Rollup merge of #126587 - Zalathar:no-mir-spans, r=oli-obk

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 instrumentation, because the coverage output is less noisy.

For internal debugging only. If future code changes would make it hard to keep supporting this flag, it should be removed at that time.

`@rustbot` label +A-code-coverage

1 files changed, 19 insertions, 14 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(