Add delimeter matching to lexer

dev_tests/src/pretty.rs

@@ -1,13 +0,0 @@
-// diagnostics_render.rs
-
-use rottlib::diagnostics::{Diagnostic};
-use rottlib::lexer::TokenizedFile;
-
-pub fn render_diagnostic(
-    diag: &Diagnostic,
-    _file: &TokenizedFile,
-    file_name: Option<&str>,
-    colors: bool,
-) {
-    diag.render(_file, file_name.unwrap_or("<default>"));
-}

dev_tests/src/uc_lexer_verify.rs

@@ -19,8 +19,6 @@ use rottlib::diagnostics::Diagnostic as Diag;
 use rottlib::lexer::TokenizedFile;
 use rottlib::parser::Parser;
 
-mod pretty;
-
 // ---------- CONFIG ----------
 const FILE_LIMIT: usize = 10000; // cap on files scanned
 const DIAG_SHOW_FIRST: usize = 12; // show first N diagnostics
@@ -35,6 +33,43 @@ const ALSO_PRINT_DEBUG_AFTER_PRETTY: bool = true;
 // chardet = "0.2"
 // encoding_rs = "0.8"
 
+fn render_diagnostic(diag: &Diag, file: &TokenizedFile<'_>, file_name: &str) {
+    diag.render(file, file_name);
+}
+
+fn render_diagnostics_window(diags: &[Diag], tf: &TokenizedFile<'_>, file_name: &str) {
+    let total = diags.len();
+    let first_n = DIAG_SHOW_FIRST.min(total);
+    let last_n = DIAG_SHOW_LAST.min(total.saturating_sub(first_n));
+
+    if total > first_n + last_n {
+        for (k, d) in diags.iter().take(first_n).enumerate() {
+            render_diagnostic(d, tf, file_name);
+            if ALSO_PRINT_DEBUG_AFTER_PRETTY {
+                eprintln!("#{}: {:#?}", k + 1, d);
+            }
+        }
+
+        eprintln!("... {} diagnostics omitted ...", total - (first_n + last_n));
+
+        let start = total - last_n;
+        for (offset, d) in diags.iter().skip(start).enumerate() {
+            let idx_global = start + offset + 1;
+            render_diagnostic(d, tf, file_name);
+            if ALSO_PRINT_DEBUG_AFTER_PRETTY {
+                eprintln!("#{idx_global}: {d:#?}");
+            }
+        }
+    } else {
+        for (k, d) in diags.iter().enumerate() {
+            render_diagnostic(d, tf, file_name);
+            if ALSO_PRINT_DEBUG_AFTER_PRETTY {
+                eprintln!("#{}: {:#?}", k + 1, d);
+            }
+        }
+    }
+}
+
 // Linux-only accurate RSS in MB. Fallback uses sysinfo.
 fn rss_mb() -> u64 {
     #[cfg(target_os = "linux")]
@@ -230,11 +265,24 @@ fn main() {
     mark("after_tokenize", t0);
 
     // If tokenization error: wait, dump tokens for the first failing file, then exit.
+    // If tokenization error: wait, print lexer diagnostics for the first failing file, then exit.
     if let Some(idx) = tk_error_idx {
-        let (bad_path, _) = &tokenized[idx];
+        let (bad_path, bad_tf) = &tokenized[idx];
-        wait_before_errors("Tokenization error found. Press Enter to dump tokens...");
+        wait_before_errors("Tokenization issues detected. Press Enter to print diagnostics...");
-        eprintln!("--- Tokenization error in: {}", bad_path.display());
+
-        //bad_tf.dump_debug_layout(); // from DebugTools
+        let fname = bad_path.display().to_string();
+
+        eprintln!("--- Tokenization issues in first failing file ---");
+        eprintln!("File: {fname}");
+
+        let diags = bad_tf.diagnostics();
+
+        if diags.is_empty() {
+            eprintln!("(no diagnostics captured)");
+        } else {
+            render_diagnostics_window(diags, bad_tf, &fname);
+        }
+
         std::process::exit(1);
     }
 
@@ -298,38 +346,8 @@ fn main() {
         if diags.is_empty() && fatal.is_none() {
             eprintln!("(no diagnostics captured)");
         } else {
-            let use_colors = is_terminal::is_terminal(io::stderr());
             let fname = path.display().to_string();
-            let total = diags.len();
+            render_diagnostics_window(&diags, tf, &fname);
-            let first_n = DIAG_SHOW_FIRST.min(total);
-            let last_n = DIAG_SHOW_LAST.min(total.saturating_sub(first_n));
-
-            if total > first_n + last_n {
-                // first window
-                for (k, d) in diags.iter().take(first_n).enumerate() {
-                    let s = pretty::render_diagnostic(d, tf, Some(&fname), use_colors);
-                    if ALSO_PRINT_DEBUG_AFTER_PRETTY {
-                        eprintln!("#{}: {:#?}", k + 1, d);
-                    }
-                }
-                eprintln!("... {} diagnostics omitted ...", total - (first_n + last_n));
-                // last window
-                let start = total - last_n;
-                for (offset, d) in diags.iter().skip(start).enumerate() {
-                    let idx_global = start + offset + 1;
-                    let s = pretty::render_diagnostic(d, tf, Some(&fname), use_colors);
-                    if ALSO_PRINT_DEBUG_AFTER_PRETTY {
-                        eprintln!("#{idx_global}: {d:#?}");
-                    }
-                }
-            } else {
-                for (k, d) in diags.iter().enumerate() {
-                    let s = pretty::render_diagnostic(d, tf, Some(&fname), use_colors);
-                    if ALSO_PRINT_DEBUG_AFTER_PRETTY {
-                        eprintln!("#{}: {:#?}", k + 1, d);
-                    }
-                }
-            }
         }
         std::process::exit(1);
     }

dev_tests/src/verify_expr.rs

@@ -7,69 +7,63 @@
 )]
 
 use rottlib::arena::Arena;
+use rottlib::diagnostics::Diagnostic;
 use rottlib::lexer::TokenizedFile;
 use rottlib::parser::Parser;
 
-mod pretty;
+/// Lexer-focused fixtures.
-
-// a * * *
-
-/// Expressions to test.
 ///
-/// Add, remove, or edit entries here.
+/// Keep these small: the goal is to inspect lexer diagnostics and delimiter
-/// Using `(&str, &str)` gives each case a human-readable label.
+/// recovery behavior, not full parser behavior.
-/// Expressions to test.
-///
-/// Add, remove, or edit entries here.
-/// Using `(&str, &str)` gives each case a human-readable label.
 const TEST_CASES: &[(&str, &str)] = &[
-    // P0022: invalid return value start
+    // L0001: invalid or unknown token
     (
-        "files/P0022_01.uc",
+        "files/L0001_01.uc",
-        "return ] ;",
+        "`",
-    ),
-    (
-        "files/P0022_02.uc",
-        "return\n    ]\n;\n",
-    ),
-    (
-        "files/P0022_03.uc",
-        "return\n}\n",
     ),
 
