Rollup merge of rust-lang#125756 - Zalathar:branch-on-bool, r=oli-obk

coverage: Optionally instrument the RHS of lazy logical operators

(This is an updated version of rust-lang#124644 and rust-lang#124402. Fixes rust-lang#124120.)

When `||` or `&&` is used outside of a branching context (such as the condition of an `if`), the rightmost value does not directly influence any branching decision, so branch coverage instrumentation does not treat it as its own true-or-false branch.

That is a correct and useful interpretation of “branch coverage”, but might be undesirable in some contexts, as described at rust-lang#124120. This PR therefore adds a new coverage level `-Zcoverage-options=condition` that behaves like branch coverage, but also adds additional branch instrumentation to the right-hand-side of lazy boolean operators.

---

As discussed at rust-lang#124120 (comment), this is mainly intended as an intermediate step towards fully-featured MC/DC instrumentation. It's likely that we'll eventually want to remove this coverage level (rather than stabilize it), either because it has been incorporated into MC/DC instrumentation, or because it's getting in the way of future MC/DC work. The main appeal of landing it now is so that work on tracking conditions can proceed concurrently with other MC/DC-related work.

``@rustbot`` label +A-code-coverage

compiler/rustc_mir_build/src/build/coverageinfo.rs

@@ -157,6 +157,63 @@ impl BranchInfoBuilder {
 }
 
 impl<'tcx> Builder<'_, 'tcx> {
+    /// If condition coverage is enabled, inject extra blocks and marker statements
+    /// that will let us track the value of the condition in `place`.
+    pub(crate) fn visit_coverage_standalone_condition(
+        &mut self,
+        mut expr_id: ExprId,     // Expression giving the span of the condition
+        place: mir::Place<'tcx>, // Already holds the boolean condition value
+        block: &mut BasicBlock,
+    ) {
+        // Bail out if condition coverage is not enabled for this function.
+        let Some(branch_info) = self.coverage_branch_info.as_mut() else { return };
+        if !self.tcx.sess.instrument_coverage_condition() {
+            return;
+        };
+
+        // Remove any wrappers, so that we can inspect the real underlying expression.
+        while let ExprKind::Use { source: inner } | ExprKind::Scope { value: inner, .. } =
+            self.thir[expr_id].kind
+        {
+            expr_id = inner;
+        }
+        // If the expression is a lazy logical op, it will naturally get branch
+        // coverage as part of its normal lowering, so we can disregard it here.
+        if let ExprKind::LogicalOp { .. } = self.thir[expr_id].kind {
+            return;
+        }
+
+        let source_info = SourceInfo { span: self.thir[expr_id].span, scope: self.source_scope };
+
+        // Using the boolean value that has already been stored in `place`, set up
+        // control flow in the shape of a diamond, so that we can place separate
+        // marker statements in the true and false blocks. The coverage MIR pass
+        // will use those markers to inject coverage counters as appropriate.
+        //
+        //          block
+        //         /     \
+        // true_block   false_block
+        //  (marker)     (marker)
+        //         \     /
+        //        join_block
+
+        let true_block = self.cfg.start_new_block();
+        let false_block = self.cfg.start_new_block();
+        self.cfg.terminate(
+            *block,
+            source_info,
+            mir::TerminatorKind::if_(mir::Operand::Copy(place), true_block, false_block),
+        );
+
+        branch_info.add_two_way_branch(&mut self.cfg, source_info, true_block, false_block);
+
+        let join_block = self.cfg.start_new_block();
+        self.cfg.goto(true_block, source_info, join_block);
+        self.cfg.goto(false_block, source_info, join_block);
+        // Any subsequent codegen in the caller should use the new join block.
+        *block = join_block;
+    }
+
     /// 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(

