coverage: Add debugging flag `-Zcoverage-options=no-mir-spans`

When set, this flag skips the code that normally extracts coverage spans from
MIR statements and terminators. That sometimes makes it easier to debug branch
coverage and MC/DC coverage, because the coverage output is less noisy.

For internal debugging only. If other code changes would make it hard to keep
supporting this flag, remove it.

2 files changed, 25 insertions, 16 deletions

diff --git a/compiler/rustc_mir_transform/src/coverage/mappings.rs b/compiler/rustc_mir_transform/src/coverage/mappings.rs
index 0e209757100..759bb7c1f9d 100644
--- a/compiler/rustc_mir_transform/src/coverage/mappings.rs
+++ b/compiler/rustc_mir_transform/src/coverage/mappings.rs
@@ -5,6 +5,7 @@ use rustc_index::bit_set::BitSet;
 use rustc_index::IndexVec;
 use rustc_middle::mir::coverage::{BlockMarkerId, BranchSpan, ConditionInfo, CoverageKind};
 use rustc_middle::mir::{self, BasicBlock, StatementKind};
+use rustc_middle::ty::TyCtxt;
 use rustc_span::Span;
 
 use crate::coverage::graph::{BasicCoverageBlock, CoverageGraph, START_BCB};
@@ -63,31 +64,35 @@ pub(super) struct ExtractedMappings {
 
 /// Extracts coverage-relevant spans from MIR, and associates them with
 /// their corresponding BCBs.
-pub(super) fn extract_all_mapping_info_from_mir(
-    mir_body: &mir::Body<'_>,
+pub(super) fn extract_all_mapping_info_from_mir<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    mir_body: &mir::Body<'tcx>,
     hir_info: &ExtractedHirInfo,
     basic_coverage_blocks: &CoverageGraph,
 ) -> ExtractedMappings {
-    if hir_info.is_async_fn {
+    let mut code_mappings = vec![];
+    let mut branch_pairs = vec![];
+    let mut mcdc_bitmap_bytes = 0;
+    let mut mcdc_branches = vec![];
+    let mut mcdc_decisions = vec![];
+
+    if hir_info.is_async_fn || tcx.sess.coverage_no_mir_spans() {
         // An async function desugars into a function that returns a future,
         // with the user code wrapped in a closure. Any spans in the desugared
         // outer function will be unhelpful, so just keep the signature span
         // and ignore all of the spans in the MIR body.
-        let mut mappings = ExtractedMappings::default();
+        //
+        // When debugging flag `-Zcoverage-options=no-mir-spans` is set, we need
+        // to give the same treatment to _all_ functions, because `llvm-cov`
+        // seems to ignore functions that don't have any ordinary code spans.
         if let Some(span) = hir_info.fn_sig_span_extended {
-            mappings.code_mappings.push(CodeMapping { span, bcb: START_BCB });
+            code_mappings.push(CodeMapping { span, bcb: START_BCB });
         }
-        return mappings;
+    } else {
+        // Extract coverage spans from MIR statements/terminators as normal.
+        extract_refined_covspans(mir_body, hir_info, basic_coverage_blocks, &mut code_mappings);
     }
 
-    let mut code_mappings = vec![];
-    let mut branch_pairs = vec![];
-    let mut mcdc_bitmap_bytes = 0;
-    let mut mcdc_branches = vec![];
-    let mut mcdc_decisions = vec![];
-
-    extract_refined_covspans(mir_body, hir_info, basic_coverage_blocks, &mut code_mappings);
-
     branch_pairs.extend(extract_branch_pairs(mir_body, hir_info, basic_coverage_blocks));
 
     extract_mcdc_mappings(
diff --git a/compiler/rustc_mir_transform/src/coverage/mod.rs b/compiler/rustc_mir_transform/src/coverage/mod.rs
index 419e39bc386..4a64d21f3d1 100644
--- a/compiler/rustc_mir_transform/src/coverage/mod.rs
+++ b/compiler/rustc_mir_transform/src/coverage/mod.rs
@@ -71,8 +71,12 @@ fn instrument_function_for_coverage<'tcx>(tcx: TyCtxt<'tcx>, mir_body: &mut mir:
 
     ////////////////////////////////////////////////////
     // Extract coverage spans and other mapping info from MIR.
-    let extracted_mappings =
-        mappings::extract_all_mapping_info_from_mir(mir_body, &hir_info, &basic_coverage_blocks);
+    let extracted_mappings = mappings::extract_all_mapping_info_from_mir(
+        tcx,
+        mir_body,
+        &hir_info,
+        &basic_coverage_blocks,
+    );
 
     ////////////////////////////////////////////////////
     // Create an optimized mix of `Counter`s and `Expression`s for the `CoverageGraph`. Ensure