cffi
FNDA:1359856,_RNCNKNvNtNtCs8lnbTqlVUHL_4pyo38internal5state12ATTACH_COUNT0s_0Csi2CMV9oxp0h_17cryptography_rust
FNDA:62,_RNCNKNvNtNtCs8lnbTqlVUHL_4pyo38internal5state12ATTACH_COUNT0s_0Cs7Va9g7vFn4f_17cryptography_cffi
FNF:1
FNH:1
DA:75,1359918
BRF:0
BRH:0
LF:1
LH:1
end_of_record

library/std/src obviously (?) does not exist inside the cryptography root, it comes from within rustc!

rustc paths properly should look like one of these: https://github.com/pyca/cryptography/blob/main/noxfile.py#L369-L372

Version it worked on

It most recently worked on: rustc 1.94.0-nightly (fa5eda1 2025-12-12)

Version with regression

It regressed on: rustc 1.94.0-nightly (0208ee0 2025-12-14)

[Urgau]

Most probably a regression from #149709.

Would be great to have a MCVE.

[alex]

I haven't attempted to reproduce outside of CI, but in theory:

git clone https://github.com/pyca/cryptography
cd cryptography
nox -e tests
cat *.lcov | grep /library/

or something thereabouts should reproduce it. Not minimal though 😭

[Urgau]

The following code reproduce the issue., but I'm having a hard time creating a reproducer in our test suite, due to rustc & compiletest normalizing the paths, which could explain why we didn't detect the regression.

use std::cell::RefCell;

thread_local! {
    pub static FOO: RefCell<Vec<u32>> = const { RefCell::new(Vec::new()) };
}

fn main() {
    FOO.with_borrow(|v| assert_eq!(v.len(), 0));
}

[alex]

Ah great (well, that's there's a MVCE, not that it can't go in the testsuite 🙃)