compiler/parser: make 'used as value' error message the same of gc

For #385

compiler/parser.go

@@ -9,6 +9,7 @@ package compiler
 import (
 	"errors"
 	"fmt"
+	"strings"
 	"unicode"
 	"unicode/utf8"
 
@@ -589,10 +590,7 @@ LABEL:
 			if cond != nil {
 				condition, _ = cond.(ast.Expression)
 				if condition == nil {
-					if a, ok := cond.(*ast.Assignment); ok && a.Type == ast.AssignmentSimple {
-						panic(syntaxError(tok.pos, "assignment %s used as value", init))
-					}
-					panic(syntaxError(tok.pos, "%s used as value", init))
+					panic(cannotUseAsValueError(pos, init))
 				}
 			}
 			var post ast.Node
@@ -611,10 +609,7 @@ LABEL:
 			if init != nil {
 				condition, _ = init.(ast.Expression)
 				if condition == nil {
-					if a, ok := init.(*ast.Assignment); ok && a.Type == ast.AssignmentSimple {
-						panic(syntaxError(tok.pos, "assignment %s used as value", init))
-					}
-					panic(syntaxError(tok.pos, "%s used as value", init))
+					panic(cannotUseAsValueError(pos, init))
 				}
 			}
 			pos.End = tok.pos.End
@@ -892,10 +887,7 @@ LABEL:
 		} else if init != nil {
 			expr, _ = init.(ast.Expression)
 			if expr == nil {
-				if a, ok := init.(*ast.Assignment); ok && a.Type == ast.AssignmentSimple {
-					panic(syntaxError(tok.pos, "assignment %s used as value", init))
-				}
-				panic(syntaxError(tok.pos, "%s used as value", init))
+				panic(cannotUseAsValueError(pos, init))
 			}
 			init = nil
 		}
@@ -1696,6 +1688,28 @@ func (p *parsing) parseAssignment(variables []ast.Expression, tok token, canBeRa
 	return node, tok
 }
 
+// cannotUseAsValueError returns a syntax error returned when node is used as
+// value at position pos.
+func cannotUseAsValueError(pos *ast.Position, node ast.Node) *SyntaxError {
+	if a, ok := node.(*ast.Assignment); ok && a.Type == ast.AssignmentSimple {
+		return syntaxError(pos, "cannot use assignment (%s) = (%s) as value",
+			printExpressions(a.Lhs), printExpressions(a.Rhs))
+	}
+	panic(syntaxError(pos, "cannot use %s as value", node))
+}
+
+// printExpressions prints expressions as a comma separated list.
+func printExpressions(expressions []ast.Expression) string {
+	var b strings.Builder
+	for i, e := range expressions {
+		if i > 0 {
+			b.WriteString(", ")
+		}
+		b.WriteString(e.String())
+	}
+	return b.String()
+}
+
 // addNode adds a node to the tree. It adds the node as a child to the current
 // parent node and sets the node as new ancestor if necessary.
 func (p *parsing) addNode(node ast.Node) {

compiler/parser_switch.go

@@ -129,14 +129,8 @@ func (p *parsing) parseSwitch(tok token, end tokenTyp) ast.Node {
 				// TODO (Gianluca): should error contain the position of the
 				// expression which caused the error instead of the token (as Go
 				// does)?
-				if !ok {
-					panic(syntaxError(tok.pos, "assignment %s used as value", assignment))
-				}
-				if ta.Type != nil {
-					panic(syntaxError(tok.pos, "%s used as value", assignment))
-				}
-				if len(assignment.Lhs) != 1 {
-					panic(syntaxError(tok.pos, "%s used as value", assignment))
+				if !ok || ta.Type != nil || len(assignment.Lhs) != 1 {
+					panic(cannotUseAsValueError(tok.pos, assignment))
 				}
 				afterSemicolon = assignment
 			} else {
@@ -160,15 +154,9 @@ func (p *parsing) parseSwitch(tok token, end tokenTyp) ast.Node {
 			if len(assignment.Rhs) != 1 {
 				panic(syntaxError(tok.pos, "unexpected %s, expecting expression", want))
 			}
-			if len(assignment.Lhs) != 1 {
-				panic(syntaxError(tok.pos, "%s used as value", assignment))
-			}
 			ta, ok := assignment.Rhs[0].(*ast.TypeAssertion)
-			if !ok {
-				panic(syntaxError(tok.pos, "assignment %s used as value", assignment))
-			}
-			if ta.Type != nil {
-				panic(syntaxError(tok.pos, "%s used as value", assignment))
+			if !ok || ta.Type != nil || len(assignment.Lhs) != 1 {
+				panic(cannotUseAsValueError(tok.pos, assignment))
 			}
 			afterSemicolon = assignment
 		}

test/compare/testdata/misc/switch2.go

@@ -21,6 +21,8 @@ func main() {
 	switch 5 { case 'a': }
 	switch 'a' { case 5: }
 	switch 5.4 { case 2i: } // ERROR `constant 2i truncated to real`
+	switch a = 1 { case true: } // ERROR `cannot use assignment (a) = (1) as value`
+	switch a := 1 { case true: } // ERROR `cannot use a := 1 as value`
 	switch 1 < 2 { case true: }
 	switch a < "b" { case true: }
 	switch a { case "b": }

test/compare/testdata/syntax/for_statement.go

@@ -22,7 +22,8 @@ func main() {
 	for ; {} // ERROR `syntax error: unexpected {, expecting for loop condition`
 	for ; true {} // ERROR `syntax error: unexpected {, expecting semicolon or newline`
 	for ; ; { break }
-	for a := 5 {} // ERROR `syntax error: a := 5 used as value`
+	for a = 5 {} // ERROR `syntax error: cannot use assignment (a) = (5) as value`
+	for a := 5 {} // ERROR `syntax error: cannot use a := 5 as value`
 	for a := 5; {} // ERROR `syntax error: unexpected {, expecting for loop condition`
 	for a := 5; true {} // ERROR `syntax error: unexpected {, expecting semicolon or newline`
 	for a := false; a; {}

test/compare/testdata/syntax/if_statement.go

@@ -44,12 +44,13 @@ func main() {
 	if f() {}
 	if <-ch {} // ERROR `non-bool <-ch (type int) used as if condition`
 	if <-chb {}
-	if ch <- 3 {} // ERROR `ch <- 3 used as value`
-	if i++ for {} // ERROR `syntax error: i++ used as value`
-	if i++ {} // ERROR `syntax error: i++ used as value`
-	if i-- {} // ERROR `syntax error: i-- used as value`
-	if i = 3 {} // ERROR `syntax error: assignment i = 3 used as value`
-	if i := 3 { _ = i } // ERROR `syntax error: i := 3 used as value`
+	if ch <- 3 {} // ERROR `cannot use ch <- 3 as value`
+	if i++ for {} // ERROR `syntax error: cannot use i++ as value`
+	if i++ {} // ERROR `syntax error: cannot use i++ as value`
+	if i-- {} // ERROR `syntax error: cannot use i-- as value`
+	if i = 3 {} // ERROR `syntax error: cannot use assignment (i) = (3) as value`
+	if a, b = 3, 6 {} // ERROR `syntax error: cannot use assignment (a, b) = (3, 6) as value`
+	if i := 3 { _ = i } // ERROR `syntax error: cannot use i := 3 as value`
 	if print(); {} // ERROR `syntax error: missing condition in if statement`
 	if <-ch; {} // ERROR `syntax error: missing condition in if statement`
 	if ch <- 3; {} // ERROR `syntax error: missing condition in if statement`