perf(parse): cut allocations and validate UTF-8 in the scan

parser.go

@@ -18,10 +18,10 @@ const ctxCheckInterval = 64
 
 // parser is a recursive-descent TOML parser producing a map[string]any tree.
 //
-// The scanner works on bytes, not runes: the input is validated UTF-8 before
-// the parser runs, every character that drives the grammar (quotes,
-// separators, newlines, bare-key characters) is ASCII, and multi-byte runes
-// matter only as string content, where they are decoded on the spot. Holding
+// The scanner works on bytes, not runes: every character that drives the
+// grammar (quotes, separators, newlines, bare-key characters) is ASCII, the
+// scan validates a multi-byte sequence where it meets one, and multi-byte
+// runes matter only as content, where they are decoded on the spot. Holding
 // the source as []rune instead would cost a conversion pass plus four bytes
 // per rune of extra memory before parsing even starts.
 type parser struct {
@@ -45,6 +45,21 @@ type parser struct {
 
 	currentPath []string
 
+	// keys interns key strings: a document that repeats a key across
+	// array-of-tables elements stores one string per distinct key instead of
+	// one per occurrence. The table is parser-local and dies with the parse;
+	// the tree keeps sharing the strings it was handed.
+	keys map[string]string
+
+	// keyBuf backs the transient single-segment result of parseKeyPath. A
+	// caller that keeps the path copies it out first, which is what
+	// retainPath does for the current section.
+	keyBuf [1]string
+
+	// absScratch backs the absolute path of a top-level key, which lives only
+	// for the statement being parsed.
+	absScratch [1]string
+
 	// wantDoc asks for the node tree the Document is built from; doc is that
 	// tree, and it stays nil when only the value tree is wanted. currentNode
 	// is the node of p.current; pending collects the comment lines since the
@@ -87,10 +102,9 @@ func (p *parser) leaveNesting() { p.depth-- }
 func (p *parser) parse() (map[string]any, error) {
 	p.root = map[string]any{}
 	p.current = p.root
-	p.headers = map[string]bool{}
-	p.frozen = map[string]bool{}
-	p.dotted = map[string]bool{}
-	p.arrays = map[string]bool{}
+	// The definition maps start unallocated: a document with no headers, no
+	// dotted keys and no inline tables never pays for them, and a nil map
+	// reads as empty. Each is created on its first write.
 	p.currentPath = nil
 	if p.wantDoc {
 		p.doc = newTable(p.root)
@@ -166,6 +180,56 @@ func (p *parser) checkCtx() error {
 	return p.ctx.Err()
 }
 
+// --- definition maps --------------------------------------------------------
+
+// The definition maps record what a document has already defined, so a later
+// statement cannot redefine it. Each is created on first write: reads on a
+// nil map answer false, which is exactly the state of a map never written.
+
+func (p *parser) markHeader(pk string) {
+	if p.headers == nil {
+		p.headers = make(map[string]bool, 4)
+	}
+	p.headers[pk] = true
+}
+
+func (p *parser) markFrozen(pk string) {
+	if p.frozen == nil {
+		p.frozen = make(map[string]bool, 4)
+	}
+	p.frozen[pk] = true
+}
+
+func (p *parser) markDotted(pk string) {
+	if p.dotted == nil {
+		p.dotted = make(map[string]bool, 4)
+	}
+	p.dotted[pk] = true
+}
+
+func (p *parser) markArray(pk string) {
+	if p.arrays == nil {
+		p.arrays = make(map[string]bool, 2)
+	}
+	p.arrays[pk] = true
+}
+
+// internKey returns the shared string for key bytes. The lookup works on the
+// bytes directly, which the compiler lets run without allocating, so a
+// repeated key costs no allocation at all and the tree stores one string per
+// distinct key.
+func (p *parser) internKey(b []byte) string {
+	if p.keys == nil {
+		p.keys = make(map[string]string, 16)
+	}
+	if s, ok := p.keys[string(b)]; ok {
+		return s
+	}
+	s := string(b)
+	p.keys[s] = s
+	return s
+}
+
 // --- table headers ---------------------------------------------------------
 
 func (p *parser) parseTableHeader() error {
@@ -176,7 +240,7 @@ func (p *parser) parseTableHeader() error {
 		p.pos++
 	}
 