compiler/rustc_mir_build/src/build/expr/into.rs

@@ -183,9 +183,13 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                         const_: Const::from_bool(this.tcx, constant),
                     },
                 );
-                let rhs = unpack!(this.expr_into_dest(destination, continuation, rhs));
+                let mut rhs_block = unpack!(this.expr_into_dest(destination, continuation, rhs));
+                // Instrument the lowered RHS's value for condition coverage.
+                // (Does nothing if condition coverage is not enabled.)
+                this.visit_coverage_standalone_condition(rhs, destination, &mut rhs_block);
+
                 let target = this.cfg.start_new_block();
-                this.cfg.goto(rhs, source_info, target);
+                this.cfg.goto(rhs_block, source_info, target);
                 this.cfg.goto(short_circuit, source_info, target);
                 target.unit()
             }

compiler/rustc_session/src/config.rs

@@ -159,7 +159,23 @@ pub enum CoverageLevel {
     Block,
     /// Also instrument branch points (includes block coverage).
     Branch,
-    /// Instrument for MC/DC. Mostly a superset of branch coverage, but might
+    /// Same as branch coverage, but also adds branch instrumentation for
+    /// certain boolean expressions that are not directly used for branching.
+    ///
+    /// For example, in the following code, `b` does not directly participate
+    /// in a branch, but condition coverage will instrument it as its own
+    /// artificial branch:
+    /// ```
+    /// # let (a, b) = (false, true);
+    /// let x = a && b;
+    /// //           ^ last operand
+    /// ```
+    ///
+    /// This level is mainly intended to be a stepping-stone towards full MC/DC
+    /// instrumentation, so it might be removed in the future when MC/DC is
+    /// sufficiently complete, or if it is making MC/DC changes difficult.
+    Condition,
+    /// Instrument for MC/DC. Mostly a superset of condition coverage, but might
     /// differ in some corner cases.
     Mcdc,
 }

compiler/rustc_session/src/options.rs

@@ -395,7 +395,7 @@ mod desc {
     pub const parse_optimization_fuel: &str = "crate=integer";
     pub const parse_dump_mono_stats: &str = "`markdown` (default) or `json`";
     pub const parse_instrument_coverage: &str = parse_bool;
-    pub const parse_coverage_options: &str = "`block` | `branch` | `mcdc`";
+    pub const parse_coverage_options: &str = "`block` | `branch` | `condition` | `mcdc`";
     pub const parse_instrument_xray: &str = "either a boolean (`yes`, `no`, `on`, `off`, etc), or a comma separated list of settings: `always` or `never` (mutually exclusive), `ignore-loops`, `instruction-threshold=N`, `skip-entry`, `skip-exit`";
     pub const parse_unpretty: &str = "`string` or `string=string`";
     pub const parse_treat_err_as_bug: &str = "either no value or a non-negative number";
@@ -961,6 +961,7 @@ mod parse {
             match option {
                 "block" => slot.level = CoverageLevel::Block,
                 "branch" => slot.level = CoverageLevel::Branch,
+                "condition" => slot.level = CoverageLevel::Condition,
                 "mcdc" => slot.level = CoverageLevel::Mcdc,
                 _ => return false,
             }

compiler/rustc_session/src/session.rs

@@ -353,6 +353,11 @@ impl Session {
             && self.opts.unstable_opts.coverage_options.level >= CoverageLevel::Branch
     }
 
+    pub fn instrument_coverage_condition(&self) -> bool {
+        self.instrument_coverage()
+            && self.opts.unstable_opts.coverage_options.level >= CoverageLevel::Condition
+    }
+
     pub fn instrument_coverage_mcdc(&self) -> bool {
         self.instrument_coverage()
             && self.opts.unstable_opts.coverage_options.level >= CoverageLevel::Mcdc

src/doc/unstable-book/src/compiler-flags/coverage-options.md

@@ -5,13 +5,16 @@ This option controls details of the coverage instrumentation performed by
 
 Multiple options can be passed, separated by commas. Valid options are:
 
