[RenjiSann]

Follow-up of #161223

Tracking issue: rust-lang/goals#638

Finalize MC/DC implementation for boolean expressions.

1. Introduce FFI structures and functions for passing MC/DC mappings to LLVM
2. Transform MC/DC MIR statements into their corresponding LLVM instructions
3. Add tests

r? @davidtwco

[rustbot]

Some changes occurred in coverage instrumentation.

cc @Zalathar

Some changes occurred in match lowering

cc @Nadrieril

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

Some changes occurred in coverage instrumentation.

cc @Zalathar

Some changes occurred in coverage tests.

cc @Zalathar

Some changes occurred in coverage instrumentation.

cc @Zalathar

Some changes occurred in coverage instrumentation.

cc @Zalathar

[rust-bors]

☔ The latest upstream changes (presumably #162009) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.

[gelintonx]

TLDR: with this stack, llvm-cov crashes (SIGILL in CoverageMapping::loadFunctionRecord) on binaries containing a matches! used as a condition inside a &&/|| decision. The same logic written as (v == 1 || v == 2) works.

Reproducer (repro.rs):

fn f(a: bool, v: u8, t: bool) -> bool {
   !a && matches!(v, 1 | 2) && t
}
fn main() {
  for (a, v, t) in [(false,1,true), (true,1,true), (false,3,true), (false,2,false)] {
   std::hint::black_box(f(a,v,t));
}
}

rustc -Cinstrument-coverage -Zcoverage-options=mcdc repro.rs              
LLVM_PROFILE_FILE=r.profraw ./repro                                       
llvm-profdata merge -o r.profdata r.profraw                               
llvm-cov show --show-mcdc ./repro -instr-profile=r.profdata   # Illegal instruction, exit code 132                          

Control: replacing matches!(v, 1 | 2) with (v == 1 || v == 2) gives exit code 0 and a normal MC/DC decision report.
Environment: this PR's head 1f916c0, stage 1 compiler (./x build --stage 1 library), x86_64 Ubuntu 24.04. llvm-cov/llvm-profdata are the LLVM 22.1.8 tools from the CI rust-dev artifact that bootstrap downloads, not a local LLVM build. I reproduced it on 2 fresh machines. This is agains current head per example before the rebase needed after #162009 (coverage struct renames in #161902).

Stack trace:

   #3 llvm::coverage::CoverageMapping::loadFunctionRecord(llvm::coverage::CoverageMappingRecord
  const&, std::optional<std::reference_wrapper<llvm::IndexedInstrProfReader>> 
  const&) (libLLVM.so.22.1-rust-1.99.0-nightly+0x825e026)                     
     #4 llvm::coverage::CoverageMapping::loadFromReaders(...) (libLLVM.so.22.1-
  rust-1.99.0-nightly+0x825e3fa)                                              
     #5 llvm::coverage::CoverageMapping::loadFromFile(...) (libLLVM.so.22.1-  
  rust-1.99.0-nightly+0x825e8d1)                                              
     #6 llvm::coverage::CoverageMapping::load(...) (libLLVM.so.22.1-rust-1.99.
  0-nightly+0x825ee6d)                                                        
     #7 (anonymous namespace)::CodeCoverageTool::load() CodeCoverage.cpp:0:0  
     #8 (anonymous namespace)::CodeCoverageTool::run(...) CodeCoverage.cpp:0:0
     #9 showMain(int, char const**) (llvm-cov+0x19d4a)   

Notes:

• Since it fails while loading the mapping every llvm-cov view fails for the whole binary. In a larger crate report, export and show without --show-mcdc flag all crashed
• These work with the same toolchain: match arms with guards, a tuple match with a _ if v guard, (a && !b && …).then_some(x), || inside a closure, h && (4..=7).contains(&t)
• With the previous implementation nightly-2025-07-15 and LLVM 20 the file doesn't crash but that version has no MC/DC decision at all only branch counters for !a and t
• I guess emitted mapping breaks an invariant loadFunctionRecord relies on but I could not manage to check it

Question:
Until pattern matching in decisions is supported should such a chain be skipped as before or should the matches! count as one opaque condition? Zaeske et al. (Arxiv:2409.08708 in 5.1) treats a nested match as its own decision and its result as a single condition of the outer one.