encoding/toml: support decoding keys named "_"

We can't blindly create an ast.Ident with such a name, as that then
refers to the "top" identifier in CUE, which cannot be used as a label:

    foo: cannot use _ as label:

We hadn't noticed this with other keys which need quoting such as "a-b"
because sticking the unquoted string in ast.Ident.Name caused cue/format
to quote the identifier correctly. It does not do this for "_" as it is
a valid identifier - it just represents top, and not a literal string.

Fixes #3350.

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: If0f2c2fd6c0d984a7c6c30e4667ab8ed097aa17b
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1198819
Unity-Result: CUE porcuepine <[email protected]>
Reviewed-by: Roger Peppe <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>

encoding/toml/decode.go

@@ -338,21 +338,13 @@ func decodeKey(key rootedKey, iter toml.Iterator) (rootedKey, []string) {
 // The "leaf" field, in this case "zzz", leaves its value as nil to be filled out.
 func inlineFields(names []string, relPos token.RelPos) (top, leaf *ast.Field) {
 	curField := &ast.Field{
-		Label: &ast.Ident{
-			// We let the caller configure whether the first field starts a new line,
-			// because that's not desirable for inline tables.
-			NamePos: token.NoPos.WithRel(relPos),
-			Name:    names[0],
-		},
+		Label: label(names[0], token.NoPos.WithRel(relPos)),
 	}
 
 	topField := curField
 	for _, elem := range names[1:] {
 		nextField := &ast.Field{
-			Label: &ast.Ident{
-				NamePos: token.NoPos.WithRel(token.Blank), // on the same line
-				Name:    elem,
-			},
+			Label: label(elem, token.NoPos.WithRel(token.Blank)), // on the same line
 		}
 		curField.Value = &ast.StructLit{Elts: []ast.Decl{nextField}}
 		curField = nextField
@@ -370,6 +362,24 @@ func quoteLabelIfNeeded(name string) string {
 	return literal.Label.Quote(name)
 }
 
+// label creates an ast.Label that represents a key with exactly the literal string name.
+// This means a quoted string literal for the key "_", as TOML never means "top",
+// as well as for any keys beginning with an underscore, as we don't want to hide any fields.
+// cue/format knows how to quote any other identifiers correctly.
+func label(name string, pos token.Pos) ast.Label {
+	if strings.HasPrefix(name, "_") {
+		return &ast.BasicLit{
+			ValuePos: pos,
+			Kind:     token.STRING,
+			Value:    literal.String.Quote(name),
+		}
+	}
+	return &ast.Ident{
+		NamePos: pos,
+		Name:    name,
+	}
+}
+
 // decodeExpr decodes a single TOML value expression, found on the right side
 // of a `key = value` line.
 func (d *Decoder) decodeExpr(key rootedKey, tnode *toml.Node) (ast.Expr, error) {

encoding/toml/decode_test.go

@@ -91,13 +91,19 @@ func TestDecoder(t *testing.T) {
 		name: "RootKeysCharacters",
 		input: `
 			a-b = "dashes"
-			a_b = "underscores"
+			a_b = "underscore unquoted"
+			_   = "underscore quoted"
+			_ab = "underscore prefix quoted"
 			123 = "numbers"
+			x._.y._ = "underscores quoted"
 		`,
 		wantCUE: `
 			"a-b": "dashes"
-			a_b:   "underscores"
+			a_b:   "underscore unquoted"
+			"_":   "underscore quoted"
+			"_ab": "underscore prefix quoted"
 			"123": "numbers"
+			x: "_": y: "_": "underscores quoted"
 		`,
 	}, {
 		name: "RootKeysQuoted",