coverage: Data structures for recording branch info during MIR building

compiler/rustc_middle/src/mir/coverage.rs

@@ -2,7 +2,7 @@
 
 use rustc_index::IndexVec;
 use rustc_macros::HashStable;
-use rustc_span::Symbol;
+use rustc_span::{Span, Symbol};
 
 use std::fmt::{self, Debug, Formatter};
 
@@ -93,7 +93,7 @@ pub enum CoverageKind {
     SpanMarker,
 
     /// Marks its enclosing basic block with an ID that can be referred to by
-    /// other data in the MIR body.
+    /// side data in [`BranchInfo`].
     ///
     /// Has no effect during codegen.
     BlockMarker { id: BlockMarkerId },
@@ -218,3 +218,22 @@ pub struct FunctionCoverageInfo {
     pub expressions: IndexVec<ExpressionId, Expression>,
     pub mappings: Vec<Mapping>,
 }
+
+/// Branch information recorded during THIR-to-MIR lowering, and stored in MIR.
+#[derive(Clone, Debug)]
+#[derive(TyEncodable, TyDecodable, Hash, HashStable, TypeFoldable, TypeVisitable)]
+pub struct BranchInfo {
+    /// 1 more than the highest-numbered [`CoverageKind::BlockMarker`] that was
+    /// injected into the MIR body. This makes it possible to allocate per-ID
+    /// data structures without having to scan the entire body first.
+    pub num_block_markers: usize,
+    pub branch_spans: Vec<BranchSpan>,
+}
+
+#[derive(Clone, Debug)]
+#[derive(TyEncodable, TyDecodable, Hash, HashStable, TypeFoldable, TypeVisitable)]
+pub struct BranchSpan {
+    pub span: Span,
+    pub true_marker: BlockMarkerId,
+    pub false_marker: BlockMarkerId,
+}

compiler/rustc_middle/src/mir/mod.rs

@@ -403,6 +403,12 @@ pub struct Body<'tcx> {
 
     pub tainted_by_errors: Option<ErrorGuaranteed>,
 
+    /// Branch coverage information collected during MIR building, to be used by
+    /// the `InstrumentCoverage` pass.
+    ///
+    /// Only present if branch coverage is enabled and this function is eligible.
+    pub coverage_branch_info: Option<Box<coverage::BranchInfo>>,
+
     /// Per-function coverage information added by the `InstrumentCoverage`
     /// pass, to be used in conjunction with the coverage statements injected
     /// into this body's blocks.
@@ -450,6 +456,7 @@ impl<'tcx> Body<'tcx> {
             is_polymorphic: false,
             injection_phase: None,
             tainted_by_errors,
+            coverage_branch_info: None,
             function_coverage_info: None,
         };
         body.is_polymorphic = body.has_non_region_param();
@@ -479,6 +486,7 @@ impl<'tcx> Body<'tcx> {
             is_polymorphic: false,
             injection_phase: None,
             tainted_by_errors: None,
+            coverage_branch_info: None,
             function_coverage_info: None,
         };
         body.is_polymorphic = body.has_non_region_param();

compiler/rustc_middle/src/mir/pretty.rs

@@ -461,6 +461,9 @@ pub fn write_mir_intro<'tcx>(
     // Add an empty line before the first block is printed.
     writeln!(w)?;
 
+    if let Some(branch_info) = &body.coverage_branch_info {
+        write_coverage_branch_info(branch_info, w)?;
+    }
     if let Some(function_coverage_info) = &body.function_coverage_info {
         write_function_coverage_info(function_coverage_info, w)?;
     }
@@ -468,6 +471,25 @@ pub fn write_mir_intro<'tcx>(
     Ok(())
 }
 
+fn write_coverage_branch_info(
+    branch_info: &coverage::BranchInfo,
+    w: &mut dyn io::Write,
+) -> io::Result<()> {
+    let coverage::BranchInfo { branch_spans, .. } = branch_info;
+
+    for coverage::BranchSpan { span, true_marker, false_marker } in branch_spans {
+        writeln!(
+            w,
+            "{INDENT}coverage branch {{ true: {true_marker:?}, false: {false_marker:?} }} => {span:?}",
+        )?;
+    }
+    if !branch_spans.is_empty() {
+        writeln!(w)?;
+    }
+
+    Ok(())
+}
+
 fn write_function_coverage_info(
     function_coverage_info: &coverage::FunctionCoverageInfo,
     w: &mut dyn io::Write,