coverage: Allow make_code_region to fail gracefully

Currently there's no way for this function to return `None`, but the next patch
will make it potentially able do so in some unknown corner-cases.

If it does fail, then skipping a span or function is better than causing an ICE
that the user might have no way to avoid without disabling coverage entirely.

Author: Zalathar

compiler/rustc_mir_transform/src/coverage/mod.rs

@@ -107,6 +107,9 @@ impl<'a, 'tcx> Instrumentor<'a, 'tcx> {
             .make_bcb_counters(&self.basic_coverage_blocks, bcb_has_coverage_spans);
 
         let mappings = self.create_mappings_and_inject_coverage_statements(&coverage_spans);
+        if mappings.is_empty() {
+            return;
+        }
 
         self.mir_body.function_coverage_info = Some(Box::new(FunctionCoverageInfo {
             function_source_hash: self.hir_info.function_source_hash,
@@ -135,18 +138,24 @@ impl<'a, 'tcx> Instrumentor<'a, 'tcx> {
 
         // Process the counters and spans associated with BCB nodes.
         for (bcb, counter_kind) in self.coverage_counters.bcb_node_counters() {
-            let spans = coverage_spans.spans_for_bcb(bcb);
-            let has_mappings = !spans.is_empty();
-
             // If this BCB has any coverage spans, add corresponding mappings to
             // the mappings table.
-            if has_mappings {
+            let has_mappings = {
                 let term = counter_kind.as_term();
-                mappings.extend(spans.iter().map(|&span| {
-                    let code_region = make_code_region(source_map, file_name, span, body_span);
-                    Mapping { code_region, term }
+                let old_mappings_len = mappings.len();
+
+                mappings.extend(coverage_spans.spans_for_bcb(bcb).iter().filter_map(|&span| {
+                    let Some(code_region) =
+                        make_code_region(source_map, file_name, span, body_span)
+                    else {
+                        debug!(?span, "Couldn't convert span to code region");
+                        return None;
+                    };
+                    Some(Mapping { code_region, term })
                 }));
-            }
+
+                mappings.len() > old_mappings_len
+            };
 
             let do_inject = match counter_kind {
                 // Counter-increment statements always need to be injected.
@@ -238,13 +247,19 @@ fn inject_statement(mir_body: &mut mir::Body<'_>, counter_kind: CoverageKind, bb
     data.statements.insert(0, statement);
 }
 
-/// Convert the Span into its file name, start line and column, and end line and column
+/// Convert the Span into its file name, start line and column, and end line and column.
+///
+/// Returns `None` if the conversion failed for some reason. There is no known example
+/// of code that would cause this to happen, but it's hard to rule out entirely
+/// (especially in the presence of complex macros or other expansions), and if it does
+/// happen then skipping a span or function is better than an ICE that the user might
+/// have no way to avoid.
 fn make_code_region(
     source_map: &SourceMap,
     file_name: Symbol,
     span: Span,
     body_span: Span,
-) -> CodeRegion {
+) -> Option<CodeRegion> {
     debug!(
         "Called make_code_region(file_name={}, span={}, body_span={})",
         file_name,
@@ -266,13 +281,13 @@ fn make_code_region(
         start_line = source_map.doctest_offset_line(&file.name, start_line);
         end_line = source_map.doctest_offset_line(&file.name, end_line);
     }
-    CodeRegion {
+    Some(CodeRegion {
         file_name,
         start_line: start_line as u32,
         start_col: start_col as u32,
         end_line: end_line as u32,
         end_col: end_col as u32,
-    }
+    })
 }
 
 fn is_eligible_for_coverage(tcx: TyCtxt<'_>, def_id: LocalDefId) -> bool {