Copy the item text in replace-query

ToRunes aliases the rune array, and the editing actions append into
t.input in place when the cursor is not at the end, so the keystrokes
edit the item. Non-ASCII items only, ASCII gets a fresh slice.

    printf '한글abcde\n' | fzf --bind 'ctrl-y:replace-query'
    ctrl-y, Left, BSpace, ctrl-u  ->  한글abcee

Runes and ToRunes are now documented read-only. A stale fold bit was the
other symptom, letting the prefilter reject an item the general path
matches.

src/terminal.go

@@ -7417,7 +7417,9 @@ func (t *Terminal) Loop() error {
 			case actReplaceQuery:
 				current := t.currentItem()
 				if current != nil {
-					t.input = current.text.ToRunes()
+					// ToRunes aliases the item text in rune mode, and the
+					// editing actions below append into t.input in place
+					t.input = append([]rune{}, current.text.ToRunes()...)
 					t.cx = len(t.input)
 				}
 			case actFatal:

src/util/chars.go

@@ -21,11 +21,9 @@ const (
 
 type Chars struct {
 	slice []byte // or []rune
-	// Written while the item is built and never after it reaches a matcher,
-	// so nothing reads these bits concurrently with a write. Prepend touches
-	// them, but only on the transient tokens inside transformItem, before
-	// item.text exists. trimLength* is kept out because TrimLength writes it
-	// lazily, long after that point.
+	// Only ever set, never cleared, so a reader racing a Prepend sees either
+	// the old or the new value and both are safe. trimLength* is kept out
+	// because TrimLength rewrites it.
 	flags           uint8
 	trimLengthKnown bool
 	trimLength      uint16
@@ -157,6 +155,8 @@ func ToChars(bytes []byte) Chars {
 	return runesToChars(runes, mayFold)
 }
 
+// RunesToChars adopts the caller's slice rather than copying it, so the caller
+// must not keep mutating it. See Runes for why.
 func RunesToChars(runes []rune) Chars {
 	mayFold := false
 	for _, r := range runes {
@@ -188,7 +188,10 @@ func (chars *Chars) MayFoldToAscii() bool {
 	return chars.flags&flagMayFold != 0
 }
 
-// Runes returns the underlying rune slice, or nil if the text is kept as bytes.
+// Runes returns the underlying rune slice, or nil if the text is kept as
+// bytes. Read only. The result aliases the text, so writing to it would change
+// the text without updating the cached fold bit, and the prefilter would then
+// reject items it should match. Copy before mutating.
 func (chars *Chars) Runes() []rune {
 	return chars.optionalRunes()
 }
@@ -340,6 +343,8 @@ func (chars *Chars) ToString() string {
 	return unsafe.String(unsafe.SliceData(chars.slice), len(chars.slice))
 }
 
+// ToRunes returns the text as runes. In rune mode the result aliases the text
+// and must not be mutated, see Runes. In byte mode it is a fresh slice.
 func (chars *Chars) ToRunes() []rune {
 	if runes := chars.optionalRunes(); runes != nil {
 		return runes

src/util/chars_test.go

@@ -257,3 +257,29 @@ func TestMayFoldFlag(t *testing.T) {
 		t.Error("Prepend of a foldable prefix must set the flag")
 	}
 }
+
+// Runes and ToRunes alias the text in rune mode, so a consumer that mutates
+// what they return changes the text without updating the cached fold bit. This
+// pins the aliasing so the read-only contract on those methods is not silently
+// dropped later.
+func TestRuneSlicesAliasTheText(t *testing.T) {
+	chars := ToChars([]byte("한글abc"))
+	runes := chars.Runes()
+	if runes == nil {
+		t.Fatal("expected rune mode")
+	}
+	if &runes[0] != &chars.ToRunes()[0] {
+		t.Error("Runes and ToRunes should return the same backing array")
+	}
+	if chars.MayFoldToAscii() {
+		t.Fatal("baseline should not be foldable")
+	}
+	// Demonstrates why callers must copy: the flag does not follow the text.
+	runes[0] = 'e'
+	if chars.MayFoldToAscii() {
+		t.Error("flag unexpectedly updated")
+	}
+	if got := chars.ToString(); got != "e글abc" {
+		t.Errorf("expected the write to reach the text, got %q", got)
+	}
+}

test/test_core.rb

@@ -479,6 +479,19 @@ class TestCore < TestInteractive
     tmux.until { |lines| assert_equal '> 10', lines[-1] }
   end
 
+  def test_bind_replace_query_does_not_mutate_item
+    tmux.send_keys "echo '한글abcde' | #{fzf('--bind=ctrl-j:replace-query,ctrl-o:clear-query')}", :Enter
+    tmux.until { |lines| assert_equal '  1/1', lines[-2] }
+    tmux.send_keys 'C-j'
+    tmux.until { |lines| assert_equal '> 한글abcde', lines[-1] }
+    # Editing away from the end used to write into the item itself
+    tmux.send_keys :Left, :BSpace
+    tmux.until { |lines| assert_equal '> 한글abce', lines[-1] }
+    tmux.send_keys 'C-o'
+    tmux.until { |lines| assert_equal '>', lines[-1] }
+    tmux.until { |lines| assert_equal '> 한글abcde', lines[-3] }
+  end
+
   def test_select_all_deselect_all_toggle_all
     tmux.send_keys "seq 100 | #{fzf('--bind ctrl-a:select-all,ctrl-d:deselect-all,ctrl-t:toggle-all --multi')}", :Enter
     tmux.until { |lines| assert_equal '  100/100 (0)', lines[-2] }