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,

compiler/rustc_interface/src/tests.rs

@@ -776,7 +776,8 @@ fn test_unstable_options_tracking_hash() {
         CoverageOptions {
             level: CoverageLevel::Mcdc,
             no_mir_spans: true,
-            discard_all_spans_in_codegen: true
+            discard_all_spans_in_codegen: true,
+            inject_unused_local_file: true,
         }
     );
     tracked!(crate_attr, vec!["abc".to_string()]);

compiler/rustc_session/src/config.rs

@@ -195,6 +195,11 @@ pub struct CoverageOptions {
     /// regression tests for #133606, because we don't have an easy way to
     /// reproduce it from actual source code.
     pub discard_all_spans_in_codegen: bool,
+
+    /// `-Zcoverage-options=inject-unused-local-file`: During codegen, add an
+    /// extra dummy entry to each function's local file table, to exercise the
+    /// code that checks for local file IDs with no mapping regions.
+    pub inject_unused_local_file: bool,
 }
 
 /// Controls whether branch coverage or MC/DC coverage is enabled.

compiler/rustc_session/src/options.rs

@@ -1413,6 +1413,7 @@ pub mod parse {
                 "mcdc" => slot.level = CoverageLevel::Mcdc,
                 "no-mir-spans" => slot.no_mir_spans = true,
                 "discard-all-spans-in-codegen" => slot.discard_all_spans_in_codegen = true,
+                "inject-unused-local-file" => slot.inject_unused_local_file = true,
                 _ => return false,
             }
         }

compiler/rustc_session/src/session.rs

@@ -371,6 +371,11 @@ impl Session {
         self.opts.unstable_opts.coverage_options.discard_all_spans_in_codegen
     }
 
+    /// True if testing flag `-Zcoverage-options=inject-unused-local-file` was passed.
+    pub fn coverage_inject_unused_local_file(&self) -> bool {
+        self.opts.unstable_opts.coverage_options.inject_unused_local_file
+    }
+
     pub fn is_sanitizer_cfi_enabled(&self) -> bool {
         self.opts.unstable_opts.sanitizer.contains(SanitizerSet::CFI)
     }

tests/coverage/unused-local-file.coverage

@@ -0,0 +1,7 @@
+   LL|       |//@ edition: 2021
+   LL|       |
+   LL|       |// Force this function to be generated in its home crate, so that it ends up
+   LL|       |// with normal coverage metadata.
+   LL|       |#[inline(never)]
+   LL|      1|pub fn external_function() {}
+

tests/coverage/unused-local-file.rs

@@ -0,0 +1,22 @@
+//! If we give LLVM a local file table for a function, but some of the entries
+//! in that table have no associated mapping regions, then an assertion failure
+//! will occur in LLVM. We therefore need to detect and skip any function that
+//! would trigger that assertion.
+//!
+//! To test that this case is handled, even before adding code that could allow
+//! it to happen organically (for expansion region support), we use a special
+//! testing-only flag to force it to occur.
+
+//@ edition: 2024
+//@ compile-flags: -Zcoverage-options=inject-unused-local-file
+
+// The `llvm-cov` tool will complain if the test binary ends up having no
+// coverage metadata at all. To prevent that, we also link to instrumented
+// code in an auxiliary crate that doesn't have the special flag set.
+
+//@ aux-build: discard_all_helper.rs
+extern crate discard_all_helper;
+
+fn main() {
+    discard_all_helper::external_function();
+}