Rollup merge of #134497 - Zalathar:spans, r=jieyouxu

coverage: Store coverage source regions as `Span` until codegen (take 2)

This is an attempt to re-land #133418:

> Historically, coverage spans were converted into line/column coordinates during the MIR instrumentation pass.

> This PR moves that conversion step into codegen, so that coverage spans spend most of their time stored as Span instead.

> In addition to being conceptually nicer, this also reduces the size of coverage mappings in MIR, because Span is smaller than 4x u32.

That PR was reverted by #133608, because in some circumstances not covered by our test suite we were emitting coverage metadata that was causing `llvm-cov` to exit with an error (#133606).

---

The implementation here is *mostly* the same, but adapted for subsequent changes in the relevant code (e.g. #134163).

I believe that the changes in #134163 should be sufficient to prevent the problem that required the original PR to be reverted. But I haven't been able to reproduce the original breakage in a regression test, and the `llvm-cov` error message is extremely unhelpful, so I can't completely rule out the possibility of this breaking again.

r? jieyouxu (reviewer of the original PR)

3 files changed, 37 insertions, 0 deletions

diff --git a/tests/coverage/auxiliary/discard_all_helper.rs b/tests/coverage/auxiliary/discard_all_helper.rs
new file mode 100644
index 00000000000..9290bac7e53
--- /dev/null
+++ b/tests/coverage/auxiliary/discard_all_helper.rs
@@ -0,0 +1,6 @@
+//@ edition: 2021
+
+// Force this function to be generated in its home crate, so that it ends up
+// with normal coverage metadata.
+#[inline(never)]
+pub fn external_function() {}
diff --git a/tests/coverage/discard-all-issue-133606.coverage b/tests/coverage/discard-all-issue-133606.coverage
new file mode 100644
index 00000000000..8f5a32f6d70
--- /dev/null
+++ b/tests/coverage/discard-all-issue-133606.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/discard-all-issue-133606.rs b/tests/coverage/discard-all-issue-133606.rs
new file mode 100644
index 00000000000..91c278b2bb8
--- /dev/null
+++ b/tests/coverage/discard-all-issue-133606.rs
@@ -0,0 +1,24 @@
+//! Regression test for <https://github.com/rust-lang/rust/issues/133606>.
+//!
+//! In rare cases, all of a function's coverage spans are discarded at a late
+//! stage during codegen. When that happens, the subsequent code needs to take
+//! special care to avoid emitting coverage metadata that would cause `llvm-cov`
+//! to fail with a fatal error.
+//!
+//! We currently don't know of a concise way to reproduce that scenario with
+//! ordinary Rust source code, so instead we set a special testing-only flag to
+//! force it to occur.
+
+//@ edition: 2021
+//@ compile-flags: -Zcoverage-options=discard-all-spans-in-codegen
+
+// 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();
+}