Rollup merge of rust-lang#124217 - Zalathar:pre-branch, r=oli-obk

coverage: Prepare for improved branch coverage

When trying to rebase my new branch coverage work (including rust-lang#124154) on top of the introduction of MC/DC coverage (rust-lang#123409), I found it a lot harder than anticipated. With the benefit of hindsight, the branch coverage code and MC/DC code have become more interdependent than I'm happy with.

This PR therefore disentangles them a bit, so that it will be easier for both areas of code to evolve independently without interference.

---

This PR also includes a few extra branch coverage tests that I had sitting around from my current branch coverage work. They mostly just demonstrate that certain language constructs listed in rust-lang#124118 currently don't have branch coverage support.

`@rustbot` label +A-code-coverage

compiler/rustc_middle/src/mir/coverage.rs

@@ -314,7 +314,9 @@ impl Default for ConditionInfo {
 #[derive(TyEncodable, TyDecodable, Hash, HashStable, TypeFoldable, TypeVisitable)]
 pub struct MCDCBranchSpan {
     pub span: Span,
-    pub condition_info: ConditionInfo,
+    /// If `None`, this actually represents a normal branch span inserted for
+    /// code that was too complex for MC/DC.
+    pub condition_info: Option<ConditionInfo>,
     pub true_marker: BlockMarkerId,
     pub false_marker: BlockMarkerId,
 }

compiler/rustc_middle/src/mir/pretty.rs

@@ -491,7 +491,7 @@ fn write_coverage_branch_info(
         writeln!(
             w,
             "{INDENT}coverage mcdc branch {{ condition_id: {:?}, true: {true_marker:?}, false: {false_marker:?} }} => {span:?}",
-            condition_info.condition_id
+            condition_info.map(|info| info.condition_id)
         )?;
     }
 

compiler/rustc_mir_build/src/build/coverageinfo.rs

@@ -7,13 +7,13 @@ use rustc_middle::mir::coverage::{
     BlockMarkerId, BranchSpan, ConditionId, ConditionInfo, CoverageKind, MCDCBranchSpan,
     MCDCDecisionSpan,
 };
-use rustc_middle::mir::{self, BasicBlock, UnOp};
+use rustc_middle::mir::{self, BasicBlock, SourceInfo, UnOp};
 use rustc_middle::thir::{ExprId, ExprKind, LogicalOp, Thir};
 use rustc_middle::ty::TyCtxt;
 use rustc_span::def_id::LocalDefId;
 use rustc_span::Span;
 
-use crate::build::Builder;
+use crate::build::{Builder, CFG};
 use crate::errors::MCDCExceedsConditionNumLimit;
 
 pub(crate) struct BranchInfoBuilder {
@@ -22,6 +22,7 @@ pub(crate) struct BranchInfoBuilder {
 
     num_block_markers: usize,
     branch_spans: Vec<BranchSpan>,
+
     mcdc_branch_spans: Vec<MCDCBranchSpan>,
     mcdc_decision_spans: Vec<MCDCDecisionSpan>,
     mcdc_state: Option<MCDCState>,
@@ -95,13 +96,7 @@ impl BranchInfoBuilder {
         }
     }
 
-    fn record_conditions_operation(&mut self, logical_op: LogicalOp, span: Span) {
-        if let Some(mcdc_state) = self.mcdc_state.as_mut() {
-            mcdc_state.record_conditions(logical_op, span);
-        }
-    }
-
-    fn fetch_condition_info(
+    fn fetch_mcdc_condition_info(
         &mut self,
         tcx: TyCtxt<'_>,
         true_marker: BlockMarkerId,
@@ -121,14 +116,9 @@ impl BranchInfoBuilder {
                 _ => {
                     // Do not generate mcdc mappings and statements for decisions with too many conditions.
                     let rebase_idx = self.mcdc_branch_spans.len() - decision.conditions_num + 1;
-                    let to_normal_branches = self.mcdc_branch_spans.split_off(rebase_idx);
-                    self.branch_spans.extend(to_normal_branches.into_iter().map(
-                        |MCDCBranchSpan { span, true_marker, false_marker, .. }| BranchSpan {
-                            span,
-                            true_marker,
-                            false_marker,
-                        },
-                    ));
+                    for branch in &mut self.mcdc_branch_spans[rebase_idx..] {
+                        branch.condition_info = None;
+                    }
 
