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Rollup merge of #120185 - Zalathar:auto-derived, r=wesleywiser

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coverage: Don't instrument `#[automatically_derived]` functions

This PR makes the coverage instrumentor detect and skip functions that have [`#[automatically_derived]`](https://doc.rust-lang.org/reference/attributes/derive.html#the-automatically_derived-attribute) on their enclosing impl block.

Most notably, this means that methods generated by built-in derives (e.g. `Clone`, `Debug`, `PartialEq`) are now ignored by coverage instrumentation, and won't appear as executed or not-executed in coverage reports.

This is a noticeable change in user-visible behaviour, but overall I think it's a net improvement. For example, we've had a few user requests for this sort of change (e.g. #105055, https://github.com/rust-lang/rust/issues/84605#issuecomment-1902069040), and I believe it's the behaviour that most users will expect/prefer by default.

It's possible to imagine situations where users would want to instrument these derived implementations, but I think it's OK to treat that as an opportunity to consider adding more fine-grained option flags to control the details of coverage instrumentation, while leaving this new behaviour as the default.

(Also note that while `-Cinstrument-coverage` is a stable feature, the exact details of coverage instrumentation are allowed to change. So we *can* make this change; the main question is whether we *should*.)

Fixes #105055.
This commit is contained in:
León Orell Valerian Liehr 2024-01-24 15:43:12 +01:00 • committed by GitHub
commit 8bd126cb18
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10 changed files with 17 additions and 97 deletions
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11
compiler/rustc_mir_transform/src/coverage/mod.rs
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@ -384,7 +384,18 @@ fn is_eligible_for_coverage(tcx: TyCtxt<'_>, def_id: LocalDefId) -> bool {
return false;
}
// Don't instrument functions with `#[automatically_derived]` on their
// enclosing impl block, on the assumption that most users won't care about
// coverage for derived impls.
if let Some(impl_of) = tcx.impl_of_method(def_id.to_def_id())
&& tcx.is_automatically_derived(impl_of)
{
trace!("InstrumentCoverage skipped for {def_id:?} (automatically derived)");
return false;
}
if tcx.codegen_fn_attrs(def_id).flags.contains(CodegenFnAttrFlags::NO_COVERAGE) {
trace!("InstrumentCoverage skipped for {def_id:?} (`#[coverage(off)]`)");
return false;
}