-    // P0023: invalid break value start
+    // L0002: unexpected closing delimiter
     (
-        "files/P0023_01.uc",
+        "files/L0002_01.uc",
-        "break ] ;",
+        "]",
-    ),
-    (
-        "files/P0023_02.uc",
-        "break\n \n\n\n\n   ]\n;\n",
-    ),
-    (
-        "files/P0023_03.uc",
-        "break\n}\n",
     ),
 
-    // P0024: goto target is missing or not a label token
+    // L0003: unclosed delimiter before later closing delimiter
+    //
+    // The `}` can still recover by matching the earlier `{`.
     (
-        "files/P0024_01.uc",
+        "files/L0003_01.uc",
-        "goto;",
+        "{\n    foo(\n}\n",
     ),
+
+    // L0004: mismatched closing delimiter
     (
-        "files/P0024_02.uc",
+        "files/L0004_01.uc",
-        "goto\n    ;\n",
+        "(]",
     ),
+
+    // L0005: unclosed delimiter at end of file
     (
-        "files/P0024_03.uc",
+        "files/L0005_01.uc",
-        "goto\n    ]\n;\n",
+        "foo(",
     ),
+
+    // Mixed recovery case:
+    //
+    // `)` recovers by matching `(` after treating `[` as unclosed;
+    // the following `]` is then unexpected.
     (
-        "files/P0024_04.uc",
+        "files/L_mixed_01.uc",
-        "goto",
+        "([)]",
     ),
 ];
 
+/// If true, also run the parser after tokenization.
+///
+/// For lexer-focused fixtures this is usually noisy, so keep it off unless you
+/// want to inspect how parser recovery behaves after lexer diagnostics.
+const RUN_PARSER: bool = false;
+
 /// If true, print the parsed expression using Debug formatting.
 const PRINT_PARSED_EXPR: bool = false;
 
@@ -82,24 +76,46 @@ fn main() {
     let mut had_any_problem = false;
 
     for (idx, (label, source)) in TEST_CASES.iter().enumerate() {
+        println!("============================================================");
+        println!("Case {}: {}", idx + 1, label);
+        println!("------------------------------------------------------------");
+
         let tf = TokenizedFile::tokenize(source);
 
-        let mut parser = Parser::new(&tf, &arena);
+        let lexer_diagnostics = tf.diagnostics();
-        let expr = parser.parse_expression();
 
-        if PRINT_PARSED_EXPR {
+        if lexer_diagnostics.is_empty() {
-            println!("Parsed expression:");
+            println!("Lexer diagnostics: none");
-            println!("{expr:#?}");
-        }
-
-        if parser.diagnostics.is_empty() {
-            println!("Diagnostics: none");
         } else {
             had_any_problem = true;
+
             if ALWAYS_PRINT_DIAGNOSTICS {
-                let use_colors = false;
+                println!("Lexer diagnostics:");
-                for (k, diag) in parser.diagnostics.iter().enumerate() {
+                for diag in lexer_diagnostics {
-                    pretty::render_diagnostic(diag, &tf, Some(label), use_colors);
+                    render_diagnostic(diag, &tf, Some(label), false);
+                }
+            }
+        }
+
+        if RUN_PARSER {
+            let mut parser = Parser::new(&tf, &arena);
+            let expr = parser.parse_expression();
+
+            if PRINT_PARSED_EXPR {
+                println!("Parsed expression:");
+                println!("{expr:#?}");
+            }
+
+            if parser.diagnostics.is_empty() {
+                println!("Parser diagnostics: none");
+            } else {
+                had_any_problem = true;
+
+                if ALWAYS_PRINT_DIAGNOSTICS {
+                    println!("Parser diagnostics:");
+                    for diag in &parser.diagnostics {
+                        render_diagnostic(diag, &tf, Some(label), false);
+                    }
                 }
             }
         }
@@ -108,10 +124,20 @@ fn main() {
     }
 
     println!("============================================================");
+
     if had_any_problem {
-        println!("Done. At least one case had tokenization or parse diagnostics.");
+        println!("Done. At least one case had lexer or parser diagnostics.");
         std::process::exit(1);
     } else {
         println!("Done. All cases completed without diagnostics.");
     }
 }
+
+fn render_diagnostic(
+    diag: &Diagnostic,
+    file: &TokenizedFile<'_>,
+    file_name: Option<&str>,
+    _colors: bool,
+) {
+    diag.render(file, file_name.unwrap_or("<default>"));
+}

rottlib/src/lexer/mod.rs

@@ -30,6 +30,7 @@ use std::ops::Range;
 
 use logos::Logos;
 
+use crate::diagnostics::{Diagnostic, DiagnosticBuilder};
 use raw_lexer::RawToken;
 
 pub use raw_lexer::BraceKind;
@@ -143,6 +144,10 @@ pub struct TokenizedFile<'src> {
     /// Records only exists for multiline tokens and ranges can be empty for
     /// lines that only contain line break boundary.
     multi_line_map: HashMap<BufferIndex, Vec<VisibleByteRange>>,
+    /// Sparse map between matched opening and closing delimiters.
+    delimiter_matches: DelimiterMatches,
+    /// Diagnostics produced during tokenization and delimiter matching.
+    diagnostics: Vec<Diagnostic>,
     /// Simple flag for marking erroneous state.
     had_errors: bool,
 }
@@ -205,6 +210,12 @@ struct Tokenizer<'src> {
     /// that started on `line_number`; it is consumed exactly once by
     /// [`Self::commit_current_line`].
     multi_line_start_line: Option<LineNumber>,
+    /// Tracks delimiter pairs and delimiter diagnostics.
+    delimiter_matcher: DelimiterMatcher,
+    /// Tracks crude line indentation for delimiter recovery diagnostics.
+    indent_tracker: IndentTracker,
+    /// Diagnostics produced while tokenizing.
+    diagnostics: Vec<Diagnostic>,
     /// Set to `true` if the lexer reported any error tokens.
     had_errors: bool,
 }
@@ -226,7 +237,8 @@ impl<'src> TokenizedFile<'src> {
                 RawToken::Error
             });
             let token_piece = make_token_data(Token::from(token), lexer.slice());
-            tokenizer.process_token_piece(token_piece);
+            let position = tokenizer.process_token_piece(token_piece);
+            tokenizer.after_token(position);
         }
         tokenizer.into_tokenized_file()
     }
@@ -244,6 +256,18 @@ impl<'src> TokenizedFile<'src> {
     pub const fn iter(&self) -> Tokens<'_, 'src> {
         Tokens::new(self)
     }
+
+    /// Returns diagnostics produced during tokenization and delimiter matching.
+    #[must_use]
+    pub fn diagnostics(&self) -> &[Diagnostic] {
+        &self.diagnostics
+    }
+
+    /// Returns the matching delimiter token, if this token is a matched delimiter.
+    #[must_use]
+    pub fn matching_delimiter(&self, position: TokenPosition) -> Option<TokenPosition> {
+        self.delimiter_matches.mate_of(position)
+    }
 }
 
 impl Line {
@@ -298,17 +322,22 @@ impl<'src> Tokenizer<'src> {
             line_number: 0,
             uncommitted_start_index: 0,
             multi_line_start_line: None,
+            delimiter_matcher: DelimiterMatcher::default(),
+            indent_tracker: IndentTracker::new(),
+            diagnostics: Vec::new(),
             had_errors: false,
         }
     }
 