                     // ConditionInfo of this branch shall also be reset.
                     condition_info = None;
@@ -144,20 +134,50 @@ impl BranchInfoBuilder {
         condition_info
     }
 
+    fn add_two_way_branch<'tcx>(
+        &mut self,
+        cfg: &mut CFG<'tcx>,
+        source_info: SourceInfo,
+        true_block: BasicBlock,
+        false_block: BasicBlock,
+    ) {
+        let true_marker = self.inject_block_marker(cfg, source_info, true_block);
+        let false_marker = self.inject_block_marker(cfg, source_info, false_block);
+
+        self.branch_spans.push(BranchSpan { span: source_info.span, true_marker, false_marker });
+    }
+
     fn next_block_marker_id(&mut self) -> BlockMarkerId {
         let id = BlockMarkerId::from_usize(self.num_block_markers);
         self.num_block_markers += 1;
         id
     }
 
+    fn inject_block_marker(
+        &mut self,
+        cfg: &mut CFG<'_>,
+        source_info: SourceInfo,
+        block: BasicBlock,
+    ) -> BlockMarkerId {
+        let id = self.next_block_marker_id();
+
+        let marker_statement = mir::Statement {
+            source_info,
+            kind: mir::StatementKind::Coverage(CoverageKind::BlockMarker { id }),
+        };
+        cfg.push(block, marker_statement);
+
+        id
+    }
+
     pub(crate) fn into_done(self) -> Option<Box<mir::coverage::BranchInfo>> {
         let Self {
             nots: _,
             num_block_markers,
             branch_spans,
             mcdc_branch_spans,
             mcdc_decision_spans,
-            ..
+            mcdc_state: _,
         } = self;
 
         if num_block_markers == 0 {
@@ -325,7 +345,7 @@ impl Builder<'_, '_> {
         mut else_block: BasicBlock,
     ) {
         // Bail out if branch coverage is not enabled for this function.
-        let Some(branch_info) = self.coverage_branch_info.as_ref() else { return };
+        let Some(branch_info) = self.coverage_branch_info.as_mut() else { return };
 
         // If this condition expression is nested within one or more `!` expressions,
         // replace it with the enclosing `!` collected by `visit_unary_not`.
@@ -335,47 +355,34 @@ impl Builder<'_, '_> {
                 std::mem::swap(&mut then_block, &mut else_block);
             }
         }
-        let source_info = self.source_info(self.thir[expr_id].span);
-
-        // Now that we have `source_info`, we can upgrade to a &mut reference.
-        let branch_info = self.coverage_branch_info.as_mut().expect("upgrading & to &mut");
-
-        let mut inject_branch_marker = |block: BasicBlock| {
-            let id = branch_info.next_block_marker_id();
-
-            let marker_statement = mir::Statement {
-                source_info,
-                kind: mir::StatementKind::Coverage(CoverageKind::BlockMarker { id }),
-            };
-            self.cfg.push(block, marker_statement);
 
-            id
-        };
-
-        let true_marker = inject_branch_marker(then_block);
-        let false_marker = inject_branch_marker(else_block);
+        let source_info = SourceInfo { span: self.thir[expr_id].span, scope: self.source_scope };
 
-        if let Some(condition_info) =
-            branch_info.fetch_condition_info(self.tcx, true_marker, false_marker)
-        {
+        // Separate path for handling branches when MC/DC is enabled.
+        if branch_info.mcdc_state.is_some() {
+            let mut inject_block_marker =
+                |block| branch_info.inject_block_marker(&mut self.cfg, source_info, block);
+            let true_marker = inject_block_marker(then_block);
+            let false_marker = inject_block_marker(else_block);
+            let condition_info =
+                branch_info.fetch_mcdc_condition_info(self.tcx, true_marker, false_marker);
             branch_info.mcdc_branch_spans.push(MCDCBranchSpan {
                 span: source_info.span,
                 condition_info,
                 true_marker,
                 false_marker,
             });
-        } else {
-            branch_info.branch_spans.push(BranchSpan {
-                span: source_info.span,
-                true_marker,
-                false_marker,
-            });
+            return;
         }
+
+        branch_info.add_two_way_branch(&mut self.cfg, source_info, then_block, else_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() {
-            branch_info.record_conditions_operation(logical_op, span);
+        if let Some(branch_info) = self.coverage_branch_info.as_mut()
+            && let Some(mcdc_state) = branch_info.mcdc_state.as_mut()
+        {
+            mcdc_state.record_conditions(logical_op, span);
         }
     }
 }

compiler/rustc_mir_transform/src/coverage/mod.rs

@@ -9,7 +9,7 @@ mod tests;
 
 use self::counters::{CounterIncrementSite, CoverageCounters};
 use self::graph::{BasicCoverageBlock, CoverageGraph};
-use self::spans::{BcbMapping, BcbMappingKind, CoverageSpans};
+use self::spans::{BcbBranchPair, BcbMapping, BcbMappingKind, CoverageSpans};
 
 use crate::MirPass;
 
