internal/encoding/yaml: use cue/literal to quote string scalars

The YAML decoder had a function to wrap literal.Quote to use the
multi-line """ form when the string had any newlines.
It turns out that cue/literal can already do this via the option
Form.WithOptionalTabIndent, which enables an "auto" multi-line mode
to use the """ form when any newlines are present.

While here, I noticed an optimization in our use of strings.Repeat
which has already been done upstream for Go 1.23.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: Iba3b9bafb0b434001173db82718b30d1e1b680ef
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1193240
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
Reviewed-by: Roger Peppe <rogpeppe@gmail.com>

cue/literal/quote.go

@@ -65,6 +65,7 @@ func tabs(n int) string {
 // WithOptionalIndent is like WithTabIndent, but only returns a multiline
 // strings if it doesn't contain any newline characters.
 func (f Form) WithOptionalTabIndent(tabs int) Form {
+	// TODO(mvdan): remove this optimization once Go 1.23 lands with https://go.dev/cl/536615
 	if tabs < len(tabIndent) {
 		f.indent = tabIndent[:tabs]
 	} else {

internal/encoding/yaml/decode.go

@@ -498,7 +498,7 @@ func (d *decoder) scalar(yn *yaml.Node) (ast.Expr, error) {
 		return &ast.BasicLit{
 			ValuePos: d.pos(yn),
 			Kind:     token.STRING,
-			Value:    quoteString(yn.Value),
+			Value:    literal.String.WithOptionalTabIndent(1).Quote(yn.Value),
 		}, nil
 
 	case binaryTag:
@@ -618,46 +618,6 @@ func (d *decoder) alias(yn *yaml.Node) (ast.Expr, error) {
 	return node, err
 }
 
-// quoteString converts a string to a CUE multiline string if needed.
-// TODO(mvdan): this is brought over from the old decoder; we should consider
-// polishing this API and moving it someplace better like cue/literal.
-func quoteString(s string) string {
-	lines := []string{}
-	last := 0
-	for i, c := range s {
-		if c == '\n' {
-			lines = append(lines, s[last:i])
-			last = i + 1
-		}
-		if c == '\r' {
-			goto quoted
-		}
-	}
-	lines = append(lines, s[last:])
-	if len(lines) >= 2 {
-		buf := []byte{}
-		buf = append(buf, `"""`+"\n"...)
-		for _, l := range lines {
-			if l == "" {
-				// no indentation for empty lines
-				buf = append(buf, '\n')
-				continue
-			}
-			buf = append(buf, '\t')
-			p := len(buf)
-			// TODO(mvdan): do not use Go's strconv for CUE syntax.
-			buf = strconv.AppendQuote(buf, l)
-			// remove quotes
-			buf[p] = '\t'
-			buf[len(buf)-1] = '\n'
-		}
-		buf = append(buf, "\t\t"+`"""`...)
-		return string(buf)
-	}
-quoted:
-	return literal.String.Quote(s)
-}
-
 func labelStr(l ast.Label) string {
 	switch l := l.(type) {
 	case *ast.Ident: