fix(frontmatter): Try alternative len code fences

Of the non-ideal error cases mentioned in #15939, this is likely the one
people will hit the most and so important for us to improve.

src/cargo/util/frontmatter.rs

@@ -95,6 +95,19 @@ impl<'s> ScriptSource<'s> {
         // Ends with a line that starts with a matching number of `-` only followed by whitespace
         let nl_fence_pattern = format!("\n{fence_pattern}");
         let Some(frontmatter_nl) = input.find_slice(nl_fence_pattern.as_str()) else {
+            for len in (2..(nl_fence_pattern.len() - 1)).rev() {
+                let Some(frontmatter_nl) = input.find_slice(&nl_fence_pattern[0..len]) else {
+                    continue;
+                };
+                let _ = input.next_slice(frontmatter_nl.start + 1);
+                let close_start = input.current_token_start();
+                let _ = input.next_slice(len);
+                let close_end = input.current_token_start();
+                return Err(FrontmatterError::new(
+                    format!("closing code fence has too few `-`"),
+                    close_start..close_end,
+                ));
+            }
             return Err(FrontmatterError::new(
                 format!("no closing `{fence_pattern}` found for frontmatter"),
                 open_start..open_end,

tests/testsuite/script/rustc_fixtures/mismatch-2.stderr

@@ -1,5 +1,5 @@
-[ERROR] no closing `----` found for frontmatter
+[ERROR] closing code fence has too few `-`
- --> script:1:1
+ --> script:3:1
   |
-1 | ----cargo
+3 | ---cargo
-  | ^^^^
+  | ^^^