     /// Handles a token span and dispatches to the appropriate handler.
-    fn process_token_piece(&mut self, token_piece: TokenData<'src>) {
+    fn process_token_piece(&mut self, token_piece: TokenData<'src>) -> TokenPosition {
+        let position = TokenPosition(self.buffer.len());
         if token_piece.token.can_span_lines() {
             self.process_multi_line_token(token_piece);
         } else {
             self.process_single_line_token(token_piece);
         }
+        position
     }
 
     /// Handles simple tokens that *never* span multiple lines, allowing us to
@@ -387,6 +416,26 @@ impl<'src> Tokenizer<'src> {
         }
     }
 
+    /// Runs post-processing that needs the token to already have a stable
+    /// [`TokenPosition`].
+    fn after_token(&mut self, position: TokenPosition) {
+        let token_piece = self.buffer[position.0];
+
+        if token_piece.token == Token::Error {
+            self.had_errors = true;
+            self.diagnostics
+                .push(diagnostic_invalid_token(position, token_piece.lexeme));
+        }
+
+        self.delimiter_matcher.observe(
+            position,
+            token_piece.token,
+            self.indent_tracker.current_indent(),
+        );
+
+        self.indent_tracker.observe_token(token_piece);
+    }
+
     /// Finishes tokenization, converting accumulated data into
     /// [`TokenizedFile`].
     fn into_tokenized_file(mut self) -> TokenizedFile<'src> {
@@ -399,6 +448,12 @@ impl<'src> Tokenizer<'src> {
             self.lines.push(Line::continued(from));
         }
 
+        let (delimiter_matches, delimiter_diagnostics) = self.delimiter_matcher.finish();
+        if !delimiter_diagnostics.is_empty() {
+            self.had_errors = true;
+        }
+        self.diagnostics.extend(delimiter_diagnostics);
+
         self.buffer.shrink_to_fit();
         self.lines.shrink_to_fit();
 