-	key, err := p.parseKeyPath()
+	first, rest, err := p.parseKeyPath()
 	if err != nil {
 		return err
 	}
@@ -193,6 +257,15 @@ func (p *parser) parseTableHeader() error {
 		p.pos++
 	}
 
+	// The key the rest of the header handling reads. parseKeyPath hands back
+	// a transient buffer for the single-segment case, the shape every
+	// repeated array-of-tables header has; anything longer is copied once.
+	key := p.keyBuf[:1]
+	key[0] = first
+	if len(rest) > 0 {
+		key = append([]string{first}, rest...)
+	}
+
 	if array {
 		tbl, elem, err := p.appendArrayTable(key)
 		if err != nil {
@@ -201,9 +274,9 @@ func (p *parser) parseTableHeader() error {
 		// A new array-of-tables element starts a fresh scope: sub-table headers
 		// and inline-table freezes from the previous element no longer apply.
 		p.resetScopeUnder(key)
-		p.arrays[pathKey(key)] = true
+		p.markArray(pathKey(key))
 		p.current = tbl
-		p.currentPath = key
+		p.currentPath = p.retainPath(key)
 		p.currentNode = elem
 		p.lastTable = elem
 		return nil
@@ -213,29 +286,52 @@ func (p *parser) parseTableHeader() error {
 	if p.headers[pk] || p.dotted[pk] || p.arrays[pk] {
 		return p.errf("table %q is defined more than once", strings.Join(key, "."))
 	}
-	p.headers[pk] = true
+	p.markHeader(pk)
 
 	tbl, node, err := p.tableAt(key)
 	if err != nil {
 		return err
 	}
 	p.current = tbl
-	p.currentPath = key
+	p.currentPath = p.retainPath(key)
 	p.currentNode = node
 	p.lastTable = node
 	return nil
 }
 
+// retainPath copies key into the parser-owned storage currentPath holds, so
+// the transient key buffer is free to serve the next statement.
+func (p *parser) retainPath(key []string) []string {
+	if cap(p.currentPath) < len(key) {
+		p.currentPath = make([]string, len(key))
+	} else {
+		p.currentPath = p.currentPath[:len(key)]
+	}
+	copy(p.currentPath, key)
+	return p.currentPath
+}
+
 // tableAt walks (creating intermediate tables) to the table named by key,
 // relative to the document root, rejecting any step into a frozen inline table.
 func (p *parser) tableAt(key []string) (map[string]any, *Table, error) {
 	cur := p.root
 	node := p.doc
-	path := make([]string, 0, len(key))
+	// The intermediate-path bookkeeping allocates only when the key actually
+	// has intermediate segments; a single-segment key checks its own name.
+	var path []string
+	if len(key) > 1 {
+		path = make([]string, 0, len(key))
+	}
 	for _, k := range key {
-		path = append(path, k)
-		if p.frozen[pathKey(path)] {
-			return nil, nil, p.errf("cannot extend inline table %q", strings.Join(path, "."))
+		if len(key) == 1 {
+			if p.frozen[k] {
+				return nil, nil, p.errf("cannot extend inline table %q", k)
+			}
+		} else {
+			path = append(path, k)
+			if p.frozen[pathKey(path)] {
+				return nil, nil, p.errf("cannot extend inline table %q", strings.Join(path, "."))
+			}
 		}
 		existing, ok := cur[k]
 		if !ok {
@@ -271,7 +367,12 @@ func (p *parser) tableAt(key []string) (map[string]any, *Table, error) {
 func (p *parser) appendArrayTable(key []string) (map[string]any, *Table, error) {
 	parent := p.root
 	node := p.doc
-	path := make([]string, 0, len(key))
+	// As in tableAt, the path slice exists only for a multi-segment key; the
+	// loop below runs for those alone.
+	var path []string
+	if len(key) > 1 {
+		path = make([]string, 0, len(key))
+	}
 	for _, k := range key[:len(key)-1] {
 		path = append(path, k)
 		if p.frozen[pathKey(path)] {
@@ -323,7 +424,7 @@ func (p *parser) appendArrayTable(key []string) (map[string]any, *Table, error)
 // --- key/value -------------------------------------------------------------
 
