`-C instrument-coverage` misses regions in const assertions

[jfrimmel]

Hey, I've run into a problem with -C instrument-coverage, const generics and assertions.

I tried this code:

const fn check_even<const N: usize>() {
    assert!(N % 2 == 0, "will fail once");
}

#[test]
fn true_branch() {
    check_even::<8>();  // even number
}

#[test]
#[should_panic = "will fail once"]
fn false_branch() {
    check_even::<9>();  // will trigger the assertion
}

I prepared a reproducer with cargo, which is not exactly a MCVE, but the code itself is rather minimal. Therefore the following commands will set up a Cargo project with the aforementioned code in the lib.rs-file.

cargo new --lib repro
RUSTFLAGS="-C instrument-coverage" cargo test --manifest-path repro/Cargo.toml --tests
llvm-profdata merge -sparse repro/default_* -o merged.profdata
llvm-cov report --instr-profile merged.profdata repro/target/debug/deps/repro-* --show-branch-summary=false  # make sure, the correct test binary is picked up

I expected to see this happen: as all branches/regions are covered, I expect 100% test coverage.

Instead, this happened: only 80% region coverage (but 100% line coverage as expected):

Filename                                Regions    Missed Regions     Cover   Functions  Missed Functions  Executed       Lines      Missed Lines     Cover
------------------------------------------------------------------------------------------------------------------------------------------------------------
/home/jfrimmel/git/repro/src/lib.rs           5                 1    80.00%           3                 0   100.00%           9                 0   100.00%
------------------------------------------------------------------------------------------------------------------------------------------------------------
TOTAL                                         5                 1    80.00%           3                 0   100.00%           9                 0   100.00%

Specifically, the region containing the assertion message is not covered, despite it clearly has to be used, since the tests would fail due to the wrong panic message.

For reference: this actually hits me in a project of mine, which aims to have 100% coverage. The only regions not covered are caused by assertions. The CI logs are located here.

Meta

rustc --version --verbose:

rustc 1.81.0 (eeb90cda1 2024-09-04)
binary: rustc
commit-hash: eeb90cda1969383f56a2637cbd3037bdf598841c
commit-date: 2024-09-04
host: x86_64-unknown-linux-gnu
release: 1.81.0
LLVM version: 18.1.7

This also reproduces with the nightlies 2024-09-18, 2024-09-09, 2024-09-01 and 2024-08-01.

@rustbot label +A-code-coverage

[jfrimmel]

I'm really not an expert in this whole area, but it seems like the coverage instrumentation thinks, that there is a "region" between the panic triggered in the assertion and the end of the function (but there is no code to be executed). Also note, that this is only an issue in the const-generics-version of the code. If I extend the code with non-const tests, alt least those new tests work as expected:

const fn check_even_const<const N: usize>() {
    assert!(N % 2 == 0, "will fail once");
}

#[test]
fn true_branch_const() {
    check_even_const::<8>();  // even number
}

#[test]
#[should_panic = "will fail once"]
fn false_branch_const() {
    check_even_const::<9>();  // will trigger the assertion
}

const fn check_even(n: usize) {
    assert!(n % 2 == 0, "will fail once");
}

#[test]
fn true_branch() {
    check_even(8);  // even number
}

#[test]
#[should_panic = "will fail once"]
fn false_branch() {
    check_even(9);  // will trigger the assertion
}

This causes a total coverage of 90% with still only one missed region (in the const part). The non-const-part works as expected and is fully covered: