coverage: Treat each match arm as a "branch" for branch coverage

Author: Zalathar

compiler/rustc_mir_build/src/build/coverageinfo.rs

@@ -33,6 +33,12 @@ struct NotInfo {
     is_flipped: bool,
 }
 
+pub(crate) struct MatchArm {
+    pub(crate) source_info: SourceInfo,
+    pub(crate) pre_binding_block: Option<BasicBlock>,
+    pub(crate) arm_block: BasicBlock,
+}
+
 #[derive(Default)]
 struct BlockMarkerGen {
     num_block_markers: usize,
@@ -150,6 +156,32 @@ impl BranchInfoBuilder {
         }
     }
 
+    pub(crate) fn add_match_arms(&mut self, cfg: &mut CFG<'_>, arms: &[MatchArm]) {
+        // Match expressions with 0-1 arms don't have any branches for their arms.
+        if arms.len() < 2 {
+            return;
+        }
+
+        // FIXME(#124118) The current implementation of branch coverage for
+        // match arms can't handle or-patterns.
+        if arms.iter().any(|arm| arm.pre_binding_block.is_none()) {
+            return;
+        }
+
+        let branch_arms = arms
+            .iter()
+            .map(|&MatchArm { source_info, pre_binding_block, arm_block }| {
+                let pre_guard_marker =
+                    self.markers.inject_block_marker(cfg, source_info, pre_binding_block.unwrap());
+                let arm_taken_marker =
+                    self.markers.inject_block_marker(cfg, source_info, arm_block);
+                BranchArm { span: source_info.span, pre_guard_marker, arm_taken_marker }
+            })
+            .collect::<Vec<_>>();
+
+        self.branch_arm_lists.push(branch_arms);
+    }
+
     pub(crate) fn into_done(self) -> Option<Box<mir::coverage::BranchInfo>> {
         let Self {
             nots: _,

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

@@ -8,7 +8,7 @@
 use crate::build::expr::as_place::PlaceBuilder;
 use crate::build::scope::DropKind;
 use crate::build::ForGuard::{self, OutsideGuard, RefWithinGuard};
-use crate::build::{BlockAnd, BlockAndExtension, Builder};
+use crate::build::{coverageinfo, BlockAnd, BlockAndExtension, Builder};
 use crate::build::{GuardFrame, GuardFrameLocal, LocalsForNode};
 use rustc_data_structures::{fx::FxIndexMap, stack::ensure_sufficient_stack};
 use rustc_hir::{BindingMode, ByRef};
@@ -473,6 +473,8 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         outer_source_info: SourceInfo,
         fake_borrow_temps: Vec<(Place<'tcx>, Local, FakeBorrowKind)>,
     ) -> BlockAnd<()> {
+        let mut coverage_match_arms = self.coverage_branch_info.is_some().then_some(vec![]);
+
         let arm_end_blocks: Vec<_> = arm_candidates
             .into_iter()
             .map(|(arm, candidate)| {
@@ -507,6 +509,8 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                         opt_scrutinee_place,
                     );
 
+                    let pre_binding_block = candidate.pre_binding_block;
+
                     let arm_block = this.bind_pattern(
                         outer_source_info,
                         candidate,
@@ -516,6 +520,14 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                         false,
                     );
 
+                    if let Some(coverage_match_arms) = coverage_match_arms.as_mut() {
+                        coverage_match_arms.push(coverageinfo::MatchArm {
+                            source_info: this.source_info(arm.pattern.span),
+                            pre_binding_block,
+                            arm_block,
+                        })
+                    }
+
                     this.fixed_temps_scope = old_dedup_scope;
 
                     if let Some(source_scope) = scope {
@@ -527,6 +539,13 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             })
             .collect();
 