 func (p *parser) parseKeyValue() error {
-	key, err := p.parseKeyPath()
+	first, rest, err := p.parseKeyPath()
 	if err != nil {
 		return err
 	}
@@ -340,47 +441,46 @@ func (p *parser) parseKeyValue() error {
 	}
 
 	dest := p.current
-	// One allocation covers the current section plus the dotted key; a
-	// top-level statement reuses it for the leaf.
-	abs := make([]string, 0, len(p.currentPath)+len(key))
-	abs = append(abs, p.currentPath...)
+	// The absolute path of the key drives the dotted-key bookkeeping and the
+	// inline-table freeze. A single top-level key needs it only for the
+	// freeze, where a one-element path sits in the parser's scratch.
+	var abs []string
+	if len(rest) > 0 || len(p.currentPath) > 0 {
+		abs = make([]string, 0, len(p.currentPath)+len(rest)+1)
+		abs = append(abs, p.currentPath...)
+		abs = append(abs, first)
+	} else {
+		abs = append(p.absScratch[:0], first)
+	}
+
 	// dests collects the map each dotted key descended into, which the node
 	// tree needs to build the matching tables around the value.
 	var dests []map[string]any
-	for _, k := range key[:len(key)-1] {
-		abs = append(abs, k)
-		if p.frozen[pathKey(abs)] {
-			return p.errf("cannot extend inline table %q", strings.Join(abs, "."))
+	leaf := first
+	if len(rest) > 0 {
+		if err := p.descendKey(&dest, first, abs, &dests); err != nil {
+			return err
 		}
-		if p.headers[pathKey(abs)] {
-			return p.errf("cannot extend table %q with a dotted key", strings.Join(abs, "."))
+		for _, k := range rest[:len(rest)-1] {
+			abs = append(abs, k)
+			if err := p.descendKey(&dest, k, abs, &dests); err != nil {
+				return err
+			}
 		}
-		p.dotted[pathKey(abs)] = true
-		existing, ok := dest[k]
-		if !ok {
-			next := map[string]any{}
-			dest[k] = next
-			dest = next
-			dests = append(dests, next)
-			continue
-		}
-		m, ok := existing.(map[string]any)
-		if !ok {
-			return p.errf("key %q is not a table", k)
-		}
-		dest = m
-		dests = append(dests, m)
+		leaf = rest[len(rest)-1]
+		abs = append(abs, leaf)
 	}
-	leaf := key[len(key)-1]
-	abs = append(abs, leaf)
 	if _, exists := dest[leaf]; exists {
 		return p.errf("duplicate key %q", leaf)
 	}
 	dest[leaf] = val
 	if p.doc != nil {
 		node := p.currentNode
-		for i, k := range key[:len(key)-1] {
-			node = node.addTable(k, dests[i])
+		if len(rest) > 0 {
+			node = node.addTable(first, dests[0])
+			for i, k := range rest[:len(rest)-1] {
+				node = node.addTable(k, dests[i+1])
+			}
 		}
 		_, inline := val.(map[string]any)
 		entry := node.addValue(leaf, val, inline)
@@ -396,6 +496,35 @@ func (p *parser) parseKeyValue() error {
 	return nil
 }
 