@@ -406,6 +461,8 @@ impl<'src> Tokenizer<'src> {
             buffer: self.buffer,
             lines: self.lines,
             multi_line_map: self.multi_line_map,
+            delimiter_matches,
+            diagnostics: self.diagnostics,
             had_errors: self.had_errors,
         }
     }
@@ -507,3 +564,409 @@ impl<'file, 'src> IntoIterator for &'file TokenizedFile<'src> {
         self.iter()
     }
 }
+
+/// Sparse map between matched delimiter tokens.
+///
+/// Stores both directions:
+///
+/// - opening delimiter -> closing delimiter
+/// - closing delimiter -> opening delimiter
+#[derive(Clone, Debug, Default, PartialEq, Eq)]
+pub struct DelimiterMatches {
+    mates: HashMap<TokenPosition, TokenPosition>,
+}
+
+impl DelimiterMatches {
+    /// Returns the matching delimiter token for `position`, if known.
+    #[must_use]
+    pub fn mate_of(&self, position: TokenPosition) -> Option<TokenPosition> {
+        self.mates.get(&position).copied()
+    }
+
+    fn insert_pair(&mut self, left: TokenPosition, right: TokenPosition) {
+        self.mates.insert(left, right);
+        self.mates.insert(right, left);
+    }
+}
+
+#[derive(Clone, Copy, Debug, Hash, PartialEq, Eq)]
+enum DelimiterKind {
+    Parenthesis,
+    Bracket,
+    Brace,
+}
+
+#[derive(Clone, Copy, Debug, Hash, PartialEq, Eq)]
+enum DelimiterSide {
+    Open,
+    Close,
+}
+
+#[derive(Clone, Copy, Debug)]
+struct OpenDelimiter {
+    kind: DelimiterKind,
+    position: TokenPosition,
+    indent: usize,
+}
+
+#[derive(Debug, Default)]
+struct DelimiterMatcher {
+    stack: Vec<OpenDelimiter>,
+    matches: DelimiterMatches,
+    diagnostics: Vec<Diagnostic>,
+    stopped_at_defaultproperties: bool,
+}
+
+impl DelimiterMatcher {
+    fn handle_defaultproperties_boundary(&mut self, defaultproperties_position: TokenPosition) {
+        while let Some(open) = self.stack.pop() {
+            self.diagnostics
+                .push(diagnostic_unclosed_delimiter_before_defaultproperties(
+                    open,
+                    defaultproperties_position,
+                ));
+        }
+    }
+
+    fn observe(&mut self, position: TokenPosition, token: Token, indent: usize) {
+        if self.stopped_at_defaultproperties {
+            return;
+        }
+
+        if token == Token::Keyword(Keyword::DefaultProperties) {
+            self.handle_defaultproperties_boundary(position);
+            self.stopped_at_defaultproperties = true;
+            return;
+        }
+
+        let Some((side, kind)) = delimiter_of(token) else {
+            return;
+        };
+
+        match side {
+            DelimiterSide::Open => {
+                self.stack.push(OpenDelimiter {
+                    kind,
+                    position,
+                    indent,
+                });
+            }
+            DelimiterSide::Close => {
+                self.handle_close(kind, position, indent);
+            }
+        }
+    }
+
+    fn handle_close(
+        &mut self,
+        close_kind: DelimiterKind,
+        close_position: TokenPosition,
+        close_indent: usize,
+    ) {
+        let Some(top) = self.stack.last().copied() else {
+            self.diagnostics
+                .push(diagnostic_unexpected_closing_delimiter(
+                    close_kind,
+                    close_position,
+                ));
+            return;
+        };
+
+        if top.kind == close_kind {
+            let open = self.stack.pop().expect("stack top was just checked");
+            self.matches.insert_pair(open.position, close_position);
+            return;
+        }
+
+        let earlier_same_kind = self.stack.iter().rposition(|open| open.kind == close_kind);
+
+        match earlier_same_kind {
+            Some(index) => {
+                let bad_open = top;
+                let recovered_open = self.stack[index];
+
+                self.diagnostics
+                    .push(diagnostic_unclosed_delimiter_before_later_close(
+                        bad_open,
+                        close_kind,
+                        close_position,
+                        recovered_open,
+                        recovered_open.indent == close_indent,
+                    ));
+
+                while self.stack.len() - 1 > index {
+                    self.stack.pop();
+                }
+
+                let open = self
+                    .stack
+                    .pop()
+                    .expect("same-kind delimiter was found on the stack");
+
+                self.matches.insert_pair(open.position, close_position);
+            }
+            None => {
+                let bad_open = self.stack.pop().expect("stack top was known to exist");
+
+                self.diagnostics
+                    .push(diagnostic_mismatched_closing_delimiter(
+                        bad_open,
+                        close_kind,
+                        close_position,
+                    ));
+            }
+        }
+    }
+
+    fn finish(mut self) -> (DelimiterMatches, Vec<Diagnostic>) {
+        if !self.stopped_at_defaultproperties {
+            while let Some(open) = self.stack.pop() {
+                self.diagnostics
+                    .push(diagnostic_unclosed_delimiter_at_eof(open));
+            }
+        }
+
+        (self.matches, self.diagnostics)
+    }
+}
+
+fn delimiter_of(token: Token) -> Option<(DelimiterSide, DelimiterKind)> {
+    match token {
+        Token::LeftParenthesis => Some((DelimiterSide::Open, DelimiterKind::Parenthesis)),
+        Token::RightParenthesis => Some((DelimiterSide::Close, DelimiterKind::Parenthesis)),
+
+        Token::LeftBracket => Some((DelimiterSide::Open, DelimiterKind::Bracket)),
+        Token::RightBracket => Some((DelimiterSide::Close, DelimiterKind::Bracket)),
+
+        Token::LeftBrace => Some((DelimiterSide::Open, DelimiterKind::Brace)),
+        Token::RightBrace => Some((DelimiterSide::Close, DelimiterKind::Brace)),
+
+        _ => None,
+    }
+}
+
+fn open_delimiter_text(kind: DelimiterKind) -> &'static str {
+    match kind {
+        DelimiterKind::Parenthesis => "(",
+        DelimiterKind::Bracket => "[",
+        DelimiterKind::Brace => "{",
+    }
+}
+
+fn close_delimiter_text(kind: DelimiterKind) -> &'static str {
+    match kind {
+        DelimiterKind::Parenthesis => ")",
+        DelimiterKind::Bracket => "]",
+        DelimiterKind::Brace => "}",
+    }
+}
+
+/// Crude indentation tracker used only for delimiter recovery diagnostics.
+///
+/// This is intentionally simple. It assumes indentation is represented by a
+/// run of whitespace tokens before the first non-trivia token on a physical
+/// line. It does not try to normalize tabs or handle every multiline trivia
+/// edge case precisely, because indentation is only a diagnostic heuristic.
+#[derive(Clone, Copy, Debug)]
+struct IndentTracker {
+    current_indent: usize,
+    before_first_nontrivia_on_line: bool,
+}
+
+impl IndentTracker {
+    fn new() -> Self {
+        Self {
+            current_indent: 0,
+            before_first_nontrivia_on_line: true,
+        }
+    }
+
+    fn current_indent(&self) -> usize {
+        self.current_indent
+    }
+
+    fn observe_token(&mut self, token_piece: TokenData<'_>) {
+        if token_is_newline(token_piece.token) {
+            self.current_indent = 0;
+            self.before_first_nontrivia_on_line = true;
+            return;
+        }
+
+        if self.before_first_nontrivia_on_line && token_is_whitespace(token_piece.token) {
+            self.current_indent += token_piece.lexeme.chars().count();
+            return;
+        }
+
+        if token_is_trivia(token_piece.token) {
+            return;
+        }
+
+        self.before_first_nontrivia_on_line = false;
+    }
+}
+
+fn token_is_newline(token: Token) -> bool {
+    matches!(token, Token::Newline)
+}
+
+fn token_is_whitespace(token: Token) -> bool {
+    matches!(token, Token::Whitespace)
+}
+
+fn token_is_trivia(token: Token) -> bool {
+    matches!(
+        token,
+        Token::Whitespace | Token::Newline | Token::LineComment | Token::BlockComment
+    )
+}
+
+fn diagnostic_invalid_token(position: TokenPosition, lexeme: &str) -> Diagnostic {
+    let shown = display_lexeme(lexeme);
+
+    DiagnosticBuilder::error(format!("invalid token: {}", shown))
+        .code("L0001")
+        .primary_label(
+            TokenSpan::new(position),
+            format!("invalid token: {}", shown),
+        )
+        .build()
+}
+
+fn diagnostic_unexpected_closing_delimiter(
+    close_kind: DelimiterKind,
+    close_position: TokenPosition,
+) -> Diagnostic {
+    let close = close_delimiter_text(close_kind);
+
+    DiagnosticBuilder::error(format!("unexpected closing delimiter: `{}`", close))
+        .code("L0002")
+        .primary_label(
+            TokenSpan::new(close_position),
+            "unexpected closing delimiter",
+        )
+        .build()
+}
+
+fn diagnostic_unclosed_delimiter_before_later_close(
+    bad_open: OpenDelimiter,
+    close_kind: DelimiterKind,
+    close_position: TokenPosition,
+    recovered_open: OpenDelimiter,
+    same_indent: bool,
+) -> Diagnostic {
+    let bad_open_text = open_delimiter_text(bad_open.kind);
+    let close_text = close_delimiter_text(close_kind);
+    let recovered_open_text = open_delimiter_text(recovered_open.kind);
+
+    let mut builder =
+        DiagnosticBuilder::error(format!("unclosed delimiter before `{}`", close_text))
+            .code("L0003")
+            .primary_label(
+                TokenSpan::new(bad_open.position),
+                format!(
+                    "this `{}` is not closed before `{}`",
+                    bad_open_text, close_text
+                ),
+            )
+            .secondary_label(
+                TokenSpan::new(close_position),
+                format!(
+                    "this `{}` is matched with the earlier `{}`",
+                    close_text, recovered_open_text
+                ),
+            );
+
+    if same_indent {
+        builder = builder.secondary_label(
+            TokenSpan::new(recovered_open.position),
+            format!(
+                "this `{}` is likely the intended match",
+                recovered_open_text,
+            ),
+        );
+    }
+
+    builder.build()
+}
+
+fn diagnostic_mismatched_closing_delimiter(
+    bad_open: OpenDelimiter,
+    close_kind: DelimiterKind,
+    close_position: TokenPosition,
+) -> Diagnostic {
+    let open = open_delimiter_text(bad_open.kind);
+    let close = close_delimiter_text(close_kind);
+
+    DiagnosticBuilder::error(format!("mismatched closing delimiter: `{}`", close))
+        .code("L0004")
+        .primary_label(
+            TokenSpan::new(close_position),
+            format!("closing delimiter does not match `{}`", open),
+        )
+        .secondary_label(
+            TokenSpan::new(bad_open.position),
+            format!("`{}` opened here", open),
+        )
+        .build()
+}
+
+fn diagnostic_unclosed_delimiter_at_eof(open: OpenDelimiter) -> Diagnostic {
+    let open_text = open_delimiter_text(open.kind);
+
+    DiagnosticBuilder::error(format!("unclosed delimiter: `{}`", open_text))
+        .code("L0005")
+        .primary_label(
+            TokenSpan::new(open.position),
+            format!("this `{}` was never closed", open_text),
+        )
+        .build()
+}
+
+fn diagnostic_unclosed_delimiter_before_defaultproperties(
+    open: OpenDelimiter,
+    defaultproperties_position: TokenPosition,
+) -> Diagnostic {
+    let open_text = open_delimiter_text(open.kind);
+
+    DiagnosticBuilder::error("unclosed delimiter before `defaultproperties`")
+        .code("L0006")
+        .primary_label(
+            TokenSpan::new(open.position),
+            format!("this `{}` is not closed before `defaultproperties`", open_text),
+        )
+        .secondary_label(
+            TokenSpan::new(defaultproperties_position),
+            "delimiter matching stops at `defaultproperties`",
+        )
+        .build()
+}
+
+fn display_lexeme(lexeme: &str) -> String {
+    if lexeme.is_empty() {
+        return "<empty>".to_string();
+    }
+
+    if lexeme == "`" {
+        return "backtick".to_string();
+    }
+
+    let escaped: String = lexeme.chars().flat_map(char::escape_default).collect();
+
+    const MAX_DISPLAY_CHARS: usize = 32;
+
+    let mut display = String::new();
+    let mut chars = escaped.chars();
+
+    for _ in 0..MAX_DISPLAY_CHARS {
+        let Some(character) = chars.next() else {
+            return format!("`{}`", display);
+        };
+        display.push(character);
+    }
+
+    if chars.next().is_some() {
+        display.push_str("...");
+    }
+
+    format!("`{}`", display)
+}

