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.

---

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.

---

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, 4 insertions, 5 deletions

diff --git a/tests/mir-opt/instrument_coverage.main.InstrumentCoverage.diff b/tests/mir-opt/instrument_coverage.main.InstrumentCoverage.diff
index e17c6ddc56e..9a8caa26307 100644
--- a/tests/mir-opt/instrument_coverage.main.InstrumentCoverage.diff
+++ b/tests/mir-opt/instrument_coverage.main.InstrumentCoverage.diff
@@ -8,12 +8,12 @@
       let mut _3: !;
   
       bb0: {
-+         Coverage::Counter(0) for /the/src/instrument_coverage.rs:11:1 - 11:11;
++         Coverage::Counter(0) for [/the/src/instrument_coverage.rs:11:1 - 11:11];
           goto -> bb1;
       }
   
       bb1: {
-+         Coverage::Expression(0) = Counter(0) + Counter(1) for /the/src/instrument_coverage.rs:12:5 - 13:17;
++         Coverage::Expression(0) = Counter(0) + Counter(1) for [/the/src/instrument_coverage.rs:12:5 - 13:17];
           falseUnwind -> [real: bb2, unwind: bb6];
       }
   
@@ -27,15 +27,14 @@
       }
   
       bb4: {
-+         Coverage::Expression(2) = Expression(1) + Zero for /the/src/instrument_coverage.rs:17:1 - 17:2;
-+         Coverage::Expression(1) = Expression(0) - Counter(1) for /the/src/instrument_coverage.rs:14:13 - 14:18;
++         Coverage::Expression(1) = Expression(0) - Counter(1) for [/the/src/instrument_coverage.rs:14:13 - 14:18, /the/src/instrument_coverage.rs:17:1 - 17:2];
           _0 = const ();
           StorageDead(_2);
           return;
       }
   
       bb5: {
-+         Coverage::Counter(1) for /the/src/instrument_coverage.rs:15:10 - 15:11;
++         Coverage::Counter(1) for [/the/src/instrument_coverage.rs:15:10 - 15:11];
           _1 = const ();
           StorageDead(_2);
           goto -> bb1;