+// descendKey walks dest into the sub-table named key on the dotted path abs,
+// recording the path in the definition maps; dests collects the maps
+// descended into.
+func (p *parser) descendKey(dest *map[string]any, key string, abs []string, dests *[]map[string]any) error {
+	ak := pathKey(abs)
+	if p.frozen[ak] {
+		return p.errf("cannot extend inline table %q", strings.Join(abs, "."))
+	}
+	if p.headers[ak] {
+		return p.errf("cannot extend table %q with a dotted key", strings.Join(abs, "."))
+	}
+	p.markDotted(ak)
+	existing, ok := (*dest)[key]
+	if !ok {
+		next := map[string]any{}
+		(*dest)[key] = next
+		*dest = next
+		*dests = append(*dests, next)
+		return nil
+	}
+	m, ok := existing.(map[string]any)
+	if !ok {
+		return p.errf("key %q is not a table", key)
+	}
+	*dest = m
+	*dests = append(*dests, m)
+	return nil
+}
+
 // takeInline returns the node of the inline table just parsed, when v is that
 // table's value, and clears it so a later value cannot pick it up.
 func (p *parser) takeInline(v any) *Table {
@@ -419,16 +548,17 @@ func (p *parser) takeArrayElems(v any) []*Table {
 }
 
 // freezeInline marks the path of an inline table (and any nested inline tables)
-// as immutable, so a later header or dotted key cannot extend it.
+// as immutable, so a later header or dotted key cannot extend it. The
+// recursion appends into the caller's path slice; the frozen map keeps the
+// joined strings, never the slice, so the backing is free to be reused.
 func (p *parser) freezeInline(path []string, val any) {
 	m, ok := val.(map[string]any)
 	if !ok {
 		return
 	}
-	p.frozen[pathKey(path)] = true
+	p.markFrozen(pathKey(path))
 	for k, v := range m {
-		child := append(append([]string{}, path...), k)
-		p.freezeInline(child, v)
+		p.freezeInline(append(path, k), v)
 	}
 }
 
@@ -438,33 +568,54 @@ func (p *parser) freezeInline(path []string, val any) {
 // fresh in the new element.
 func (p *parser) resetScopeUnder(key []string) {
 	prefix := pathKey(key) + "\x00"
-	for _, m := range []map[string]bool{p.headers, p.frozen, p.dotted, p.arrays} {
-		for k := range m {
-			if strings.HasPrefix(k, prefix) {
-				delete(m, k)
-			}
+	p.resetMapUnder(p.headers, prefix)
+	p.resetMapUnder(p.frozen, prefix)
+	p.resetMapUnder(p.dotted, prefix)
+	p.resetMapUnder(p.arrays, prefix)
+}
+
+// resetMapUnder deletes the entries m holds under prefix. An empty or
+// unallocated map holds none, so the common case walks nothing.
+func (p *parser) resetMapUnder(m map[string]bool, prefix string) {
+	if len(m) == 0 {
+		return
+	}
+	for k := range m {
+		if strings.HasPrefix(k, prefix) {
+			delete(m, k)
 		}
 	}
 }
 
-// parseKeyPath parses a dotted key into its components.
-func (p *parser) parseKeyPath() ([]string, error) {
-	var parts []string
+// parseKeyPath parses a dotted key. The first component comes back directly
+// and the rest as a usually nil slice, because a single-component key is the
+// common shape and a fresh slice per statement is what the allocation profile
+// showed. The single-key slice a caller sees is parser-owned and transient.
+func (p *parser) parseKeyPath() (string, []string, error) {
+	p.skipInline()
+	first, err := p.parseKeyComponent()
+	if err != nil {
+		return "", nil, err
+	}
+	p.skipInline()
+	if p.eof() || p.peek() != '.' {
+		return first, nil, nil
+	}
+	p.pos++
+	var rest []string
 	for {
 		p.skipInline()
 		part, err := p.parseKeyComponent()
 		if err != nil {
-			return nil, err
+			return "", nil, err
 		}
-		parts = append(parts, part)
+		rest = append(rest, part)
 		p.skipInline()
-		if !p.eof() && p.peek() == '.' {
-			p.pos++
-			continue
+		if p.eof() || p.peek() != '.' {
+			return first, rest, nil
 		}
-		break
+		p.pos++
 	}
-	return parts, nil
 }
 
