coverage: Detect unused local file IDs to avoid an LLVM assertion

This case can't actually happen yet (other than via a testing flag), because
currently all of a function's spans must belong to the same file and expansion.
But this will be an important edge case when adding expansion region support.

1 files changed, 21 insertions, 5 deletions

diff --git a/compiler/rustc_codegen_llvm/src/coverageinfo/ffi.rs b/compiler/rustc_codegen_llvm/src/coverageinfo/ffi.rs
index f6000e72840..c207df2fb0b 100644
--- a/compiler/rustc_codegen_llvm/src/coverageinfo/ffi.rs
+++ b/compiler/rustc_codegen_llvm/src/coverageinfo/ffi.rs
@@ -155,6 +155,20 @@ pub(crate) struct Regions {
 impl Regions {
     /// Returns true if none of this structure's tables contain any regions.
     pub(crate) fn has_no_regions(&self) -> bool {
+        // Every region has a span, so if there are no spans then there are no regions.
+        self.all_cov_spans().next().is_none()
+    }
+
+    pub(crate) fn all_cov_spans(&self) -> impl Iterator<Item = &CoverageSpan> {
+        macro_rules! iter_cov_spans {
+            ( $( $regions:expr ),* $(,)? ) => {
+                std::iter::empty()
+                $(
+                    .chain( $regions.iter().map(|region| &region.cov_span) )
+                )*
+            }
+        }
+
         let Self {
             code_regions,
             expansion_regions,
@@ -163,11 +177,13 @@ impl Regions {
             mcdc_decision_regions,
         } = self;
 
-        code_regions.is_empty()
-            && expansion_regions.is_empty()
-            && branch_regions.is_empty()
-            && mcdc_branch_regions.is_empty()
-            && mcdc_decision_regions.is_empty()
+        iter_cov_spans!(
+            code_regions,
+            expansion_regions,
+            branch_regions,
+            mcdc_branch_regions,
+            mcdc_decision_regions,
+        )
     }
 }