Rollup merge of #118852 - Zalathar:no-spans, r=cjgillot

coverage: Skip instrumenting a function if no spans were extracted from MIR

The immediate symptoms of #118643 were fixed by #118666, but some users reported that their builds now encounter another coverage-related ICE:

```
error: internal compiler error: compiler/rustc_codegen_llvm/src/coverageinfo/mapgen.rs:98:17: A used function should have had coverage mapping data but did not: (...)
```

I was able to reproduce at least one cause of this error: if no relevant spans could be extracted from a function, but the function contains `CoverageKind::SpanMarker` statements, then codegen still thinks the function is instrumented and complains about the fact that it has no coverage spans.

This PR prevents that from happening in two ways:
- If we didn't extract any relevant spans from MIR, skip instrumenting the entire function and don't create a `FunctionCoverateInfo` for it.
- If coverage codegen sees a `CoverageKind::SpanMarker` statement, skip it early and avoid creating `func_coverage`.

---

Fixes #118850.

compiler/rustc_codegen_llvm/src/coverageinfo/mod.rs

@@ -85,6 +85,14 @@ impl<'tcx> CoverageInfoBuilderMethods<'tcx> for Builder<'_, '_, 'tcx> {
 
         let bx = self;
 
+        match coverage.kind {
+            // Marker statements have no effect during codegen,
+            // so return early and don't create `func_coverage`.
+            CoverageKind::SpanMarker => return,
+            // Match exhaustively to ensure that newly-added kinds are classified correctly.
+            CoverageKind::CounterIncrement { .. } | CoverageKind::ExpressionUsed { .. } => {}
+        }
+
         let Some(function_coverage_info) =
             bx.tcx.instance_mir(instance.def).function_coverage_info.as_deref()
         else {
@@ -100,9 +108,9 @@ impl<'tcx> CoverageInfoBuilderMethods<'tcx> for Builder<'_, '_, 'tcx> {
 
         let Coverage { kind } = coverage;
         match *kind {
-            // Span markers are only meaningful during MIR instrumentation,
+            CoverageKind::SpanMarker => unreachable!(
-            // and have no effect during codegen.
+                "unexpected marker statement {kind:?} should have caused an early return"
-            CoverageKind::SpanMarker => {}
+            ),
             CoverageKind::CounterIncrement { id } => {
                 func_coverage.mark_counter_id_seen(id);
                 // We need to explicitly drop the `RefMut` before calling into `instrprof_increment`,

compiler/rustc_mir_transform/src/coverage/mod.rs

@@ -99,12 +99,15 @@ impl<'a, 'tcx> Instrumentor<'a, 'tcx> {
     fn inject_counters(&'a mut self) {
         ////////////////////////////////////////////////////
         // Compute coverage spans from the `CoverageGraph`.
-        let coverage_spans = CoverageSpans::generate_coverage_spans(
+        let Some(coverage_spans) = CoverageSpans::generate_coverage_spans(
             self.mir_body,
             self.fn_sig_span,
             self.body_span,
             &self.basic_coverage_blocks,
-        );
+        ) else {
+            // No relevant spans were found in MIR, so skip instrumenting this function.
+            return;
+        };
 
         ////////////////////////////////////////////////////
         // Create an optimized mix of `Counter`s and `Expression`s for the `CoverageGraph`. Ensure

compiler/rustc_mir_transform/src/coverage/spans.rs

@@ -15,12 +15,16 @@ pub(super) struct CoverageSpans {
 }
 
 impl CoverageSpans {
+    /// Extracts coverage-relevant spans from MIR, and associates them with
+    /// their corresponding BCBs.
+    ///
+    /// Returns `None` if no coverage-relevant spans could be extracted.
     pub(super) fn generate_coverage_spans(
         mir_body: &mir::Body<'_>,
         fn_sig_span: Span,
         body_span: Span,
         basic_coverage_blocks: &CoverageGraph,
-    ) -> Self {
+    ) -> Option<Self> {
         let coverage_spans = CoverageSpansGenerator::generate_coverage_spans(
             mir_body,
             fn_sig_span,
@@ -28,13 +32,17 @@ impl CoverageSpans {
             basic_coverage_blocks,
         );
 
+        if coverage_spans.is_empty() {
+            return None;
+        }
+
         // Group the coverage spans by BCB, with the BCBs in sorted order.
         let mut bcb_to_spans = IndexVec::from_elem_n(Vec::new(), basic_coverage_blocks.num_nodes());
         for CoverageSpan { bcb, span, .. } in coverage_spans {
             bcb_to_spans[bcb].push(span);
         }
 
-        Self { bcb_to_spans }
+        Some(Self { bcb_to_spans })
     }
 
     pub(super) fn bcb_has_coverage_spans(&self, bcb: BasicCoverageBlock) -> bool {

tests/coverage/no_spans_if_not.cov-map

@@ -0,0 +1,8 @@
+Function name: no_spans_if_not::main
+Raw bytes (9): 0x[01, 01, 00, 01, 01, 0b, 01, 02, 02]
+Number of files: 1
+- file 0 => global file 1
+Number of expressions: 0
+Number of file 0 mappings: 1
+- Code(Counter(0)) at (prev + 11, 1) to (start + 2, 2)
+

tests/coverage/no_spans_if_not.coverage

@@ -0,0 +1,30 @@
+   LL|       |// edition: 2021
+   LL|       |
+   LL|       |// If the span extractor can't find any relevant spans for a function,
+   LL|       |// but the function contains coverage span-marker statements (e.g. inserted
+   LL|       |// for `if !`), coverage codegen may think that it is instrumented and
+   LL|       |// consequently complain that it has no spans.
+   LL|       |//
+   LL|       |// Regression test for <https://github.com/rust-lang/rust/issues/118850>,
+   LL|       |// "A used function should have had coverage mapping data but did not".
+   LL|       |
+   LL|      1|fn main() {
+   LL|      1|    affected_function();
+   LL|      1|}
+   LL|       |
+   LL|       |macro_rules! macro_that_defines_a_function {
+   LL|       |    (fn $name:ident () $body:tt) => {
+   LL|       |        fn $name () $body
+   LL|       |    }
+   LL|       |}
+   LL|       |
+   LL|       |macro_that_defines_a_function! {
+   LL|       |    fn affected_function() {
+   LL|       |        if !false {
+   LL|       |            ()
+   LL|       |        } else {
+   LL|       |            ()
+   LL|       |        }
+   LL|       |    }
+   LL|       |}
+

tests/coverage/no_spans_if_not.rs

@@ -0,0 +1,29 @@
+// edition: 2021
+
+// If the span extractor can't find any relevant spans for a function,
+// but the function contains coverage span-marker statements (e.g. inserted
+// for `if !`), coverage codegen may think that it is instrumented and
+// consequently complain that it has no spans.
+//
+// Regression test for <https://github.com/rust-lang/rust/issues/118850>,
+// "A used function should have had coverage mapping data but did not".
+
+fn main() {
+    affected_function();
+}
+
+macro_rules! macro_that_defines_a_function {
+    (fn $name:ident () $body:tt) => {
+        fn $name () $body
+    }
+}
+
+macro_that_defines_a_function! {
+    fn affected_function() {
+        if !false {
+            ()
+        } else {
+            ()
+        }
+    }
+}