Map: core / syntax

core/src/syntax/ turns source text into the surface AST — the only place that sees raw bytes and bare words.

• lexer.rs — lex(source) -> Result<Vec<(Token, Span)>, LexError>; the Token enum.
• parser.rs — parse(source) -> Result<Vec<Stmt>, ParseError>; parse_with carries a FileId. A digit glued to a comparison operator inside $[…] ($[2>3], lexed as the file-descriptor redirect 2>) earns a diagnostic that names the shape and asks for spaces, not a bare “redirect” token. << is the here-string redirect; the bash spellings (<<<, a glued heredoc <<EOF) earn targeted diagnostics naming ral’s form.
• ast.rs — the surface AST. Ast is the expression node and Stmt the statement node; the enum is deliberately wide and flat (ast-stays-flat).
• group.rs — pre-pass detecting mutually recursive binding groups, consumed by the elaborator.
• quote.rs — bare-word classification (is_bare_word, quote_word, quote_word_if_needed), shared with the REPL.
• tag.rs, free_refs.rs — variant-label tagging and free-reference scans.

Recursion is bounded by a single shared depth cap, so adversarial nesting rejects cleanly instead of overflowing the stack. One NESTING_DEPTH_LIMIT governs both stages.

• The lexer enforces it at scan_token_group, the sole recursion through bracketed token groups.
• The parser’s three mutually-recursive sub-grammars each descend through exactly one guarded chokepoint, all counting against Parser::nested’s shared depth: values through parse_primary, arithmetic through parse_expr_atom (the unary prefixes - / not and parenthesised sub-expressions bottom out here), patterns through parse_pattern (list and map patterns recurse back through it per element).

A command’s pipe modes are the modal projection of its declared type, read once: a builtin’s boundary PipeSpec is sig_pipe_spec (the streaming reducer reducer_spec) in typecheck::builtins, and the static checker walks a prelude function’s body for its modes. With the runtime engine retired (unconditional-mode-pass), there is no shell-free fallback and no hand-maintained byte-mode residue: the baked prelude’s schemes and the IR’s ground wires carry the real inferred modes for every prelude export.

The five reserved control operators are recognised at this layer; everything else is a library binding. See docs/SPEC.md §2–§4 for the grammar.