coverage: Avoid creating malformed macro name spans

compiler/rustc_mir_transform/src/coverage/spans.rs

@@ -381,6 +381,12 @@ impl<'a> CoverageSpansGenerator<'a> {
 
         let merged_prefix_len = self.curr_original_span.lo() - curr.span.lo();
         let after_macro_bang = merged_prefix_len + BytePos(visible_macro.as_str().len() as u32 + 1);
+        if self.curr().span.lo() + after_macro_bang > self.curr().span.hi() {
+            // Something is wrong with the macro name span;
+            // return now to avoid emitting malformed mappings.
+            // FIXME(#117788): Track down why this happens.
+            return;
+        }
         let mut macro_name_cov = curr.clone();
         self.curr_mut().span = curr.span.with_lo(curr.span.lo() + after_macro_bang);
         macro_name_cov.span =

tests/coverage/auxiliary/macro_name_span_helper.rs

@@ -0,0 +1,10 @@
+// edition: 2021
+
+#[macro_export]
+macro_rules! macro_that_defines_a_function {
+    (fn $name:ident () $body:tt) => {
+        fn $name () -> () $body
+    }
+}
+
+// Non-executable comment.

tests/coverage/macro_name_span.cov-map

@@ -0,0 +1,16 @@
+Function name: macro_name_span::affected_function
+Raw bytes (9): 0x[01, 01, 00, 01, 01, 06, 1b, 00, 20]
+Number of files: 1
+- file 0 => global file 1
+Number of expressions: 0
+Number of file 0 mappings: 1
+- Code(Counter(0)) at (prev + 6, 27) to (start + 0, 32)
+
+Function name: macro_name_span::main
+Raw bytes (9): 0x[01, 02, 00, 01, 01, 0b, 01, 02, 02]
+Number of files: 1
+- file 0 => global file 2
+Number of expressions: 0
+Number of file 0 mappings: 1
+- Code(Counter(0)) at (prev + 11, 1) to (start + 2, 2)
+

tests/coverage/macro_name_span.coverage

@@ -0,0 +1,39 @@
+$DIR/auxiliary/macro_name_span_helper.rs:
+   LL|       |// edition: 2021
+   LL|       |
+   LL|       |#[macro_export]
+   LL|       |macro_rules! macro_that_defines_a_function {
+   LL|       |    (fn $name:ident () $body:tt) => {
+   LL|      1|        fn $name () -> () $body
+   LL|       |    }
+   LL|       |}
+   LL|       |
+   LL|       |// Non-executable comment.
+
+$DIR/macro_name_span.rs:
+   LL|       |// edition: 2021
+   LL|       |
+   LL|       |// Regression test for <https://github.com/rust-lang/rust/issues/117788>.
+   LL|       |// Under some circumstances, the heuristics that detect macro name spans can
+   LL|       |// get confused and produce incorrect spans beyond the bounds of the span
+   LL|       |// being processed.
+   LL|       |
+   LL|       |// aux-build: macro_name_span_helper.rs
+   LL|       |extern crate macro_name_span_helper;
+   LL|       |
+   LL|      1|fn main() {
+   LL|      1|    affected_function();
+   LL|      1|}
+   LL|       |
+   LL|       |macro_rules! macro_with_an_unreasonably_and_egregiously_long_name {
+   LL|       |    () => {
+   LL|       |        println!("hello");
+   LL|       |    };
+   LL|       |}
+   LL|       |
+   LL|       |macro_name_span_helper::macro_that_defines_a_function! {
+   LL|       |    fn affected_function() {
+   LL|       |        macro_with_an_unreasonably_and_egregiously_long_name!();
+   LL|       |    }
+   LL|       |}
+

tests/coverage/macro_name_span.rs

@@ -0,0 +1,25 @@
+// edition: 2021
+
+// Regression test for <https://github.com/rust-lang/rust/issues/117788>.
+// Under some circumstances, the heuristics that detect macro name spans can
+// get confused and produce incorrect spans beyond the bounds of the span
+// being processed.
+
+// aux-build: macro_name_span_helper.rs
+extern crate macro_name_span_helper;
+
+fn main() {
+    affected_function();
+}
+
+macro_rules! macro_with_an_unreasonably_and_egregiously_long_name {
+    () => {
+        println!("hello");
+    };
+}
+
+macro_name_span_helper::macro_that_defines_a_function! {
+    fn affected_function() {
+        macro_with_an_unreasonably_and_egregiously_long_name!();
+    }
+}