rottlib/src/parser/cursor.rs

@@ -384,4 +384,31 @@ impl<'src, 'arena> Parser<'src, 'arena> {
             }
         }
     }
+
+    /// Returns the first significant token after `position`, without consuming it.
+    ///
+    /// This buffers through the cursor, so trivia is still recorded normally and
+    /// insignificant tokens are skipped consistently with the rest of the parser.
+    ///
+    /// Returns `None` if the stream ends before a later significant token is found.
+    #[must_use]
+    pub(crate) fn peek_token_after_position(&mut self, position: TokenPosition) -> Option<Token> {
+        let mut lookahead = 0usize;
+
+        loop {
+            self.cursor
+                .ensure_lookahead_available(lookahead, &mut self.trivia);
+
+            let Some((token_position, token_data)) = self.cursor.lookahead_buffer.get(lookahead)
+            else {
+                return None;
+            };
+
+            if *token_position > position {
+                return Some(token_data.token);
+            }
+
+            lookahead += 1;
+        }
+    }
 }

rottlib/src/parser/grammar/expression/control_flow/for_loop.rs

@@ -122,6 +122,8 @@ impl<'src, 'arena> Parser<'src, 'arena> {
                 )
                 // Header recovery must not consume the next `;`;
                 // it belongs to the surrounding `for` header.
+                // That's why `SyncLevel` match is preferable to syncing onto
+                // matching delimiter - latter wouldn't stop at preceding `;`.
                 .sync_error_until(self, SyncLevel::CloseParenthesis)
                 .unwrap_or_fallback(self),
             )
@@ -216,7 +218,7 @@ impl<'src, 'arena> Parser<'src, 'arena> {
             )
             .widen_error_span_from(for_keyword_position)
             .related_token("for_header_start", left_parenthesis_position)
-            .sync_error_at(self, SyncLevel::CloseParenthesis)
+            .sync_error_at_matching_delimiter(self, left_parenthesis_position)
             .ok_or_report(self);
     }
 

rottlib/src/parser/grammar/expression/control_flow/mod.rs

@@ -97,27 +97,13 @@ impl<'src, 'arena> Parser<'src, 'arena> {
         else {
             return None;
         };
-        let mut nesting_depth: usize = 1;
+
-        let mut lookahead_token_offset: usize = 1;
+        let right_parenthesis_position =
-        while let Some(lookahead_token) = self.peek_token_at(lookahead_token_offset) {
+            self.file().matching_delimiter(left_parenthesis_position)?;
-            match lookahead_token {
+
-                Token::LeftParenthesis => nesting_depth += 1,
+        self.peek_token_after_position(right_parenthesis_position)
-                Token::RightParenthesis => {
+            .is_some_and(|token| token.is_valid_identifier_name())
-                    if nesting_depth == 1 {
+            .then_some(left_parenthesis_position)
-                        return self
-                            .peek_token_at(lookahead_token_offset + 1)
-                            .is_some_and(|token| token.is_valid_identifier_name())
-                            .then_some(left_parenthesis_position);
-                    }
-                    nesting_depth -= 1;
-                }
-                _ => (),
-            }
-            lookahead_token_offset += 1;
-        }
-        // Recovery is left to normal expression parsing when the closing `)`
-        // is missing.
-        None
     }
 
     /// Parses a control-flow condition.

rottlib/src/parser/grammar/expression/primary/mod.rs

@@ -196,7 +196,7 @@ impl<'src, 'arena> Parser<'src, 'arena> {
                 .widen_error_span_from(left_parenthesis_position)
                 .extend_blame_to_next_token(self)
                 .related_token("left_parenthesis", left_parenthesis_position)
-                .sync_error_until(self, SyncLevel::CloseParenthesis)
+                .sync_error_at_matching_delimiter(self, left_parenthesis_position)
                 .fallback(self);
         };
         let inner_expression = self.parse_expression();
@@ -206,7 +206,7 @@ impl<'src, 'arena> Parser<'src, 'arena> {
                 ParseErrorKind::ParenthesizedExpressionMissingClosingParenthesis,
             )
             .widen_error_span_from(left_parenthesis_position)
-            .sync_error_at(self, SyncLevel::CloseParenthesis)
+            .sync_error_at_matching_delimiter(self, left_parenthesis_position)
             .extend_blame_start_to_covered_start()
             .related_token("left_parenthesis", left_parenthesis_position)
             .unwrap_or_fallback(self);