-- `block`, `branch`, `mcdc`:
+- `block`, `branch`, `condition`, `mcdc`:
   Sets the level of coverage instrumentation.
   Setting the level will override any previously-specified level.
   - `block` (default):
     Blocks in the control-flow graph will be instrumented for coverage.
   - `branch`:
     In addition to block coverage, also enables branch coverage instrumentation.
+  - `condition`:
+    In addition to branch coverage, also instruments some boolean expressions
+    as branches, even if they are not directly used as branch conditions.
   - `mcdc`:
-    In addition to block and branch coverage, also enables MC/DC instrumentation.
+    In addition to condition coverage, also enables MC/DC instrumentation.
     (Branch coverage instrumentation may differ in some cases.)

tests/coverage/condition/conditions.cov-map

@@ -0,0 +1,152 @@
+Function name: conditions::assign_3_and_or
+Raw bytes (69): 0x[01, 01, 07, 07, 11, 09, 0d, 01, 05, 05, 09, 16, 1a, 05, 09, 01, 05, 09, 01, 1c, 01, 00, 2f, 03, 01, 09, 00, 0a, 01, 00, 0d, 00, 0e, 20, 05, 1a, 00, 0d, 00, 0e, 05, 00, 12, 00, 13, 20, 09, 16, 00, 12, 00, 13, 13, 00, 17, 00, 18, 20, 0d, 11, 00, 17, 00, 18, 03, 01, 05, 01, 02]
+Number of files: 1
+- file 0 => global file 1
+Number of expressions: 7
+- expression 0 operands: lhs = Expression(1, Add), rhs = Counter(4)
+- expression 1 operands: lhs = Counter(2), rhs = Counter(3)
+- expression 2 operands: lhs = Counter(0), rhs = Counter(1)
+- expression 3 operands: lhs = Counter(1), rhs = Counter(2)
+- expression 4 operands: lhs = Expression(5, Sub), rhs = Expression(6, Sub)
+- expression 5 operands: lhs = Counter(1), rhs = Counter(2)
+- expression 6 operands: lhs = Counter(0), rhs = Counter(1)
+Number of file 0 mappings: 9
+- Code(Counter(0)) at (prev + 28, 1) to (start + 0, 47)
+- Code(Expression(0, Add)) at (prev + 1, 9) to (start + 0, 10)
+    = ((c2 + c3) + c4)
+- Code(Counter(0)) at (prev + 0, 13) to (start + 0, 14)
+- Branch { true: Counter(1), false: Expression(6, Sub) } at (prev + 0, 13) to (start + 0, 14)
+    true  = c1
+    false = (c0 - c1)
+- Code(Counter(1)) at (prev + 0, 18) to (start + 0, 19)
+- Branch { true: Counter(2), false: Expression(5, Sub) } at (prev + 0, 18) to (start + 0, 19)
+    true  = c2
+    false = (c1 - c2)
+- Code(Expression(4, Add)) at (prev + 0, 23) to (start + 0, 24)
+    = ((c1 - c2) + (c0 - c1))
+- Branch { true: Counter(3), false: Counter(4) } at (prev + 0, 23) to (start + 0, 24)
+    true  = c3
+    false = c4
+- Code(Expression(0, Add)) at (prev + 1, 5) to (start + 1, 2)
+    = ((c2 + c3) + c4)
+
+Function name: conditions::assign_3_or_and
+Raw bytes (73): 0x[01, 01, 09, 05, 07, 0b, 11, 09, 0d, 01, 05, 01, 05, 22, 11, 01, 05, 22, 11, 01, 05, 09, 01, 17, 01, 00, 2f, 03, 01, 09, 00, 0a, 01, 00, 0d, 00, 0e, 20, 05, 22, 00, 0d, 00, 0e, 22, 00, 12, 00, 13, 20, 1e, 11, 00, 12, 00, 13, 1e, 00, 17, 00, 18, 20, 09, 0d, 00, 17, 00, 18, 03, 01, 05, 01, 02]
