cmd/vet: polish output of shadow test

This commit modifies the style of a error message in case of -shadow.

Previously such a message would look like:
foo.go:42: declaration of err shadows declaration at shadow.go:13:

Changes of the commit include highlighting the variable name and
removing the ": "(space intended) at the end of the line:
foo.go:42: declaration of "err" shadows declaration at shadow.go:13

Fixes #14585.

Change-Id: Ia6a6bf396668dcba9a24f025a08d8826db31f434
Reviewed-on: https://go-review.googlesource.com/20093Reviewed-by: Rob Pike <r@golang.org>

src/cmd/vet/main.go

@@ -435,17 +435,17 @@ func (f *File) loc(pos token.Pos) string {
 	// expression instead of the inner part with the actual error, the
 	// precision can mislead.
 	posn := f.fset.Position(pos)
-	return fmt.Sprintf("%s:%d: ", posn.Filename, posn.Line)
+	return fmt.Sprintf("%s:%d", posn.Filename, posn.Line)
 }
 
 // Warn reports an error but does not set the exit code.
 func (f *File) Warn(pos token.Pos, args ...interface{}) {
-	fmt.Fprint(os.Stderr, f.loc(pos)+fmt.Sprintln(args...))
+	fmt.Fprintf(os.Stderr, "%s: %s", f.loc(pos), fmt.Sprintln(args...))
 }
 
 // Warnf reports a formatted error but does not set the exit code.
 func (f *File) Warnf(pos token.Pos, format string, args ...interface{}) {
-	fmt.Fprintf(os.Stderr, f.loc(pos)+format+"\n", args...)
+	fmt.Fprintf(os.Stderr, "%s: %s\n", f.loc(pos), fmt.Sprintf(format, args...))
 }
 
 // walkFile walks the file's tree.

src/cmd/vet/shadow.go

@@ -232,7 +232,7 @@ func checkShadowing(f *File, ident *ast.Ident) {
 		// the shadowing identifier.
 		span, ok := f.pkg.spans[shadowed]
 		if !ok {
-			f.Badf(ident.Pos(), "internal error: no range for %s", ident.Name)
+			f.Badf(ident.Pos(), "internal error: no range for %q", ident.Name)
 			return
 		}
 		if !span.contains(ident.Pos()) {
@@ -241,6 +241,6 @@ func checkShadowing(f *File, ident *ast.Ident) {
 	}
 	// Don't complain if the types differ: that implies the programmer really wants two different things.
 	if types.Identical(obj.Type(), shadowed.Type()) {
-		f.Badf(ident.Pos(), "declaration of %s shadows declaration at %s", obj.Name(), f.loc(shadowed.Pos()))
+		f.Badf(ident.Pos(), "declaration of %q shadows declaration at %s", obj.Name(), f.loc(shadowed.Pos()))
 	}
 }

src/cmd/vet/testdata/shadow.go

@@ -17,7 +17,7 @@ func ShadowRead(f *os.File, buf []byte) (err error) {
 		_ = err
 	}
 	if f != nil {
-		_, err := f.Read(buf) // ERROR "declaration of err shadows declaration at testdata/shadow.go:13"
+		_, err := f.Read(buf) // ERROR "declaration of .err. shadows declaration at testdata/shadow.go:13"
 		if err != nil {
 			return err
 		}
@@ -25,8 +25,8 @@ func ShadowRead(f *os.File, buf []byte) (err error) {
 		_ = i
 	}
 	if f != nil {
-		x := one()               // ERROR "declaration of x shadows declaration at testdata/shadow.go:14"
-		var _, err = f.Read(buf) // ERROR "declaration of err shadows declaration at testdata/shadow.go:13"
+		x := one()               // ERROR "declaration of .x. shadows declaration at testdata/shadow.go:14"
+		var _, err = f.Read(buf) // ERROR "declaration of .err. shadows declaration at testdata/shadow.go:13"
 		if x == 1 && err != nil {
 			return err
 		}
@@ -46,7 +46,7 @@ func ShadowRead(f *os.File, buf []byte) (err error) {
 	if shadowTemp := shadowTemp; true { // OK: obviously intentional idiomatic redeclaration
 		var f *os.File // OK because f is not mentioned later in the function.
 		// The declaration of x is a shadow because x is mentioned below.
-		var x int // ERROR "declaration of x shadows declaration at testdata/shadow.go:14"
+		var x int // ERROR "declaration of .x. shadows declaration at testdata/shadow.go:14"
 		_, _, _ = x, f, shadowTemp
 	}
 	// Use a couple of variables to trigger shadowing errors.