 func (p *parser) parseKeyComponent() (string, error) {
@@ -493,11 +644,18 @@ func (p *parser) parseKeyComponent() (string, error) {
 			}
 			break
 		}
+		// The stopping byte decides the message: a multi-byte sequence that
+		// does not decode names that, before any grammar message can.
+		if !p.eof() && p.peek() >= utf8.RuneSelf {
+			if r, size := utf8.DecodeRune(p.src[p.pos:]); r == utf8.RuneError && size == 1 {
+				return "", p.errf("invalid UTF-8 in key")
+			}
+		}
 		if p.pos == start {
 			r, _ := utf8.DecodeRune(p.src[p.pos:])
 			return "", p.errf("invalid key character %q", string(r))
 		}
-		return string(p.src[start:p.pos]), nil
+		return p.internKey(p.src[start:p.pos]), nil
 	}
 }
 
@@ -544,6 +702,9 @@ func (p *parser) parseAtom() (any, error) {
 	if tok == "" {
 		return nil, p.errf("expected a value")
 	}
+	if hasHighByte(tok) && !utf8.ValidString(tok) {
+		return nil, p.errf("invalid UTF-8 in value")
+	}
 	// A date may be followed by a space and a time, forming one date-time.
 	if isDateToken(tok) && !p.eof() && p.peek() == ' ' {
 		if next, ok := p.peekAt(1); ok && next >= '0' && next <= '9' {
@@ -576,6 +737,17 @@ func (p *parser) scanBareToken() {
 	}
 }
 
+// hasHighByte reports whether s holds any byte outside ASCII, the cheap gate
+// in front of a full UTF-8 check.
+func hasHighByte(s string) bool {
+	for i := range len(s) {
+		if s[i] >= utf8.RuneSelf {
+			return true
+		}
+	}
+	return false
+}
+
 // --- strings ---------------------------------------------------------------
 
 func (p *parser) parseBasicString() (string, error) {
@@ -583,7 +755,36 @@ func (p *parser) parseBasicString() (string, error) {
 		return p.parseMultilineString('"', true)
 	}
 	p.pos++ // opening quote
+	start := p.pos
+	// A run of plain characters up to the closing quote needs no builder, only
+	// one copy at the end; escapes, controls and multi-byte runes fall through
+	// to the builder loop, which validates them on the spot.
+	for p.pos < len(p.src) {
+		c := p.src[p.pos]
+		if c == '"' {
+			s := string(p.src[start:p.pos])
+			p.pos++
+			return s, nil
+		}
+		if c == '\\' || c == '\n' || c == '\r' || c >= utf8.RuneSelf ||
+			(c < 0x20 && c != '\t') || c == 0x7f {
+			break
+		}
+		p.pos++
+	}
+	if p.eof() {
+		return "", p.errf("unterminated string")
+	}
 	var b strings.Builder
+	b.Grow(p.pos - start)
+	b.Write(p.src[start:p.pos])
+	return p.parseBasicStringRest(&b)
+}
+
+// parseBasicStringRest continues a basic string whose fast scan has met a byte
+// it does not handle: an escape, a control character, a multi-byte rune, or a
+// bare newline, which the loop rejects.
+func (p *parser) parseBasicStringRest(b *strings.Builder) (string, error) {
 	for {
 		if p.eof() {
 			return "", p.errf("unterminated string")
@@ -605,7 +806,7 @@ func (p *parser) parseBasicString() (string, error) {
 			}
 			b.WriteRune(r)
 		default:
-			if err := p.writeContentRune(&b); err != nil {
+			if err := p.writeContentRune(b); err != nil {
 				return "", err
 			}
 		}
@@ -617,7 +818,27 @@ func (p *parser) parseLiteralString() (string, error) {
 		return p.parseMultilineString('\'', false)
 	}
 	p.pos++ // opening quote
+	start := p.pos
+	// The same fast scan as the basic string, without the escape case.
+	for p.pos < len(p.src) {
+		c := p.src[p.pos]
+		if c == '\'' {
+			s := string(p.src[start:p.pos])
+			p.pos++
+			return s, nil
+		}
+		if c == '\n' || c == '\r' || c >= utf8.RuneSelf ||
+			(c < 0x20 && c != '\t') || c == 0x7f {
+			break
+		}
+		p.pos++
+	}
+	if p.eof() {
+		return "", p.errf("unterminated literal string")
+	}
 	var b strings.Builder
+	b.Grow(p.pos - start)
+	b.Write(p.src[start:p.pos])
 	for {
 		if p.eof() {
 			return "", p.errf("unterminated literal string")
@@ -641,8 +862,8 @@ func (p *parser) parseLiteralString() (string, error) {
 
