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Rollup merge of #125700 - Zalathar:limit-overflow, r=nnethercote

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coverage: Avoid overflow when the MC/DC condition limit is exceeded

Fix for the test failure seen in https://github.com/rust-lang/rust/pull/124571#issuecomment-2099620869.

If we perform this subtraction first, it can sometimes overflow to -1 before the addition can bring its value back to 0.

That behaviour seems to be benign, but it nevertheless causes test failures in compiler configurations that check for overflow.

``@rustbot`` label +A-code-coverage
This commit is contained in:
Matthias Krüger 2024-05-29 20:12:33 +02:00 • committed by GitHub
commit 9a61146765
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6 changed files with 313 additions and 2 deletions
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5
compiler/rustc_mir_build/src/build/coverageinfo/mcdc.rs
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@ -217,12 +217,13 @@ impl MCDCInfoBuilder {
}
_ => {
// Do not generate mcdc mappings and statements for decisions with too many conditions.
let rebase_idx = self.branch_spans.len() - decision.conditions_num + 1;
// Therefore, first erase the condition info of the (N-1) previous branch spans.
let rebase_idx = self.branch_spans.len() - (decision.conditions_num - 1);
for branch in &mut self.branch_spans[rebase_idx..] {
branch.condition_info = None;
}
// ConditionInfo of this branch shall also be reset.
// Then, erase this last branch span's info too, for a total of N.
condition_info = None;
tcx.dcx().emit_warn(MCDCExceedsConditionNumLimit {