coverage: Regression test for markers in a function with no spans

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 {
+            ()
+        }
+    }
+}