 // writeContentRune appends the rune at the cursor to b and advances past it.
 // An ASCII byte, which includes every control character the grammar forbids,
-// is checked and written directly; a multi-byte rune is decoded and can never
-// be a control character.
+// is checked and written directly; a multi-byte rune is decoded, and a
+// sequence that does not decode is the UTF-8 error reported where it sits.
 func (p *parser) writeContentRune(b *strings.Builder) error {
 	c := p.peek()
 	if c < utf8.RuneSelf {
@@ -654,6 +875,9 @@ func (p *parser) writeContentRune(b *strings.Builder) error {
 		return nil
 	}
 	r, size := utf8.DecodeRune(p.src[p.pos:])
+	if r == utf8.RuneError && size == 1 {
+		return p.errf("invalid UTF-8 in string")
+	}
 	p.pos += size
 	b.WriteRune(r)
 	return nil
@@ -831,7 +1055,10 @@ func (p *parser) parseArray() (val any, err error) {
 	}
 	defer p.leaveNesting()
 	p.pos++ // '['
-	arr := []any{}
+	// A small presize covers the arrays documents actually hold, and trades a
+	// little capacity on tiny arrays for the growth chain an append-from-nil
+	// costs per array.
+	arr := make([]any, 0, 4)
 	// elems carries the node of each element that is an inline table, so the
 	// caller can keep its key order; the entries are nil for other values.
 	var elems []*Table
@@ -886,7 +1113,9 @@ func (p *parser) parseInlineTable() (val any, err error) {
 	defer p.leaveNesting()
 	p.pos++ // '{'
 	tbl := map[string]any{}
-	assigned := map[string]bool{}
+	// assigned tracks the dotted paths written into this table. It is created
+	// on the first key, so an empty inline table allocates nothing for it.
+	var assigned map[string]bool
 	// The inline table is a node of its own, so the keys keep their order; the
 	// caller picks the node up when the table parses.
 	var node *Table
@@ -913,7 +1142,7 @@ func (p *parser) parseInlineTable() (val any, err error) {
 		if err := p.skipNestedSpace(); err != nil {
 			return nil, err
 		}
-		key, err := p.parseKeyPath()
+		first, rest, err := p.parseKeyPath()
 		if err != nil {
 			return nil, err
 		}
@@ -929,39 +1158,46 @@ func (p *parser) parseInlineTable() (val any, err error) {
 		}
 
