cmd/compile/internal/syntax: better error when an assignment is used in value context

The error message is now positioned at the statement position (which is
an identifing token, such as the '=' for assignments); and in case of
assignments it emphasizes the assignment by putting the Lhs and Rhs
in parentheses. Finally, the wording is changed from "use of * as value"
to the stronger "cannot use * as value" (for which there is precedent
elsewhere in the parser).

Fixes #36858.

Change-Id: Ic3f101bba50f58e3a1d9b29645066634631f2d61
Reviewed-on: https://go-review.googlesource.com/c/go/+/218337
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/syntax/parser.go

@@ -1886,11 +1886,16 @@ done:
 		// which turns an expression into an assignment. Provide
 		// a more explicit error message in that case to prevent
 		// further confusion.
-		str := String(s)
+		var str string
 		if as, ok := s.(*AssignStmt); ok && as.Op == 0 {
-			str = "assignment " + str
+			// Emphasize Lhs and Rhs of assignment with parentheses to highlight '='.
+			// Do it always - it's not worth going through the trouble of doing it
+			// only for "complex" left and right sides.
+			str = "assignment (" + String(as.Lhs) + ") = (" + String(as.Rhs) + ")"
+		} else {
+			str = String(s)
 		}
-		p.syntaxError(fmt.Sprintf("%s used as value", str))
+		p.syntaxErrorAt(s.Pos(), fmt.Sprintf("cannot use %s as value", str))
 	}
 
 	p.xnest = outer

src/cmd/compile/internal/syntax/testdata/issue23385.src

@@ -6,7 +6,12 @@
 
 package p
 
-func f() {
-	if true || 0 = 1 /* ERROR assignment .* used as value */ {
+func _() {
+	if true || 0 /* ERROR cannot use assignment .* as value */ = 1 {
+	}
+}
+
+func _(a, b string) {
+	if a == "a" && b /* ERROR cannot use assignment .* as value */ = "b" {
 	}
 }

test/fixedbugs/issue18915.go

@@ -10,12 +10,12 @@
 package p
 
 func _() {
-	if a := 10 { // ERROR "a := 10 used as value"
+	if a := 10 { // ERROR "cannot use a := 10 as value"
 	}
 
-	for b := 10 { // ERROR "b := 10 used as value"
+	for b := 10 { // ERROR "cannot use b := 10 as value"
 	}
 
-	switch c := 10 { // ERROR "c := 10 used as value"
+	switch c := 10 { // ERROR "cannot use c := 10 as value"
 	}
 }

test/syntax/chan1.go

@@ -10,7 +10,7 @@ var c chan int
 var v int
 
 func main() {
-	if c <- v { // ERROR "used as value"
+	if c <- v { // ERROR "cannot use c <- v as value"
 	}
 }
 

test/syntax/typesw.go

@@ -7,7 +7,7 @@
 package main
 
 func main() {
-	switch main() := interface{}(nil).(type) {	// ERROR "invalid variable name|used as value"
+	switch main() := interface{}(nil).(type) {	// ERROR "invalid variable name|cannot use .* as value"
 	default:
 	}
 }