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.

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,
+        )
     }
 }
 

compiler/rustc_codegen_llvm/src/coverageinfo/mapgen/covfun.rs

@@ -11,6 +11,7 @@ use rustc_abi::Align;
 use rustc_codegen_ssa::traits::{
     BaseTypeCodegenMethods as _, ConstCodegenMethods, StaticCodegenMethods,
 };
+use rustc_index::IndexVec;
 use rustc_middle::mir::coverage::{
     BasicCoverageBlock, CovTerm, CoverageIdsInfo, Expression, FunctionCoverageInfo, Mapping,
     MappingKind, Op,
@@ -125,6 +126,12 @@ fn fill_region_tables<'tcx>(
 
     let local_file_id = covfun.virtual_file_mapping.push_file(&source_file);
 
+    // If this testing flag is set, add an extra unused entry to the local
+    // file table, to help test the code for detecting unused file IDs.
+    if tcx.sess.coverage_inject_unused_local_file() {
+        covfun.virtual_file_mapping.push_file(&source_file);
+    }
+
     // In rare cases, _all_ of a function's spans are discarded, and coverage
     // codegen needs to handle that gracefully to avoid #133606.
     // It's hard for tests to trigger this organically, so instead we set
@@ -177,6 +184,19 @@ fn fill_region_tables<'tcx>(
     }
 }
 
+/// LLVM requires all local file IDs to have at least one mapping region.
+/// If that's not the case, skip this function, to avoid an assertion failure
+/// (or worse) in LLVM.
+fn check_local_file_table(covfun: &CovfunRecord<'_>) -> bool {
+    let mut local_file_id_seen =
+        IndexVec::<u32, _>::from_elem_n(false, covfun.virtual_file_mapping.local_file_table.len());
+    for cov_span in covfun.regions.all_cov_spans() {
+        local_file_id_seen[cov_span.file_id] = true;
+    }
+
+    local_file_id_seen.into_iter().all(|seen| seen)
+}
+
 /// Generates the contents of the covfun record for this function, which
 /// contains the function's coverage mapping data. The record is then stored
 /// as a global variable in the `__llvm_covfun` section.
@@ -185,6 +205,10 @@ pub(crate) fn generate_covfun_record<'tcx>(
     global_file_table: &GlobalFileTable,
     covfun: &CovfunRecord<'tcx>,
 ) {
+    if !check_local_file_table(covfun) {
+        return;
+    }
+
     let &CovfunRecord {
         mangled_function_name,
         source_hash,