coverage: Regression test for unwrapping prev when there are no spans

tests/coverage/no_spans.cov-map

@@ -0,0 +1,8 @@
+Function name: no_spans::affected_function::{closure#0}
+Raw bytes (9): 0x[01, 01, 00, 01, 01, 1b, 0c, 00, 0e]
+Number of files: 1
+- file 0 => global file 1
+Number of expressions: 0
+Number of file 0 mappings: 1
+- Code(Counter(0)) at (prev + 27, 12) to (start + 0, 14)
+

tests/coverage/no_spans.coverage

@@ -0,0 +1,30 @@
+   LL|       |#![feature(coverage_attribute)]
+   LL|       |// edition: 2021
+   LL|       |
+   LL|       |// If the span extractor can't find any relevant spans for a function, the
+   LL|       |// refinement loop will terminate with nothing in its `prev` slot. If the
+   LL|       |// subsequent code tries to unwrap `prev`, it will panic.
+   LL|       |//
+   LL|       |// This scenario became more likely after #118525 started discarding spans that
+   LL|       |// can't be un-expanded back to within the function body.
+   LL|       |//
+   LL|       |// Regression test for "invalid attempt to unwrap a None some_prev", as seen
+   LL|       |// in issues such as #118643 and #118662.
+   LL|       |
+   LL|       |#[coverage(off)]
+   LL|       |fn main() {
+   LL|       |    affected_function()();
+   LL|       |}
+   LL|       |
+   LL|       |macro_rules! macro_that_defines_a_function {
+   LL|       |    (fn $name:ident () $body:tt) => {
+   LL|       |        fn $name () -> impl Fn() $body
+   LL|       |    }
+   LL|       |}
+   LL|       |
+   LL|       |macro_that_defines_a_function! {
+   LL|       |    fn affected_function() {
+   LL|      1|        || ()
+   LL|       |    }
+   LL|       |}
+

tests/coverage/no_spans.rs

@@ -0,0 +1,29 @@
+#![feature(coverage_attribute)]
+// edition: 2021
+
+// If the span extractor can't find any relevant spans for a function, the
+// refinement loop will terminate with nothing in its `prev` slot. If the
+// subsequent code tries to unwrap `prev`, it will panic.
+//
+// This scenario became more likely after #118525 started discarding spans that
+// can't be un-expanded back to within the function body.
+//
+// Regression test for "invalid attempt to unwrap a None some_prev", as seen
+// in issues such as #118643 and #118662.
+
+#[coverage(off)]
+fn main() {
+    affected_function()();
+}
+
+macro_rules! macro_that_defines_a_function {
+    (fn $name:ident () $body:tt) => {
+        fn $name () -> impl Fn() $body
+    }
+}
+
+macro_that_defines_a_function! {
+    fn affected_function() {
+        || ()
+    }
+}