rottlib/src/parser/grammar/expression/primary/new.rs

@@ -277,7 +277,7 @@ impl<'src, 'arena> Parser<'src, 'arena> {
                 error = error.related("first_extra_argument", span);
             }
             error
-                .sync_error_until(self, SyncLevel::CloseParenthesis)
+                .sync_error_until_matching_delimiter(self, state.left_parenthesis_position)
                 .report_error(self);
         }
     }
@@ -296,7 +296,7 @@ impl<'src, 'arena> Parser<'src, 'arena> {
             .related_token("new_keyword", state.new_keyword_position)
             .related_token("left_parenthesis", state.left_parenthesis_position);
         let class_specifier_parse_action = if self.next_token_definitely_cannot_start_expression() {
-            error = error.sync_error_at(self, SyncLevel::CloseParenthesis);
+            error = error.sync_error_at_matching_delimiter(self, state.left_parenthesis_position);
             // Skipping the class specifier avoids cascading errors when
             // the next token cannot start an expression anyway.
             NewClassSpecifierParseAction::Skip

rottlib/src/parser/recovery.rs

@@ -138,6 +138,22 @@ impl SyncLevel {
             }
         }
     }
+
+    fn fallback_sync_level_for_delimiter_start(token: Option<Token>) -> SyncLevel {
+        match token {
+            Some(Token::LeftParenthesis) => SyncLevel::CloseParenthesis,
+            Some(Token::LeftBracket) => SyncLevel::CloseBracket,
+            Some(Token::LeftBrace) => SyncLevel::BlockBoundary,
+            _ => SyncLevel::CloseParenthesis,
+        }
+    }
+
+    fn is_opening_delimiter(token: Token) -> bool {
+        matches!(
+            token,
+            Token::LeftParenthesis | Token::LeftBracket | Token::LeftBrace
+        )
+    }
 }
 
 impl<'src, 'arena> Parser<'src, 'arena> {
@@ -171,6 +187,105 @@ impl<'src, 'arena> Parser<'src, 'arena> {
         }
     }
 
+    /// Skips tokens until a token with exactly `level` is found, then consumes
+    /// that token.
+    ///
+    /// This mirrors the behavior used by [`ResultRecoveryExt::sync_error_at`]:
+    /// stronger sync tokens can stop [`Self::recover_until`], but they are not
+    /// consumed unless they are exactly the requested level.
+    fn recover_at_sync_level(&mut self, level: SyncLevel) {
+        self.recover_until(level);
+
+        if self
+            .peek_token()
+            .and_then(SyncLevel::for_token)
+            .is_some_and(|next_level| next_level == level)
+        {
+            self.advance();
+        }
+    }
+
+    /// Recovers up to the lexer-produced matching delimiter for
+    /// `delimiter_start`, if possible, but does not consume it.
+    ///
+    /// If `delimiter_start` is not an opening delimiter, if no match is known,
+    /// or if the parser has already moved past the matching delimiter, this
+    /// falls back to ordinary sync-level recovery inferred from
+    /// `delimiter_start`.
+    pub(crate) fn recover_until_matching_delimiter_or_sync(
+        &mut self,
+        delimiter_start: TokenPosition,
+    ) {
+        let start_token = self.file().token_at(delimiter_start).map(|data| data.token);
+        let fallback_level = SyncLevel::fallback_sync_level_for_delimiter_start(start_token);
+
+        let Some(start_token) = start_token else {
+            self.recover_until(fallback_level);
+            return;
+        };
+
+        if !SyncLevel::is_opening_delimiter(start_token) {
+            self.recover_until(fallback_level);
+            return;
+        }
+
+        let Some(target) = self.file().matching_delimiter(delimiter_start) else {
+            self.recover_until(fallback_level);
+            return;
+        };
+
+        if self.peek_position_or_eof() > target {
+            self.recover_until(fallback_level);
+            return;
+        }
+
+        while self.peek_position_or_eof() < target {
+            self.advance();
+        }
+    }
+
+    /// Recovers by using the lexer-produced matching delimiter for
+    /// `delimiter_start`, if possible.
+    ///
+    /// If `delimiter_start` is not an opening delimiter, if no match is known,
+    /// or if the parser has already moved past the matching delimiter, this
+    /// falls back to ordinary sync-level recovery inferred from
+    /// `delimiter_start`.
+    pub(crate) fn recover_at_matching_delimiter_or_sync(&mut self, delimiter_start: TokenPosition) {
+        let start_token = self.file().token_at(delimiter_start).map(|data| data.token);
+        let fallback_level = SyncLevel::fallback_sync_level_for_delimiter_start(start_token);
+
+        let Some(start_token) = start_token else {
+            self.recover_at_sync_level(fallback_level);
+            return;
+        };
+
+        if !SyncLevel::is_opening_delimiter(start_token) {
+            self.recover_at_sync_level(fallback_level);
+            return;
+        }
+
+        let Some(target) = self.file().matching_delimiter(delimiter_start) else {
+            self.recover_at_sync_level(fallback_level);
+            return;
+        };
+
+        if self.peek_position_or_eof() > target {
+            self.recover_at_sync_level(fallback_level);
+            return;
+        }
+
+        while self.peek_position_or_eof() < target {
+            self.advance();
+        }
+
+        if self.peek_position_or_eof() == target {
+            self.advance();
+        } else {
+            self.recover_at_sync_level(fallback_level);
+        }
+    }
+
     /// Reports `error` and returns the recovery fallback for `T`.
     ///
     /// This is the primitive used when parsing must keep going with a
@@ -234,6 +349,39 @@ pub trait ResultRecoveryExt<'src, 'arena, T>: Sized {
     #[must_use]
     fn sync_error_at(self, parser: &mut Parser<'src, 'arena>, level: SyncLevel) -> Self;
 
+    /// Extends the right end of the error span up to, but not including, the
+    /// known matching closing delimiter for `delimiter_start`, if that delimiter
+    /// is known and has not already been passed.
+    ///
+    /// If no usable delimiter match exists, falls back to ordinary
+    /// [`SyncLevel`]-based recovery. The fallback level is inferred from the
+    /// token at `delimiter_start`.
+    #[must_use]
+    fn sync_error_until_matching_delimiter(
+        self,
+        parser: &mut Parser<'src, 'arena>,
+        delimiter_start: TokenPosition,
+    ) -> Self;
+
+    /// Extends the right end of the error span to include the known matching
+    /// closing delimiter for `delimiter_start`, if that delimiter is known and
+    /// has not already been passed.
+    ///
+    /// If no usable delimiter match exists, falls back to ordinary
+    /// [`SyncLevel`]-based recovery. The fallback level is inferred from the
+    /// token at `delimiter_start`:
+    ///
+    /// - `(` -> [`SyncLevel::CloseParenthesis`]
+    /// - `[` -> [`SyncLevel::CloseBracket`]
+    /// - `{` -> [`SyncLevel::BlockBoundary`]
+    /// - anything else -> [`SyncLevel::CloseParenthesis`]
+    #[must_use]
+    fn sync_error_at_matching_delimiter(
+        self,
+        parser: &mut Parser<'src, 'arena>,
+        delimiter_start: TokenPosition,
+    ) -> Self;
+
     /// Either returns expected value or its best effort fallback.
     #[must_use]
     fn unwrap_or_fallback(self, parser: &mut Parser<'src, 'arena>) -> T
@@ -305,6 +453,36 @@ impl<'src, 'arena, T> ResultRecoveryExt<'src, 'arena, T> for ParseResult<'src, '
         self
     }
 