+        if let Some(coverage_match_arms) = coverage_match_arms {
+            self.coverage_branch_info
+                .as_mut()
+                .expect("checked when creating `coverage_match_arms`")
+                .add_match_arms(&mut self.cfg, &coverage_match_arms);
+        }
+
         // all the arm blocks will rejoin here
         let end_block = self.cfg.start_new_block();
 

tests/coverage/branch/guard-simple.cov-map

@@ -1,24 +1,35 @@
 Function name: guard_simple::never_taken
-Raw bytes (56): 0x[01, 01, 04, 01, 05, 11, 09, 0f, 0d, 05, 09, 08, 01, 08, 01, 02, 1e, 20, 05, 02, 02, 0e, 00, 1e, 05, 00, 22, 00, 24, 11, 01, 0e, 00, 1e, 20, 09, 06, 00, 0e, 00, 1e, 09, 00, 22, 00, 24, 0d, 01, 0e, 00, 10, 0b, 02, 01, 00, 02]
+Raw bytes (80): 0x[01, 01, 09, 07, 01, 17, 05, 0d, 09, 01, 05, 17, 11, 0d, 09, 11, 09, 23, 0d, 05, 09, 0a, 01, 08, 01, 02, 1e, 20, 01, 02, 02, 09, 00, 0a, 20, 05, 0e, 00, 0e, 00, 1e, 05, 00, 22, 00, 24, 20, 11, 12, 01, 09, 00, 0a, 11, 00, 0e, 00, 1e, 20, 09, 1a, 00, 0e, 00, 1e, 09, 00, 22, 00, 24, 0d, 01, 0e, 00, 10, 1f, 02, 01, 00, 02]
 Number of files: 1
 - file 0 => global file 1
-Number of expressions: 4
-- expression 0 operands: lhs = Counter(0), rhs = Counter(1)
-- expression 1 operands: lhs = Counter(4), rhs = Counter(2)
-- expression 2 operands: lhs = Expression(3, Add), rhs = Counter(3)
-- expression 3 operands: lhs = Counter(1), rhs = Counter(2)
-Number of file 0 mappings: 8
+Number of expressions: 9
+- expression 0 operands: lhs = Expression(1, Add), rhs = Counter(0)
+- expression 1 operands: lhs = Expression(5, Add), rhs = Counter(1)
+- expression 2 operands: lhs = Counter(3), rhs = Counter(2)
+- expression 3 operands: lhs = Counter(0), rhs = Counter(1)
+- expression 4 operands: lhs = Expression(5, Add), rhs = Counter(4)
+- expression 5 operands: lhs = Counter(3), rhs = Counter(2)
+- expression 6 operands: lhs = Counter(4), rhs = Counter(2)
+- expression 7 operands: lhs = Expression(8, Add), rhs = Counter(3)
+- expression 8 operands: lhs = Counter(1), rhs = Counter(2)
+Number of file 0 mappings: 10
 - Code(Counter(0)) at (prev + 8, 1) to (start + 2, 30)
-- Branch { true: Counter(1), false: Expression(0, Sub) } at (prev + 2, 14) to (start + 0, 30)
+- Branch { true: Counter(0), false: Expression(0, Sub) } at (prev + 2, 9) to (start + 0, 10)
+    true  = c0
+    false = (((c3 + c2) + c1) - c0)
+- Branch { true: Counter(1), false: Expression(3, Sub) } at (prev + 0, 14) to (start + 0, 30)
     true  = c1
     false = (c0 - c1)
 - Code(Counter(1)) at (prev + 0, 34) to (start + 0, 36)
-- Code(Counter(4)) at (prev + 1, 14) to (start + 0, 30)
-- Branch { true: Counter(2), false: Expression(1, Sub) } at (prev + 0, 14) to (start + 0, 30)
+- Branch { true: Counter(4), false: Expression(4, Sub) } at (prev + 1, 9) to (start + 0, 10)
+    true  = c4
+    false = ((c3 + c2) - c4)
+- Code(Counter(4)) at (prev + 0, 14) to (start + 0, 30)
+- Branch { true: Counter(2), false: Expression(6, Sub) } at (prev + 0, 14) to (start + 0, 30)
     true  = c2
     false = (c4 - c2)
 - Code(Counter(2)) at (prev + 0, 34) to (start + 0, 36)
 - Code(Counter(3)) at (prev + 1, 14) to (start + 0, 16)
-- Code(Expression(2, Add)) at (prev + 2, 1) to (start + 0, 2)
+- Code(Expression(7, Add)) at (prev + 2, 1) to (start + 0, 2)
     = ((c1 + c2) + c3)
 

