cue/parser: allow for as label for required field

This adds support for using `for` in field labels when they are
marked as required with '!'.

Fixes #3783

Change-Id: I77b063250f79e26c9eb33c706cc970639a5e69b1
Signed-off-by: haoqixu <[email protected]>
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1209270
Reviewed-by: Daniel Martí <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>

Author: haoqixu

Diff for: cue/parser/parser.go

@@ -1083,7 +1083,7 @@ func (p *parser) parseComprehensionClauses(first bool) (clauses []ast.Clause, c
 			forPos := p.expect(token.FOR)
 			if first {
 				switch p.tok {
-				case token.COLON, token.BIND, token.OPTION,
+				case token.COLON, token.BIND, token.OPTION, token.NOT,
 					token.COMMA, token.EOF:
 					return nil, c
 				}

Diff for: cue/parser/parser_test.go

@@ -72,6 +72,18 @@ func TestParse(t *testing.T) {
 		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 optional labels",
+		`if?: 0, 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 required labels",
+		`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}}}`,
 	}, {
 		"keywords as alias",
 		`if=foo: 0