 		dest := tbl
-		path := make([]string, 0, len(key))
+		var path []string
 		var dests []map[string]any
-		for _, k := range key[:len(key)-1] {
-			path = append(path, k)
-			if assigned[pathKey(path)] {
-				return nil, p.errf("key %q is already defined", strings.Join(path, "."))
+		leaf := first
+		if len(rest) > 0 {
+			path = append(p.absScratch[:0], first)
+			d, err := p.descendInline(&dest, first, path, assigned)
+			if err != nil {
+				return nil, err
 			}
-			existing, ok := dest[k]
-			if !ok {
-				m := map[string]any{}
-				dest[k] = m
-				dest = m
-				dests = append(dests, m)
-				continue
+			dests = append(dests, d)
+			for _, k := range rest[:len(rest)-1] {
+				path = append(path, k)
+				d, err := p.descendInline(&dest, k, path, assigned)
+				if err != nil {
+					return nil, err
+				}
+				dests = append(dests, d)
 			}
-			m, isMap := existing.(map[string]any)
-			if !isMap {
-				return nil, p.errf("key %q is already defined", k)
-			}
-			dest = m
-			dests = append(dests, m)
+			leaf = rest[len(rest)-1]
+			path = append(path, leaf)
 		}
-		leaf := key[len(key)-1]
-		path = append(path, leaf)
 		if _, exists := dest[leaf]; exists {
 			return nil, p.errf("duplicate key %q in inline table", leaf)
 		}
 		dest[leaf] = val
-		assigned[pathKey(path)] = true
+		if assigned == nil {
+			assigned = make(map[string]bool, 4)
+		}
+		if len(rest) == 0 {
+			assigned[first] = true
+		} else {
+			assigned[pathKey(path)] = true
+		}
 		if node != nil {
 			child := node
-			for i, k := range key[:len(key)-1] {
-				child = child.addTable(k, dests[i])
+			if len(rest) > 0 {
+				child = child.addTable(first, dests[0])
+				for i, k := range rest[:len(rest)-1] {
+					child = child.addTable(k, dests[i+1])
+				}
 			}
 			_, inline := val.(map[string]any)
 			entry := child.addValue(leaf, val, inline)
@@ -998,6 +1234,28 @@ func (p *parser) parseInlineTable() (val any, err error) {
 	}
 }
 
+// descendInline walks dest into the sub-table named key inside an inline
+// table, rejecting a dotted segment the table has already defined.
+func (p *parser) descendInline(dest *map[string]any, key string, path []string, assigned map[string]bool) (map[string]any, error) {
+	pk := pathKey(path)
+	if assigned[pk] {
+		return nil, p.errf("key %q is already defined", strings.Join(path, "."))
+	}
+	existing, ok := (*dest)[key]
+	if !ok {
+		m := map[string]any{}
+		(*dest)[key] = m
+		*dest = m
+		return m, nil
+	}
+	m, isMap := existing.(map[string]any)
+	if !isMap {
+		return nil, p.errf("key %q is already defined", key)
+	}
+	*dest = m
+	return m, nil
+}
+
 // --- scanning helpers ------------------------------------------------------
 
 func (p *parser) eof() bool  { return p.pos >= len(p.src) }
@@ -1121,8 +1379,14 @@ func (p *parser) skipComment() (string, error) {
 			p.pos++
 		case c < 0x20 || c == 0x7f:
 			return "", p.errf("control character U+%04X is not allowed in a comment", c)
-		default:
+		case c < utf8.RuneSelf:
 			p.pos++
+		default:
+			r, size := utf8.DecodeRune(p.src[p.pos:])
+			if r == utf8.RuneError && size == 1 {
+				return "", p.errf("invalid UTF-8 in comment")
+			}
+			p.pos += size
 		}
 	}
 	return commentText(string(p.src[start:p.pos])), nil
@@ -1176,7 +1440,10 @@ func (p *parser) expectLineEnd() error {
 		p.pos++
 		return nil
 	}
-	r, _ := utf8.DecodeRune(p.src[p.pos:])
+	r, size := utf8.DecodeRune(p.src[p.pos:])
+	if r == utf8.RuneError && size == 1 {
+		return p.errf("invalid UTF-8 after value")
+	}
 	return p.errf("unexpected %q after value", string(r))
 }
 
@@ -1185,7 +1452,8 @@ func (p *parser) errf(format string, args ...any) error {
 }
 
 // pathKey joins key components with a NUL separator so a dotted path can be
-// used as a map key for tracking defined tables.
+// used as a map key for tracking defined tables. A single component comes
+// back as it is, with no join and no copy.
 func pathKey(parts []string) string {
 	return strings.Join(parts, "\x00")
 }