tests/coverage/branch/guard-simple.coverage

@@ -10,11 +10,13 @@
    LL|      1|        _ if black_box(false) => {}
                                                ^0
   ------------------
+  |  Branch (LL:9): [True: 1, False: 0]
   |  Branch (LL:14): [True: 0, False: 1]
   ------------------
    LL|      1|        _ if black_box(false) => {}
                                                ^0
   ------------------
+  |  Branch (LL:9): [True: 1, False: 0]
   |  Branch (LL:14): [True: 0, False: 1]
   ------------------
    LL|      1|        _ => {}

tests/coverage/branch/guard.cov-map

@@ -1,32 +1,48 @@
 Function name: guard::branch_match_guard
-Raw bytes (85): 0x[01, 01, 06, 19, 0d, 05, 09, 0f, 15, 13, 11, 17, 0d, 05, 09, 0d, 01, 0c, 01, 01, 10, 1d, 03, 0b, 00, 0c, 15, 01, 14, 02, 0a, 0d, 03, 0e, 00, 0f, 19, 00, 14, 00, 19, 20, 0d, 02, 00, 14, 00, 1e, 0d, 00, 1d, 02, 0a, 11, 03, 0e, 00, 0f, 1d, 00, 14, 00, 19, 20, 11, 09, 00, 14, 00, 1e, 11, 00, 1d, 02, 0a, 17, 03, 0e, 02, 0a, 0b, 04, 01, 00, 02]
+Raw bytes (120): 0x[01, 01, 0d, 1b, 0d, 33, 11, 05, 09, 03, 19, 19, 0d, 1b, 1d, 33, 11, 05, 09, 05, 09, 2b, 15, 2f, 11, 33, 0d, 05, 09, 10, 01, 0c, 01, 01, 10, 1d, 03, 0b, 00, 0c, 20, 15, 03, 01, 09, 00, 10, 15, 00, 14, 02, 0a, 20, 19, 0e, 03, 09, 00, 10, 0d, 00, 0e, 00, 0f, 19, 00, 14, 00, 19, 20, 0d, 12, 00, 14, 00, 1e, 0d, 00, 1d, 02, 0a, 20, 1d, 16, 03, 09, 00, 10, 11, 00, 0e, 00, 0f, 1d, 00, 14, 00, 19, 20, 11, 09, 00, 14, 00, 1e, 11, 00, 1d, 02, 0a, 33, 03, 0e, 02, 0a, 27, 04, 01, 00, 02]
 Number of files: 1
 - file 0 => global file 1
-Number of expressions: 6
-- expression 0 operands: lhs = Counter(6), rhs = Counter(3)
-- expression 1 operands: lhs = Counter(1), rhs = Counter(2)
-- expression 2 operands: lhs = Expression(3, Add), rhs = Counter(5)
-- expression 3 operands: lhs = Expression(4, Add), rhs = Counter(4)
-- expression 4 operands: lhs = Expression(5, Add), rhs = Counter(3)
-- expression 5 operands: lhs = Counter(1), rhs = Counter(2)
-Number of file 0 mappings: 13
+Number of expressions: 13
+- expression 0 operands: lhs = Expression(6, Add), rhs = Counter(3)
+- expression 1 operands: lhs = Expression(12, Add), rhs = Counter(4)
+- expression 2 operands: lhs = Counter(1), rhs = Counter(2)
+- expression 3 operands: lhs = Expression(0, Add), rhs = Counter(6)
+- expression 4 operands: lhs = Counter(6), rhs = Counter(3)
+- expression 5 operands: lhs = Expression(6, Add), rhs = Counter(7)
+- expression 6 operands: lhs = Expression(12, Add), rhs = Counter(4)
+- expression 7 operands: lhs = Counter(1), rhs = Counter(2)
+- expression 8 operands: lhs = Counter(1), rhs = Counter(2)
+- expression 9 operands: lhs = Expression(10, Add), rhs = Counter(5)
+- expression 10 operands: lhs = Expression(11, Add), rhs = Counter(4)