+    fn sync_error_until_matching_delimiter(
+        mut self,
+        parser: &mut Parser<'src, 'arena>,
+        delimiter_start: TokenPosition,
+    ) -> Self {
+        if let Err(ref mut error) = self {
+            parser.recover_until_matching_delimiter_or_sync(delimiter_start);
+            error.covered_span.end = std::cmp::max(
+                error.covered_span.end,
+                parser.last_consumed_position_or_start(),
+            );
+        }
+        self
+    }
+
+    fn sync_error_at_matching_delimiter(
+        mut self,
+        parser: &mut Parser<'src, 'arena>,
+        delimiter_start: TokenPosition,
+    ) -> Self {
+        if let Err(ref mut error) = self {
+            parser.recover_at_matching_delimiter_or_sync(delimiter_start);
+            error.covered_span.end = std::cmp::max(
+                error.covered_span.end,
+                parser.last_consumed_position_or_start(),
+            );
+        }
+        self
+    }
+
     fn unwrap_or_fallback(self, parser: &mut Parser<'src, 'arena>) -> T
     where
         T: RecoveryFallback<'src, 'arena>,
@@ -390,6 +568,32 @@ impl<'src, 'arena> ResultRecoveryExt<'src, 'arena, ()> for ParseError {
         self
     }
 
+    fn sync_error_until_matching_delimiter(
+        mut self,
+        parser: &mut Parser<'src, 'arena>,
+        delimiter_start: TokenPosition,
+    ) -> Self {
+        parser.recover_until_matching_delimiter_or_sync(delimiter_start);
+        self.covered_span.end = std::cmp::max(
+            self.covered_span.end,
+            parser.last_consumed_position_or_start(),
+        );
+        self
+    }
+
+    fn sync_error_at_matching_delimiter(
+        mut self,
+        parser: &mut Parser<'src, 'arena>,
+        delimiter_start: TokenPosition,
+    ) -> Self {
+        parser.recover_at_matching_delimiter_or_sync(delimiter_start);
+        self.covered_span.end = std::cmp::max(
+            self.covered_span.end,
+            parser.last_consumed_position_or_start(),
+        );
+        self
+    }
+
     fn unwrap_or_fallback(self, parser: &mut Parser<'src, 'arena>) {
         parser.report_error(self);
     }