+Number of files: 1
+- file 0 => global file 1
+Number of expressions: 9
+- expression 0 operands: lhs = Counter(1), rhs = Expression(1, Add)
+- expression 1 operands: lhs = Expression(2, Add), rhs = Counter(4)
+- expression 2 operands: lhs = Counter(2), rhs = Counter(3)
+- expression 3 operands: lhs = Counter(0), rhs = Counter(1)
+- expression 4 operands: lhs = Counter(0), rhs = Counter(1)
+- expression 5 operands: lhs = Expression(8, Sub), rhs = Counter(4)
+- expression 6 operands: lhs = Counter(0), rhs = Counter(1)
+- expression 7 operands: lhs = Expression(8, Sub), rhs = Counter(4)
+- expression 8 operands: lhs = Counter(0), rhs = Counter(1)
+Number of file 0 mappings: 9
+- Code(Counter(0)) at (prev + 23, 1) to (start + 0, 47)
+- Code(Expression(0, Add)) at (prev + 1, 9) to (start + 0, 10)
+    = (c1 + ((c2 + c3) + c4))
+- Code(Counter(0)) at (prev + 0, 13) to (start + 0, 14)
+- Branch { true: Counter(1), false: Expression(8, Sub) } at (prev + 0, 13) to (start + 0, 14)
+    true  = c1
+    false = (c0 - c1)
+- Code(Expression(8, Sub)) at (prev + 0, 18) to (start + 0, 19)
+    = (c0 - c1)
+- Branch { true: Expression(7, Sub), false: Counter(4) } at (prev + 0, 18) to (start + 0, 19)
+    true  = ((c0 - c1) - c4)
+    false = c4
+- Code(Expression(7, Sub)) at (prev + 0, 23) to (start + 0, 24)
+    = ((c0 - c1) - c4)
+- Branch { true: Counter(2), false: Counter(3) } at (prev + 0, 23) to (start + 0, 24)
+    true  = c2
+    false = c3
+- Code(Expression(0, Add)) at (prev + 1, 5) to (start + 1, 2)
+    = (c1 + ((c2 + c3) + c4))
+
+Function name: conditions::assign_and
+Raw bytes (51): 0x[01, 01, 04, 07, 0e, 09, 0d, 01, 05, 01, 05, 07, 01, 0d, 01, 00, 21, 03, 01, 09, 00, 0a, 01, 00, 0d, 00, 0e, 20, 05, 0e, 00, 0d, 00, 0e, 05, 00, 12, 00, 13, 20, 09, 0d, 00, 12, 00, 13, 03, 01, 05, 01, 02]
+Number of files: 1
+- file 0 => global file 1
+Number of expressions: 4
+- expression 0 operands: lhs = Expression(1, Add), rhs = Expression(3, Sub)
+- expression 1 operands: lhs = Counter(2), rhs = Counter(3)
+- expression 2 operands: lhs = Counter(0), rhs = Counter(1)
+- expression 3 operands: lhs = Counter(0), rhs = Counter(1)
+Number of file 0 mappings: 7
+- Code(Counter(0)) at (prev + 13, 1) to (start + 0, 33)
+- Code(Expression(0, Add)) at (prev + 1, 9) to (start + 0, 10)
+    = ((c2 + c3) + (c0 - c1))
+- Code(Counter(0)) at (prev + 0, 13) to (start + 0, 14)
+- Branch { true: Counter(1), false: Expression(3, Sub) } at (prev + 0, 13) to (start + 0, 14)
+    true  = c1
+    false = (c0 - c1)
+- Code(Counter(1)) at (prev + 0, 18) to (start + 0, 19)
+- Branch { true: Counter(2), false: Counter(3) } at (prev + 0, 18) to (start + 0, 19)
+    true  = c2
+    false = c3