@@ -141,21 +141,25 @@ fn create_mappings<'tcx>(
 
     let mut mappings = Vec::new();
 
-    mappings.extend(coverage_spans.all_bcb_mappings().filter_map(
+    mappings.extend(coverage_spans.mappings.iter().filter_map(
         |BcbMapping { kind: bcb_mapping_kind, span }| {
             let kind = match *bcb_mapping_kind {
                 BcbMappingKind::Code(bcb) => MappingKind::Code(term_for_bcb(bcb)),
-                BcbMappingKind::Branch { true_bcb, false_bcb } => MappingKind::Branch {
-                    true_term: term_for_bcb(true_bcb),
-                    false_term: term_for_bcb(false_bcb),
-                },
-                BcbMappingKind::MCDCBranch { true_bcb, false_bcb, condition_info } => {
-                    MappingKind::MCDCBranch {
+                BcbMappingKind::MCDCBranch { true_bcb, false_bcb, condition_info: None } => {
+                    MappingKind::Branch {
                         true_term: term_for_bcb(true_bcb),
                         false_term: term_for_bcb(false_bcb),
-                        mcdc_params: condition_info,
                     }
                 }
+                BcbMappingKind::MCDCBranch {
+                    true_bcb,
+                    false_bcb,
+                    condition_info: Some(mcdc_params),
+                } => MappingKind::MCDCBranch {
+                    true_term: term_for_bcb(true_bcb),
+                    false_term: term_for_bcb(false_bcb),
+                    mcdc_params,
+                },
                 BcbMappingKind::MCDCDecision { bitmap_idx, conditions_num, .. } => {
                     MappingKind::MCDCDecision(DecisionInfo { bitmap_idx, conditions_num })
                 }
@@ -165,6 +169,16 @@ fn create_mappings<'tcx>(
         },
     ));
 
+    mappings.extend(coverage_spans.branch_pairs.iter().filter_map(
+        |&BcbBranchPair { span, true_bcb, false_bcb }| {
+            let true_term = term_for_bcb(true_bcb);
+            let false_term = term_for_bcb(false_bcb);
+            let kind = MappingKind::Branch { true_term, false_term };
+            let code_region = make_code_region(source_map, file_name, span, body_span)?;
+            Some(Mapping { kind, code_region })
+        },
+    ));
+
     mappings
 }
 
@@ -233,7 +247,7 @@ fn inject_mcdc_statements<'tcx>(
 
     // Inject test vector update first because `inject_statement` always insert new statement at head.
     for (end_bcbs, bitmap_idx) in
-        coverage_spans.all_bcb_mappings().filter_map(|mapping| match &mapping.kind {
+        coverage_spans.mappings.iter().filter_map(|mapping| match &mapping.kind {
             BcbMappingKind::MCDCDecision { end_bcbs, bitmap_idx, .. } => {
                 Some((end_bcbs, *bitmap_idx))
             }
@@ -247,9 +261,9 @@ fn inject_mcdc_statements<'tcx>(
     }
 
     for (true_bcb, false_bcb, condition_id) in
-        coverage_spans.all_bcb_mappings().filter_map(|mapping| match mapping.kind {
+        coverage_spans.mappings.iter().filter_map(|mapping| match mapping.kind {
             BcbMappingKind::MCDCBranch { true_bcb, false_bcb, condition_info } => {
-                Some((true_bcb, false_bcb, condition_info.condition_id))
+                Some((true_bcb, false_bcb, condition_info?.condition_id))
             }
             _ => None,
         })