rottlib/tests/lexer_diagnostics.rs

@@ -0,0 +1,350 @@
+use std::collections::HashMap;
+
+use rottlib::diagnostics::{Diagnostic, Severity};
+use rottlib::lexer::{TokenPosition, TokenSpan, TokenizedFile};
+
+#[derive(Debug)]
+struct ExpectedLabel {
+    span: TokenSpan,
+    message: &'static str,
+}
+
+#[derive(Debug)]
+struct ExpectedDiagnostic<'a> {
+    headline: &'static str,
+    severity: Severity,
+    code: Option<&'static str>,
+    primary_label: Option<ExpectedLabel>,
+    secondary_labels: &'a [ExpectedLabel],
+    help: Option<&'static str>,
+    notes: &'a [&'static str],
+}
+
+#[track_caller]
+fn assert_diagnostic(actual: &Diagnostic, expected: &ExpectedDiagnostic<'_>) {
+    assert_eq!(actual.headline(), expected.headline);
+    assert_eq!(actual.severity(), expected.severity);
+    assert_eq!(actual.code(), expected.code);
+    assert_eq!(actual.help(), expected.help);
+
+    match (actual.primary_label(), expected.primary_label.as_ref()) {
+        (None, None) => {}
+        (Some(actual), Some(expected)) => {
+            assert_eq!(actual.span, expected.span);
+            assert_eq!(actual.message, expected.message);
+        }
+        _ => panic!("primary label mismatch"),
+    }
+
+    let actual_secondary = actual.secondary_labels();
+    assert_eq!(actual_secondary.len(), expected.secondary_labels.len());
+
+    for (actual, expected) in actual_secondary
+        .iter()
+        .zip(expected.secondary_labels.iter())
+    {
+        assert_eq!(actual.span, expected.span);
+        assert_eq!(actual.message, expected.message);
+    }
+
+    let actual_notes = actual.notes();
+    assert_eq!(actual_notes.len(), expected.notes.len());
+
+    for (actual, expected) in actual_notes.iter().zip(expected.notes.iter()) {
+        assert_eq!(actual, expected);
+    }
+}
+
+#[derive(Debug, Clone, Copy)]
+struct Fixture {
+    label: &'static str,
+    source: &'static str,
+}
+
+type FixtureRun = Vec<Diagnostic>;
+
+struct FixtureRuns {
+    runs: HashMap<&'static str, FixtureRun>,
+}
+
+impl FixtureRuns {
+    #[track_caller]
+    fn get(&self, label: &str) -> Option<Vec<Diagnostic>> {
+        self.runs.get(label).cloned()
+    }
+
+    #[track_caller]
+    fn get_any(&self, label: &str) -> Diagnostic {
+        self.runs
+            .get(label)
+            .map(|fixture_run| fixture_run[0].clone())
+            .unwrap_or_else(|| panic!("no fixture run for `{label}`"))
+    }
+
+    #[track_caller]
+    fn get_by_code(&self, label: &str, code: &str) -> Diagnostic {
+        self.runs
+            .get(label)
+            .unwrap_or_else(|| panic!("no fixture run for `{label}`"))
+            .iter()
+            .find(|diagnostic| diagnostic.code() == Some(code))
+            .unwrap_or_else(|| panic!("no `{code}` diagnostic in fixture `{label}`"))
+            .clone()
+    }
+}
+
+const fn span(position: usize) -> TokenSpan {
+    TokenSpan {
+        start: TokenPosition(position),
+        end: TokenPosition(position),
+    }
+}
+
+const LEXER_FIXTURES: &[Fixture] = &[
+    Fixture {
+        label: "files/L0001_01.uc",
+        source: "`",
+    },
+    Fixture {
+        label: "files/L0002_01.uc",
+        source: "]",
+    },
+    Fixture {
+        label: "files/L0003_01.uc",
+        source: "{\n    foo(\n}\n",
+    },
+    Fixture {
+        label: "files/L0004_01.uc",
+        source: "(]",
+    },
+    Fixture {
+        label: "files/L0005_01.uc",
+        source: "foo(",
+    },
+    Fixture {
+        label: "files/L_mixed_01.uc",
+        source: "([)]",
+    },
+];
+
+fn run_fixture(fixture: &'static Fixture) -> FixtureRun {
+    let file = TokenizedFile::tokenize(fixture.source);
+    file.diagnostics().to_vec()
+}
+
+fn run_fixtures(fixtures: &'static [Fixture]) -> FixtureRuns {
+    let mut runs = HashMap::new();
+
+    for fixture in fixtures {
+        runs.insert(fixture.label, run_fixture(fixture));
+    }
+
+    FixtureRuns { runs }
+}
+
+#[test]
+fn check_lexer_diagnostic_counts() {
+    let runs = run_fixtures(LEXER_FIXTURES);
+
+    assert_eq!(runs.get("files/L0001_01.uc").unwrap().len(), 1);
+    assert_eq!(runs.get("files/L0002_01.uc").unwrap().len(), 1);
+    assert_eq!(runs.get("files/L0003_01.uc").unwrap().len(), 1);
+    assert_eq!(runs.get("files/L0004_01.uc").unwrap().len(), 1);
+    assert_eq!(runs.get("files/L0005_01.uc").unwrap().len(), 1);
+    assert_eq!(runs.get("files/L_mixed_01.uc").unwrap().len(), 2);
+}
+
+#[test]
+fn check_l0001_invalid_token() {
+    let runs = run_fixtures(LEXER_FIXTURES);
+
+    assert_diagnostic(
+        &runs.get_any("files/L0001_01.uc"),
+        &ExpectedDiagnostic {
+            headline: "invalid token: backtick",
+            severity: Severity::Error,
+            code: Some("L0001"),
+            primary_label: Some(ExpectedLabel {
+                span: span(0),
+                message: "invalid token: backtick",
+            }),
+            secondary_labels: &[],
+            help: None,
+            notes: &[],
+        },
+    );
+}
+
+#[test]
+fn check_l0002_unexpected_closing_delimiter() {
+    let runs = run_fixtures(LEXER_FIXTURES);
+
+    assert_diagnostic(
+        &runs.get_any("files/L0002_01.uc"),
+        &ExpectedDiagnostic {
+            headline: "unexpected closing delimiter: `]`",
+            severity: Severity::Error,
+            code: Some("L0002"),
+            primary_label: Some(ExpectedLabel {
+                span: span(0),
+                message: "unexpected closing delimiter",
+            }),
+            secondary_labels: &[],
+            help: None,
+            notes: &[],
+        },
+    );
+}
+
+#[test]
+fn check_l0003_unclosed_delimiter_before_later_close() {
+    let runs = run_fixtures(LEXER_FIXTURES);
+
+    assert_diagnostic(
+        &runs.get_any("files/L0003_01.uc"),
+        &ExpectedDiagnostic {
+            headline: "unclosed delimiter before `}`",
+            severity: Severity::Error,
+            code: Some("L0003"),
+            primary_label: Some(ExpectedLabel {
+                span: span(4),
+                message: "this `(` is not closed before `}`",
+            }),
+            secondary_labels: &[
+                ExpectedLabel {
+                    span: span(6),
+                    message: "this `}` is matched with the earlier `{`",
+                },
+                ExpectedLabel {
+                    span: span(0),
+                    message: "this `{` is likely the intended match",
+                },
+            ],
+            help: None,
+            notes: &[],
+        },
+    );
+}
+
+#[test]
+fn check_l0004_mismatched_closing_delimiter() {
+    let runs = run_fixtures(LEXER_FIXTURES);
+
+    assert_diagnostic(
+        &runs.get_any("files/L0004_01.uc"),
+        &ExpectedDiagnostic {
+            headline: "mismatched closing delimiter: `]`",
+            severity: Severity::Error,
+            code: Some("L0004"),
+            primary_label: Some(ExpectedLabel {
+                span: span(1),
+                message: "closing delimiter does not match `(`",
+            }),
+            secondary_labels: &[ExpectedLabel {
+                span: span(0),
+                message: "`(` opened here",
+            }],
+            help: None,
+            notes: &[],
+        },
+    );
+}
+
+#[test]
+fn check_l0005_unclosed_delimiter_at_eof() {
+    let runs = run_fixtures(LEXER_FIXTURES);
+
+    assert_diagnostic(
+        &runs.get_any("files/L0005_01.uc"),
+        &ExpectedDiagnostic {
+            headline: "unclosed delimiter: `(`",
+            severity: Severity::Error,
+            code: Some("L0005"),
+            primary_label: Some(ExpectedLabel {
+                span: span(1),
+                message: "this `(` was never closed",
+            }),
+            secondary_labels: &[],
+            help: None,
+            notes: &[],
+        },
+    );
+}
+
+#[test]
+fn check_mixed_recovery_diagnostics() {
+    let runs = run_fixtures(LEXER_FIXTURES);
+
+    assert_diagnostic(
+        &runs.get_by_code("files/L_mixed_01.uc", "L0003"),
+        &ExpectedDiagnostic {
+            headline: "unclosed delimiter before `)`",
+            severity: Severity::Error,
+            code: Some("L0003"),
+            primary_label: Some(ExpectedLabel {
+                span: span(1),
+                message: "this `[` is not closed before `)`",
+            }),
+            secondary_labels: &[
+                ExpectedLabel {
+                    span: span(2),
+                    message: "this `)` is matched with the earlier `(`",
+                },
+                ExpectedLabel {
+                    span: span(0),
+                    message: "this `(` is likely the intended match",
+                },
+            ],
+            help: None,
+            notes: &[],
+        },
+    );
+
+    assert_diagnostic(
+        &runs.get_by_code("files/L_mixed_01.uc", "L0002"),
+        &ExpectedDiagnostic {
+            headline: "unexpected closing delimiter: `]`",
+            severity: Severity::Error,
+            code: Some("L0002"),
+            primary_label: Some(ExpectedLabel {
+                span: span(3),
+                message: "unexpected closing delimiter",
+            }),
+            secondary_labels: &[],
+            help: None,
+            notes: &[],
+        },
+    );
+}
+
+#[test]
+fn check_recovered_delimiter_matches_are_stored() {
+    let file = TokenizedFile::tokenize("{\n    foo(\n}\n");
+
+    assert_eq!(
+        file.matching_delimiter(TokenPosition(0)),
+        Some(TokenPosition(6))
+    );
+    assert_eq!(
+        file.matching_delimiter(TokenPosition(6)),
+        Some(TokenPosition(0))
+    );
+    assert_eq!(file.matching_delimiter(TokenPosition(4)), None);
+}
+
+#[test]
+fn check_mixed_recovery_delimiter_matches_are_stored() {
+    let file = TokenizedFile::tokenize("([)]");
+
+    assert_eq!(
+        file.matching_delimiter(TokenPosition(0)),
+        Some(TokenPosition(2))
+    );
+    assert_eq!(
+        file.matching_delimiter(TokenPosition(2)),
+        Some(TokenPosition(0))
+    );
+
+    assert_eq!(file.matching_delimiter(TokenPosition(1)), None);
+    assert_eq!(file.matching_delimiter(TokenPosition(3)), None);
+}