coverage: Add branch coverage support for let-else

Author: Zalathar

compiler/rustc_mir_build/src/build/coverageinfo.rs

@@ -8,7 +8,7 @@ use rustc_middle::mir::coverage::{
     MCDCDecisionSpan,
 };
 use rustc_middle::mir::{self, BasicBlock, SourceInfo, UnOp};
-use rustc_middle::thir::{ExprId, ExprKind, LogicalOp, Thir};
+use rustc_middle::thir::{ExprId, ExprKind, LogicalOp, Pat, Thir};
 use rustc_middle::ty::TyCtxt;
 use rustc_span::def_id::LocalDefId;
 use rustc_span::Span;
@@ -360,7 +360,7 @@ impl MCDCState {
     }
 }
 
-impl Builder<'_, '_> {
+impl<'tcx> Builder<'_, 'tcx> {
     /// If branch coverage is enabled, inject marker statements into `then_block`
     /// and `else_block`, and record their IDs in the table of branch spans.
     pub(crate) fn visit_coverage_branch_condition(
@@ -407,6 +407,25 @@ impl Builder<'_, '_> {
         branch_info.add_two_way_branch(&mut self.cfg, source_info, then_block, else_block);
     }
 
+    /// If branch coverage is enabled, inject marker statements into `true_block`
+    /// and `false_block`, and record their IDs in the table of branches.
+    ///
+    /// Used to instrument let-else for branch coverage.
+    pub(crate) fn visit_coverage_conditional_let(
+        &mut self,
+        pattern: &Pat<'tcx>, // Pattern that has been matched when the true path is taken
+        true_block: BasicBlock,
+        false_block: BasicBlock,
+    ) {
+        // Bail out if branch coverage is not enabled for this function.
+        let Some(branch_info) = self.coverage_branch_info.as_mut() else { return };
+
+        // FIXME(#124144) This may need special handling when MC/DC is enabled.
+
+        let source_info = SourceInfo { span: pattern.span, scope: self.source_scope };
+        branch_info.add_two_way_branch(&mut self.cfg, source_info, true_block, false_block);
+    }
+
     pub(crate) fn visit_coverage_branch_operation(&mut self, logical_op: LogicalOp, span: Span) {
         if let Some(branch_info) = self.coverage_branch_info.as_mut()
             && let Some(mcdc_state) = branch_info.mcdc_state.as_mut()

compiler/rustc_mir_build/src/build/matches/mod.rs

@@ -2452,6 +2452,10 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 None,
                 true,
             );
+
+            // If branch coverage is enabled, record this branch.
+            this.visit_coverage_conditional_let(pattern, matching, failure);
+
             this.break_for_else(failure, this.source_info(initializer_span));
             matching.unit()
         });

tests/coverage/branch/let-else.cov-map

@@ -1,13 +1,16 @@
 Function name: let_else::let_else
-Raw bytes (38): 0x[01, 01, 02, 05, 09, 09, 02, 06, 01, 0c, 01, 01, 10, 02, 03, 0e, 00, 0f, 05, 00, 13, 00, 18, 09, 01, 09, 01, 0f, 02, 04, 05, 00, 0b, 07, 01, 01, 00, 02]
+Raw bytes (45): 0x[01, 01, 02, 05, 09, 09, 02, 07, 01, 0c, 01, 01, 10, 20, 02, 09, 03, 09, 00, 10, 02, 00, 0e, 00, 0f, 05, 00, 13, 00, 18, 09, 01, 09, 01, 0f, 02, 04, 05, 00, 0b, 07, 01, 01, 00, 02]
 Number of files: 1
 - file 0 => global file 1
 Number of expressions: 2
 - expression 0 operands: lhs = Counter(1), rhs = Counter(2)
 - expression 1 operands: lhs = Counter(2), rhs = Expression(0, Sub)
-Number of file 0 mappings: 6
+Number of file 0 mappings: 7
 - Code(Counter(0)) at (prev + 12, 1) to (start + 1, 16)
-- Code(Expression(0, Sub)) at (prev + 3, 14) to (start + 0, 15)
+- Branch { true: Expression(0, Sub), false: Counter(2) } at (prev + 3, 9) to (start + 0, 16)
+    true  = (c1 - c2)
+    false = c2
+- Code(Expression(0, Sub)) at (prev + 0, 14) to (start + 0, 15)
     = (c1 - c2)
 - Code(Counter(1)) at (prev + 0, 19) to (start + 0, 24)
 - Code(Counter(2)) at (prev + 1, 9) to (start + 1, 15)

tests/coverage/branch/let-else.coverage

@@ -14,6 +14,9 @@
    LL|       |
    LL|      3|    let Some(x) = value else {
                            ^2
+  ------------------
+  |  Branch (LL:9): [True: 2, False: 1]
+  ------------------
    LL|      1|        say("none");
    LL|      1|        return;
    LL|       |    };