Rollup merge of #127607 - Zalathar:normalize-hint, r=wesleywiser

compiletest: Better error message for bad `normalize-*` headers

Follow-up to #126777.

Example of the new error message in context:
```text
---- [ui] tests/ui/rfcs/rfc-2632-const-trait-impl/effects/minicore.rs stdout ----
thread '[ui] tests/ui/rfcs/rfc-2632-const-trait-impl/effects/minicore.rs' panicked at src/tools/compiletest/src/header.rs:1001:13:
couldn't parse custom normalization rule: `normalize-stderr-test ".*note: .*\n\n" -> ""`
help: expected syntax is: `normalize-stderr-test: "REGEX" -> "REPLACEMENT"`
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```

src/tools/compiletest/src/header.rs

@@ -991,13 +991,19 @@ impl Config {
     }
 
     fn parse_custom_normalization(&self, line: &str, prefix: &str) -> Option<(String, String)> {
-        if parse_cfg_name_directive(self, line, prefix).outcome == MatchOutcome::Match {
-            let (regex, replacement) = parse_normalize_rule(line)
-                .unwrap_or_else(|| panic!("couldn't parse custom normalization rule: `{line}`"));
-            Some((regex, replacement))
-        } else {
-            None
+        let parsed = parse_cfg_name_directive(self, line, prefix);
+        if parsed.outcome != MatchOutcome::Match {
+            return None;
         }
+        let name = parsed.name.expect("successful match always has a name");
+
+        let Some((regex, replacement)) = parse_normalize_rule(line) else {
+            panic!(
+                "couldn't parse custom normalization rule: `{line}`\n\
+                help: expected syntax is: `{prefix}-{name}: \"REGEX\" -> \"REPLACEMENT\"`"
+            );
+        };
+        Some((regex, replacement))
     }
 
     fn parse_name_directive(&self, line: &str, directive: &str) -> bool {