+- Code(Expression(0, Add)) at (prev + 1, 5) to (start + 1, 2)
+    = ((c2 + c3) + (c0 - c1))
+
+Function name: conditions::assign_or
+Raw bytes (51): 0x[01, 01, 04, 07, 0d, 05, 09, 01, 05, 01, 05, 07, 01, 12, 01, 00, 20, 03, 01, 09, 00, 0a, 01, 00, 0d, 00, 0e, 20, 05, 0e, 00, 0d, 00, 0e, 0e, 00, 12, 00, 13, 20, 09, 0d, 00, 12, 00, 13, 03, 01, 05, 01, 02]
+Number of files: 1
+- file 0 => global file 1
+Number of expressions: 4
+- expression 0 operands: lhs = Expression(1, Add), rhs = Counter(3)
+- expression 1 operands: lhs = Counter(1), rhs = Counter(2)
+- expression 2 operands: lhs = Counter(0), rhs = Counter(1)
+- expression 3 operands: lhs = Counter(0), rhs = Counter(1)
+Number of file 0 mappings: 7
+- Code(Counter(0)) at (prev + 18, 1) to (start + 0, 32)
+- Code(Expression(0, Add)) at (prev + 1, 9) to (start + 0, 10)
+    = ((c1 + c2) + c3)
+- Code(Counter(0)) at (prev + 0, 13) to (start + 0, 14)
+- Branch { true: Counter(1), false: Expression(3, Sub) } at (prev + 0, 13) to (start + 0, 14)
+    true  = c1
+    false = (c0 - c1)
+- Code(Expression(3, Sub)) at (prev + 0, 18) to (start + 0, 19)
+    = (c0 - c1)
+- Branch { true: Counter(2), false: Counter(3) } at (prev + 0, 18) to (start + 0, 19)
+    true  = c2
+    false = c3
+- Code(Expression(0, Add)) at (prev + 1, 5) to (start + 1, 2)
+    = ((c1 + c2) + c3)
+
+Function name: conditions::foo
+Raw bytes (9): 0x[01, 01, 00, 01, 01, 21, 01, 02, 02]
+Number of files: 1
+- file 0 => global file 1
+Number of expressions: 0
+Number of file 0 mappings: 1
+- Code(Counter(0)) at (prev + 33, 1) to (start + 2, 2)
+
+Function name: conditions::func_call
+Raw bytes (39): 0x[01, 01, 03, 01, 05, 0b, 02, 09, 0d, 05, 01, 25, 01, 01, 0a, 20, 05, 02, 01, 09, 00, 0a, 05, 00, 0e, 00, 0f, 20, 09, 0d, 00, 0e, 00, 0f, 07, 01, 01, 00, 02]
+Number of files: 1
+- file 0 => global file 1
+Number of expressions: 3
+- expression 0 operands: lhs = Counter(0), rhs = Counter(1)
+- expression 1 operands: lhs = Expression(2, Add), rhs = Expression(0, Sub)
+- expression 2 operands: lhs = Counter(2), rhs = Counter(3)
+Number of file 0 mappings: 5
+- Code(Counter(0)) at (prev + 37, 1) to (start + 1, 10)
+- Branch { true: Counter(1), false: Expression(0, Sub) } at (prev + 1, 9) to (start + 0, 10)
+    true  = c1
+    false = (c0 - c1)
+- Code(Counter(1)) at (prev + 0, 14) to (start + 0, 15)
+- Branch { true: Counter(2), false: Counter(3) } at (prev + 0, 14) to (start + 0, 15)
+    true  = c2
+    false = c3
+- Code(Expression(1, Add)) at (prev + 1, 1) to (start + 0, 2)
+    = ((c2 + c3) + (c0 - c1))
+
+Function name: conditions::simple_assign
+Raw bytes (9): 0x[01, 01, 00, 01, 01, 08, 01, 03, 02]
+Number of files: 1
+- file 0 => global file 1
+Number of expressions: 0
+Number of file 0 mappings: 1
+- Code(Counter(0)) at (prev + 8, 1) to (start + 3, 2)
+