cue/parser: allow if as a label for a required field

The field `if!: 123` resulted in a parse error because
the token `if` followed by `!` was understood as the start of
a comprehension with a negated expression, like:

    if !boolexpr { ... }

To differentiate the two cases, we need to check if the following
token is ':', meaning that we are declaring a required field.
Teach the parser how to peek one token past the current one.

As a reminder, the CUE language allows using keywords as labels.
All other keywords already worked and had tests, including for
required fields; only `if` was broken for the reason outlined above.

Change-Id: I1ea9f3b82499b9c9b5e896c1d3857be86d7ca2b8
Signed-off-by: haoqixu <[email protected]>
Co-authored-by: Daniel Martí <[email protected]>
Signed-off-by: Daniel Martí <[email protected]>
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1209287
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Paul Jolly <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>

Author: haoqixu

cue/parser/parser.go

@@ -43,11 +43,20 @@ type parser struct {
 	leadComment *ast.CommentGroup
 	comments    *commentState
 
-	// Next token
+	// Next token, filled by [parser.next0].
 	pos token.Pos   // token position
 	tok token.Token // one token look-ahead
 	lit string      // token literal
 
+	// Token after next, filled by [parser.peek].
+	peekToken struct {
+		scanned bool
+
+		pos token.Pos
+		tok token.Token
+		lit string
+	}
+
 	// Error recovery
 	// (used to limit the number of calls to sync... functions
 	// w/o making scanning progress - avoids potential endless
@@ -269,9 +278,26 @@ func (p *parser) next0() {
 		}
 	}
 
+	// We had peeked one token, effectively scanning it early; use it now.
+	if p.peekToken.scanned {
+		p.pos, p.tok, p.lit = p.peekToken.pos, p.peekToken.tok, p.peekToken.lit
+		p.peekToken.scanned = false
+		return
+	}
+
 	p.pos, p.tok, p.lit = p.scanner.Scan()
 }
 
+// peek scans one more token as a look-ahead and stores it in [parser.peekToken].
+// Peeking multiple tokens ahead is not supported.
+func (p *parser) peek() {
+	if p.peekToken.scanned {
+		panic("can only peek one token at a time")
+	}
+	p.peekToken.pos, p.peekToken.tok, p.peekToken.lit = p.scanner.Scan()
+	p.peekToken.scanned = true
+}
+
 // Consume a comment and return it and the line on which it ends.
 func (p *parser) consumeComment() (comment *ast.Comment, endline int) {
 	endline = p.file.Line(p.pos)
@@ -1116,6 +1142,11 @@ func (p *parser) parseComprehensionClauses(first bool) (clauses []ast.Clause, c
 				case token.COLON, token.BIND, token.OPTION,
 					token.COMMA, token.EOF:
 					return nil, c
+				case token.NOT:
+					p.peek()
+					if p.peekToken.tok == token.COLON {
+						return nil, c
+					}
 				}
 			}
 

cue/parser/parser_test.go

@@ -80,10 +80,10 @@ func TestParse(t *testing.T) {
 		`if?: 0, for?: 1, in?: 2, where?: 3, div?: 4, quo?: 5, func?: 6, for?: {if?: {func?: {let?: 3}}}`,
 	}, {
 		"keywords as required labels",
-		`for!: 1, in!: 2, where!: 3, div!: 4, quo!: 5, func!: 6
+		`if!: 0, for!: 1, in!: 2, where!: 3, div!: 4, quo!: 5, func!: 6
 		for!: if!: func!: let!: 3
 		`,
-		`for!: 1, in!: 2, where!: 3, div!: 4, quo!: 5, func!: 6, for!: {if!: {func!: {let!: 3}}}`,
+		`if!: 0, for!: 1, in!: 2, where!: 3, div!: 4, quo!: 5, func!: 6, for!: {if!: {func!: {let!: 3}}}`,
 	}, {
 		"keywords as alias",
 		`if=foo: 0