compiler/rustc_mir_transform/src/coverage/spans.rs

@@ -15,13 +15,17 @@ mod from_mir;
 pub(super) enum BcbMappingKind {
     /// Associates an ordinary executable code span with its corresponding BCB.
     Code(BasicCoverageBlock),
-    /// Associates a branch span with BCBs for its true and false arms.
-    Branch { true_bcb: BasicCoverageBlock, false_bcb: BasicCoverageBlock },
+
+    // Ordinary branch mappings are stored separately, so they don't have a
+    // variant in this enum.
+    //
     /// Associates a mcdc branch span with condition info besides fields for normal branch.
     MCDCBranch {
         true_bcb: BasicCoverageBlock,
         false_bcb: BasicCoverageBlock,
-        condition_info: ConditionInfo,
+        /// If `None`, this actually represents a normal branch mapping inserted
+        /// for code that was too complex for MC/DC.
+        condition_info: Option<ConditionInfo>,
     },
     /// Associates a mcdc decision with its join BCB.
     MCDCDecision { end_bcbs: BTreeSet<BasicCoverageBlock>, bitmap_idx: u32, conditions_num: u16 },
@@ -33,9 +37,20 @@ pub(super) struct BcbMapping {
     pub(super) span: Span,
 }
 
+/// This is separate from [`BcbMappingKind`] to help prepare for larger changes
+/// that will be needed for improved branch coverage in the future.
+/// (See <https://github.com/rust-lang/rust/pull/124217>.)
+#[derive(Debug)]
+pub(super) struct BcbBranchPair {
+    pub(super) span: Span,
+    pub(super) true_bcb: BasicCoverageBlock,
+    pub(super) false_bcb: BasicCoverageBlock,
+}
+
 pub(super) struct CoverageSpans {
     bcb_has_mappings: BitSet<BasicCoverageBlock>,
-    mappings: Vec<BcbMapping>,
+    pub(super) mappings: Vec<BcbMapping>,
+    pub(super) branch_pairs: Vec<BcbBranchPair>,
     test_vector_bitmap_bytes: u32,
 }
 
@@ -44,10 +59,6 @@ impl CoverageSpans {
         self.bcb_has_mappings.contains(bcb)
     }
 
-    pub(super) fn all_bcb_mappings(&self) -> impl Iterator<Item = &BcbMapping> {
-        self.mappings.iter()
-    }
-
     pub(super) fn test_vector_bitmap_bytes(&self) -> u32 {
         self.test_vector_bitmap_bytes
     }
@@ -63,6 +74,7 @@ pub(super) fn generate_coverage_spans(
     basic_coverage_blocks: &CoverageGraph,
 ) -> Option<CoverageSpans> {
     let mut mappings = vec![];
+    let mut branch_pairs = vec![];
 
     if hir_info.is_async_fn {
         // An async function desugars into a function that returns a future,
@@ -84,14 +96,20 @@ pub(super) fn generate_coverage_spans(
             BcbMapping { kind: BcbMappingKind::Code(bcb), span }
         }));
 
-        mappings.extend(from_mir::extract_branch_mappings(
+        branch_pairs.extend(from_mir::extract_branch_pairs(
+            mir_body,
+            hir_info,
+            basic_coverage_blocks,
+        ));
+
+        mappings.extend(from_mir::extract_mcdc_mappings(
             mir_body,
             hir_info.body_span,
             basic_coverage_blocks,
         ));
     }
 
-    if mappings.is_empty() {
+    if mappings.is_empty() && branch_pairs.is_empty() {
         return None;
     }
 
@@ -104,8 +122,7 @@ pub(super) fn generate_coverage_spans(
     for BcbMapping { kind, span: _ } in &mappings {
         match *kind {
             BcbMappingKind::Code(bcb) => insert(bcb),
-            BcbMappingKind::Branch { true_bcb, false_bcb }
-            | BcbMappingKind::MCDCBranch { true_bcb, false_bcb, .. } => {
+            BcbMappingKind::MCDCBranch { true_bcb, false_bcb, .. } => {
                 insert(true_bcb);
                 insert(false_bcb);
             }
@@ -118,8 +135,12 @@ pub(super) fn generate_coverage_spans(
             }
         }
     }
+    for &BcbBranchPair { true_bcb, false_bcb, .. } in &branch_pairs {
+        insert(true_bcb);
+        insert(false_bcb);
+    }
 
-    Some(CoverageSpans { bcb_has_mappings, mappings, test_vector_bitmap_bytes })
+    Some(CoverageSpans { bcb_has_mappings, mappings, branch_pairs, test_vector_bitmap_bytes })
 }
 
 #[derive(Debug)]