+- expression 11 operands: lhs = Expression(12, Add), rhs = Counter(3)
+- expression 12 operands: lhs = Counter(1), rhs = Counter(2)
+Number of file 0 mappings: 16
 - Code(Counter(0)) at (prev + 12, 1) to (start + 1, 16)
 - Code(Counter(7)) at (prev + 3, 11) to (start + 0, 12)
-- Code(Counter(5)) at (prev + 1, 20) to (start + 2, 10)
-- Code(Counter(3)) at (prev + 3, 14) to (start + 0, 15)
+- Branch { true: Counter(5), false: Expression(0, Add) } at (prev + 1, 9) to (start + 0, 16)
+    true  = c5
+    false = (((c1 + c2) + c4) + c3)
+- Code(Counter(5)) at (prev + 0, 20) to (start + 2, 10)
+- Branch { true: Counter(6), false: Expression(3, Sub) } at (prev + 3, 9) to (start + 0, 16)
+    true  = c6
+    false = ((((c1 + c2) + c4) + c3) - c6)
+- Code(Counter(3)) at (prev + 0, 14) to (start + 0, 15)
 - Code(Counter(6)) at (prev + 0, 20) to (start + 0, 25)
-- Branch { true: Counter(3), false: Expression(0, Sub) } at (prev + 0, 20) to (start + 0, 30)
+- Branch { true: Counter(3), false: Expression(4, Sub) } at (prev + 0, 20) to (start + 0, 30)
     true  = c3
     false = (c6 - c3)
 - Code(Counter(3)) at (prev + 0, 29) to (start + 2, 10)
-- Code(Counter(4)) at (prev + 3, 14) to (start + 0, 15)
+- Branch { true: Counter(7), false: Expression(5, Sub) } at (prev + 3, 9) to (start + 0, 16)
+    true  = c7
+    false = (((c1 + c2) + c4) - c7)
+- Code(Counter(4)) at (prev + 0, 14) to (start + 0, 15)
 - Code(Counter(7)) at (prev + 0, 20) to (start + 0, 25)
 - Branch { true: Counter(4), false: Counter(2) } at (prev + 0, 20) to (start + 0, 30)
     true  = c4
     false = c2
 - Code(Counter(4)) at (prev + 0, 29) to (start + 2, 10)
-- Code(Expression(5, Add)) at (prev + 3, 14) to (start + 2, 10)
+- Code(Expression(12, Add)) at (prev + 3, 14) to (start + 2, 10)
     = (c1 + c2)
-- Code(Expression(2, Add)) at (prev + 4, 1) to (start + 0, 2)
+- Code(Expression(9, Add)) at (prev + 4, 1) to (start + 0, 2)
     = ((((c1 + c2) + c3) + c4) + c5)
 

tests/coverage/branch/guard.coverage

@@ -14,17 +14,22 @@
    LL|       |
    LL|      1|    match x {
    LL|      1|        Some(0) => {
+  ------------------
+  |  Branch (LL:9): [True: 1, False: 3]
+  ------------------
    LL|      1|            println!("zero");
    LL|      1|        }
    LL|      3|        Some(x) if x % 2 == 0 => {
                            ^2
   ------------------
+  |  Branch (LL:9): [True: 3, False: 0]
   |  Branch (LL:20): [True: 2, False: 1]
   ------------------
    LL|      2|            println!("is nonzero and even");
    LL|      2|        }
    LL|      1|        Some(x) if x % 3 == 0 => {
   ------------------
+  |  Branch (LL:9): [True: 1, False: 0]
   |  Branch (LL:20): [True: 1, False: 0]
   ------------------
    LL|      1|            println!("is nonzero and odd, but divisible by 3");