coverage: Store all of a function's mappings in function coverage info

Previously, mappings were attached to individual coverage statements in MIR.
That necessitated special handling in MIR optimizations to avoid deleting those
statements, since otherwise codegen would be unable to reassemble the original
list of mappings.

With this change, a function's list of mappings is now attached to its MIR
body, and survives intact even if individual statements are deleted by
optimizations.

compiler/rustc_mir_transform/src/coverage/mod.rs

@@ -104,6 +104,7 @@ struct Instrumentor<'a, 'tcx> {
     function_source_hash: u64,
     basic_coverage_blocks: CoverageGraph,
     coverage_counters: CoverageCounters,
+    mappings: Vec<Mapping>,
 }
 
 impl<'a, 'tcx> Instrumentor<'a, 'tcx> {
@@ -144,6 +145,7 @@ impl<'a, 'tcx> Instrumentor<'a, 'tcx> {
             function_source_hash,
             basic_coverage_blocks,
             coverage_counters,
+            mappings: Vec::new(),
         }
     }
 
@@ -216,6 +218,7 @@ impl<'a, 'tcx> Instrumentor<'a, 'tcx> {
             function_source_hash: self.function_source_hash,
             num_counters: self.coverage_counters.num_counters(),
             num_expressions: self.coverage_counters.num_expressions(),
+            mappings: std::mem::take(&mut self.mappings),
         }));
     }
 
@@ -232,18 +235,16 @@ impl<'a, 'tcx> Instrumentor<'a, 'tcx> {
                 bug!("Every BasicCoverageBlock should have a Counter or Expression");
             });
 
-            // Convert the coverage spans into a vector of code regions to be
-            // associated with this BCB's coverage statement.
-            let code_regions = spans
-                .iter()
-                .map(|&span| make_code_region(source_map, file_name, span, body_span))
-                .collect::<Vec<_>>();
+            let term = counter_kind.as_term();
+            self.mappings.extend(spans.iter().map(|&span| {
+                let code_region = make_code_region(source_map, file_name, span, body_span);
+                Mapping { code_region, term }
+            }));
 
             inject_statement(
                 self.mir_body,
                 self.make_mir_coverage_kind(&counter_kind),
                 self.bcb_leader_bb(bcb),
-                code_regions,
             );
         }
     }
@@ -301,7 +302,6 @@ impl<'a, 'tcx> Instrumentor<'a, 'tcx> {
                         self.mir_body,
                         self.make_mir_coverage_kind(&counter_kind),
                         inject_to_bb,
-                        Vec::new(),
                     );
                 }
                 BcbCounter::Expression { .. } => inject_intermediate_expression(
@@ -329,7 +329,7 @@ impl<'a, 'tcx> Instrumentor<'a, 'tcx> {
 
     fn make_mir_coverage_kind(&self, counter_kind: &BcbCounter) -> CoverageKind {
         match *counter_kind {
-            BcbCounter::Counter { id } => CoverageKind::Counter { id },
+            BcbCounter::Counter { id } => CoverageKind::CounterIncrement { id },
             BcbCounter::Expression { id, lhs, op, rhs } => {
                 CoverageKind::Expression { id, lhs, op, rhs }
             }
@@ -360,18 +360,13 @@ fn inject_edge_counter_basic_block(
     new_bb
 }
 
-fn inject_statement(
-    mir_body: &mut mir::Body<'_>,
-    counter_kind: CoverageKind,
-    bb: BasicBlock,
-    code_regions: Vec<CodeRegion>,
-) {
-    debug!("  injecting statement {counter_kind:?} for {bb:?} at code regions: {code_regions:?}");
+fn inject_statement(mir_body: &mut mir::Body<'_>, counter_kind: CoverageKind, bb: BasicBlock) {
+    debug!("  injecting statement {counter_kind:?} for {bb:?}");
     let data = &mut mir_body[bb];
     let source_info = data.terminator().source_info;
     let statement = Statement {
         source_info,
-        kind: StatementKind::Coverage(Box::new(Coverage { kind: counter_kind, code_regions })),
+        kind: StatementKind::Coverage(Box::new(Coverage { kind: counter_kind })),
     };
     data.statements.insert(0, statement);
 }
@@ -385,10 +380,7 @@ fn inject_intermediate_expression(mir_body: &mut mir::Body<'_>, expression: Cove
     let source_info = data.terminator().source_info;
     let statement = Statement {
         source_info,
-        kind: StatementKind::Coverage(Box::new(Coverage {
-            kind: expression,
-            code_regions: Vec::new(),
-        })),
+        kind: StatementKind::Coverage(Box::new(Coverage { kind: expression })),
     };
     data.statements.push(statement);
 }