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.

2 files changed, 29 insertions, 0 deletions

diff --git a/tests/coverage/unused-local-file.coverage b/tests/coverage/unused-local-file.coverage
new file mode 100644
index 00000000000..8f5a32f6d70
--- /dev/null
+++ b/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() {}
+
diff --git a/tests/coverage/unused-local-file.rs b/tests/coverage/unused-local-file.rs
new file mode 100644
index 00000000000..cf43c62d703
--- /dev/null
+++ b/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();
+}