cmd/compile/internal/gc: reword "declared and not used" error message

"declared and not used" is technically correct, but might confuse
the user. Switching "and" to "but" will hopefully create the
contrast for the users: they did one thing (declaration), but
not the other --- actually using the variable.

This new message is still not ideal (specifically, declared is not
entirely precise here), but at least it matches the other parsers
and is one step in the right direction.

Change-Id: I725c7c663535f9ab9725c4b0bf35b4fa74b0eb20
Reviewed-on: https://go-review.googlesource.com/c/go/+/203282
Run-TryBot: Minux Ma <minux@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/gc/swt.go

@@ -32,7 +32,7 @@ func typecheckTypeSwitch(n *Node) {
 	// declaration itself. So if there are no cases, we won't
 	// notice that it went unused.
 	if v := n.Left.Left; v != nil && !v.isBlank() && n.List.Len() == 0 {
-		yyerrorl(v.Pos, "%v declared and not used", v.Sym)
+		yyerrorl(v.Pos, "%v declared but not used", v.Sym)
 	}
 
 	var defCase, nilCase *Node

src/cmd/compile/internal/gc/walk.go

@@ -50,10 +50,10 @@ func walk(fn *Node) {
 			if defn.Left.Name.Used() {
 				continue
 			}
-			yyerrorl(defn.Left.Pos, "%v declared and not used", ln.Sym)
+			yyerrorl(defn.Left.Pos, "%v declared but not used", ln.Sym)
 			defn.Left.Name.SetUsed(true) // suppress repeats
 		} else {
-			yyerrorl(ln.Pos, "%v declared and not used", ln.Sym)
+			yyerrorl(ln.Pos, "%v declared but not used", ln.Sym)
 		}
 	}
 

test/fixedbugs/bug373.go

@@ -9,7 +9,7 @@
 package foo
 
 func f(x interface{}) {
-	switch t := x.(type) {  // ERROR "declared and not used"
+	switch t := x.(type) {  // ERROR "declared but not used"
 	case int:
 	}
 }

test/fixedbugs/issue21317.go

@@ -48,8 +48,8 @@ func main() {
 		log.Fatalf("expected cmd/compile to fail")
 	}
 	wantErrs := []string{
-		"7:9: n declared and not used",
+		"7:9: n declared but not used",
-		"7:12: err declared and not used",
+		"7:12: err declared but not used",
 	}
 	outStr := string(out)
 	for _, want := range wantErrs {

test/fixedbugs/issue23116.go

@@ -10,6 +10,6 @@ func f(x interface{}) {
 	switch x.(type) {
 	}
 
-	switch t := x.(type) { // ERROR "declared and not used"
+	switch t := x.(type) { // ERROR "declared but not used"
 	}
 }

test/fixedbugs/issue29870b.go

@@ -10,5 +10,5 @@
 package main
 
 func _() {
-	x := 7 // ERROR "x declared and not used"
+	x := 7 // ERROR "x declared but not used"
 }

test/typeswitch2b.go

@@ -11,9 +11,9 @@ package main
 
 func notused(x interface{}) {
 	// The first t is in a different scope than the 2nd t; it cannot
-	// be accessed (=> declared and not used error); but it is legal
+	// be accessed (=> declared but not used error); but it is legal
 	// to declare it.
-	switch t := 0; t := x.(type) { // ERROR "declared and not used"
+	switch t := 0; t := x.(type) { // ERROR "declared but not used"
 	case int:
 		_ = t // this is using the t of "t := x.(type)"
 	}