coverage: Add `CoverageKind::SpanMarker` for including extra spans in MIR

There are cases where coverage instrumentation wants to show a span for some
syntax element, but there is no MIR node that naturally carries that span, so
the instrumentor can't see it.

MIR building can now use this new kind of coverage statement to deliberately
include those spans in MIR, attached to a dummy statement that has no other
effect.

1 files changed, 12 insertions, 3 deletions

diff --git a/compiler/rustc_mir_transform/src/coverage/spans/from_mir.rs b/compiler/rustc_mir_transform/src/coverage/spans/from_mir.rs
index e1531f2c239..6f7d8d9dd75 100644
--- a/compiler/rustc_mir_transform/src/coverage/spans/from_mir.rs
+++ b/compiler/rustc_mir_transform/src/coverage/spans/from_mir.rs
@@ -92,13 +92,13 @@ fn is_closure(statement: &Statement<'_>) -> bool {
 /// If the MIR `Statement` has a span contributive to computing coverage spans,
 /// return it; otherwise return `None`.
 fn filtered_statement_span(statement: &Statement<'_>) -> Option<Span> {
+    use mir::coverage::CoverageKind;
+
     match statement.kind {
         // These statements have spans that are often outside the scope of the executed source code
         // for their parent `BasicBlock`.
         StatementKind::StorageLive(_)
         | StatementKind::StorageDead(_)
-        // Coverage should not be encountered, but don't inject coverage coverage
-        | StatementKind::Coverage(_)
         // Ignore `ConstEvalCounter`s
         | StatementKind::ConstEvalCounter
         // Ignore `Nop`s
@@ -122,9 +122,13 @@ fn filtered_statement_span(statement: &Statement<'_>) -> Option<Span> {
         // If and when the Issue is resolved, remove this special case match pattern:
         StatementKind::FakeRead(box (FakeReadCause::ForGuardBinding, _)) => None,
 
-        // Retain spans from all other statements
+        // Retain spans from most other statements.
         StatementKind::FakeRead(box (_, _)) // Not including `ForGuardBinding`
         | StatementKind::Intrinsic(..)
+        | StatementKind::Coverage(box mir::Coverage {
+            // The purpose of `SpanMarker` is to be matched and accepted here.
+            kind: CoverageKind::SpanMarker
+        })
         | StatementKind::Assign(_)
         | StatementKind::SetDiscriminant { .. }
         | StatementKind::Deinit(..)
@@ -133,6 +137,11 @@ fn filtered_statement_span(statement: &Statement<'_>) -> Option<Span> {
         | StatementKind::AscribeUserType(_, _) => {
             Some(statement.source_info.span)
         }
+
+        StatementKind::Coverage(box mir::Coverage {
+            // These coverage statements should not exist prior to coverage instrumentation.
+            kind: CoverageKind::CounterIncrement { .. } | CoverageKind::ExpressionUsed { .. }
+        }) => bug!("Unexpected coverage statement found during coverage instrumentation: {statement:?}"),
     }
 }