coverage: Regression test for a span extraction inconsistency

tests/coverage/closure_unit_return.cov-map

@@ -0,0 +1,34 @@
+Function name: closure_unit_return::explicit_unit
+Raw bytes (14): 0x[01, 01, 00, 02, 01, 07, 01, 01, 10, 01, 05, 05, 02, 02]
+Number of files: 1
+- file 0 => global file 1
+Number of expressions: 0
+Number of file 0 mappings: 2
+- Code(Counter(0)) at (prev + 7, 1) to (start + 1, 16)
+- Code(Counter(0)) at (prev + 5, 5) to (start + 2, 2)
+
+Function name: closure_unit_return::explicit_unit::{closure#0} (unused)
+Raw bytes (9): 0x[01, 01, 00, 01, 00, 08, 16, 02, 06]
+Number of files: 1
+- file 0 => global file 1
+Number of expressions: 0
+Number of file 0 mappings: 1
+- Code(Zero) at (prev + 8, 22) to (start + 2, 6)
+
+Function name: closure_unit_return::implicit_unit
+Raw bytes (14): 0x[01, 01, 00, 02, 01, 10, 01, 01, 13, 01, 03, 06, 04, 02]
+Number of files: 1
+- file 0 => global file 1
+Number of expressions: 0
+Number of file 0 mappings: 2
+- Code(Counter(0)) at (prev + 16, 1) to (start + 1, 19)
+- Code(Counter(0)) at (prev + 3, 6) to (start + 4, 2)
+
+Function name: closure_unit_return::implicit_unit::{closure#0} (unused)
+Raw bytes (9): 0x[01, 01, 00, 01, 00, 11, 16, 02, 06]
+Number of files: 1
+- file 0 => global file 1
+Number of expressions: 0
+Number of file 0 mappings: 1
+- Code(Zero) at (prev + 17, 22) to (start + 2, 6)
+

tests/coverage/closure_unit_return.coverage

@@ -0,0 +1,30 @@
+   LL|       |#![feature(coverage_attribute)]
+   LL|       |// edition: 2021
+   LL|       |
+   LL|       |// Regression test for an inconsistency between functions that return the value
+   LL|       |// of their trailing expression, and functions that implicitly return `()`.
+   LL|       |
+   LL|      1|fn explicit_unit() {
+   LL|      1|    let closure = || {
+   LL|      0|        ();
+   LL|      0|    };
+   LL|       |
+   LL|      1|    drop(closure);
+   LL|      1|    () // explicit return of trailing value
+   LL|      1|}
+   LL|       |
+   LL|      1|fn implicit_unit() {
+   LL|      1|    let closure = || {
+   LL|      0|        ();
+   LL|      1|    };
+   LL|      1|
+   LL|      1|    drop(closure);
+   LL|      1|    // implicit return of `()`
+   LL|      1|}
+   LL|       |
+   LL|       |#[coverage(off)]
+   LL|       |fn main() {
+   LL|       |    explicit_unit();
+   LL|       |    implicit_unit();
+   LL|       |}
+

tests/coverage/closure_unit_return.rs

@@ -0,0 +1,29 @@
+#![feature(coverage_attribute)]
+// edition: 2021
+
+// Regression test for an inconsistency between functions that return the value
+// of their trailing expression, and functions that implicitly return `()`.
+
+fn explicit_unit() {
+    let closure = || {
+        ();
+    };
+
+    drop(closure);
+    () // explicit return of trailing value
+}
+
+fn implicit_unit() {
+    let closure = || {
+        ();
+    };
+
+    drop(closure);
+    // implicit return of `()`
+}
+
+#[coverage(off)]
+fn main() {
+    explicit_unit();
+    implicit_unit();
+}