feat: hint at unterminated strings in unknown prefix errors

When encountering 'unknown literal prefix' errors, check for unbalanced
quotes in recent code and suggest checking for unterminated string literals.

Author: avrabe

.claude/settings.local.json

@@ -0,0 +1,8 @@
+{
+  "permissions": {
+    "allow": [
+      "mcp__sequential-thinking__sequentialthinking"
+    ],
+    "deny": []
+  }
+}

crates/parser/src/lexed_str.rs

@@ -149,6 +149,13 @@ impl<'a> Converter<'a> {
         }
     }
 
+    /// Simple check for recent unterminated string literal
+    fn has_recent_unterminated_string(&self) -> bool {
+        let start = self.offset.saturating_sub(500);
+        let recent_text = &self.res.text[start..self.offset];
+        recent_text.chars().filter(|&c| c == '"').count() % 2 == 1
+    }
+
     fn finalize_with_eof(mut self) -> LexedStr<'a> {
         self.res.push(EOF, self.offset);
         self.res
@@ -267,7 +274,15 @@ impl<'a> Converter<'a> {
                 rustc_lexer::TokenKind::Unknown => ERROR,
                 rustc_lexer::TokenKind::UnknownPrefix if token_text == "builtin" => IDENT,
                 rustc_lexer::TokenKind::UnknownPrefix => {
-                    errors.push("unknown literal prefix".into());
+                    let error_msg = if self.has_recent_unterminated_string() {
+                        format!(
+                            "unknown literal prefix `{}` (note: check for unterminated string literal)",
+                            token_text
+                        )
+                    } else {
+                        "unknown literal prefix".into()
+                    };
+                    errors.push(error_msg);
                     IDENT
                 }
                 rustc_lexer::TokenKind::Eof => EOF,