Auto merge of #115301 - Zalathar:regions-vec, r=davidtwco

coverage: Allow each coverage statement to have multiple code regions

The original implementation of coverage instrumentation was built around the assumption that a coverage counter/expression would be associated with *up to one* code region. When it was discovered that *multiple* regions would sometimes need to share a counter, a workaround was found: for the remaining regions, the instrumentor would create a fresh expression that adds zero  to the existing counter/expression.

That got the job done, but resulted in some awkward code, and produces unnecessarily complicated coverage maps in the final binary.

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This PR removes that tension by changing `StatementKind::Coverage`'s code region field from `Option<CodeRegion>` to `Vec<CodeRegion>`.

The changes on the codegen side are fairly straightforward. As long as each `CoverageKind::Counter` only injects one `llvm.instrprof.increment`, the rest of coverage codegen is happy to handle multiple regions mapped to the same counter/expression, with only minor option-to-vec adjustments.

On the instrumentor/mir-transform side, we can get rid of the code that creates extra (x + 0) expressions. Instead we gather all of the code regions associated with a single BCB, and inject them all into one coverage statement.

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There are several patches here but they can be divided in to three phases:
- Preparatory work
- Actually switching over to multiple regions per coverage statement
- Cleaning up

So viewing the patches individually may be easier.

1 files changed, 2 insertions, 2 deletions

diff --git a/compiler/rustc_mir_transform/src/coverage/query.rs b/compiler/rustc_mir_transform/src/coverage/query.rs
index 56365c5d474..2c0164e765c 100644
--- a/compiler/rustc_mir_transform/src/coverage/query.rs
+++ b/compiler/rustc_mir_transform/src/coverage/query.rs
@@ -93,8 +93,8 @@ fn coverageinfo<'tcx>(tcx: TyCtxt<'tcx>, instance_def: ty::InstanceDef<'tcx>) ->
 fn covered_code_regions(tcx: TyCtxt<'_>, def_id: DefId) -> Vec<&CodeRegion> {
     let body = mir_body(tcx, def_id);
     all_coverage_in_mir_body(body)
-        // Not all coverage statements have an attached code region.
-        .filter_map(|coverage| coverage.code_region.as_ref())
+        // Coverage statements have a list of code regions (possibly empty).
+        .flat_map(|coverage| coverage.code_regions.as_slice())
         .collect()
 }