gopls/internal/lsp/safetoken: fix error message

This change uses [x:y] notation in the error message to indicate
an inclusive interval, which is what the logic actually implements.
We also include the file name.

Also, put the inequalities in lo <= x <= hi form for clarity,
and inline/simplify the token.File.Pos/Offset calls to avoid
the redundant double-check.

Updates golang/go#57484

Change-Id: I63e10d0e6659aae2613b5b7d51e87a8a4bfb225d
Reviewed-on: https://go-review.googlesource.com/c/tools/+/459637
Run-TryBot: Alan Donovan <adonovan@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

gopls/internal/lsp/safetoken/safetoken.go

@@ -11,26 +11,26 @@ import (
 	"go/token"
 )
 
-// Offset returns tok.Offset(pos), but first checks that the pos is in range
+// Offset returns f.Offset(pos), but first checks that the pos is in range
 // for the given file.
-func Offset(tf *token.File, pos token.Pos) (int, error) {
-	if !InRange(tf, pos) {
-		return -1, fmt.Errorf("pos %v is not in range for file [%v:%v)", pos, tf.Base(), tf.Base()+tf.Size())
+func Offset(f *token.File, pos token.Pos) (int, error) {
+	if !InRange(f, pos) {
+		return -1, fmt.Errorf("pos %d is not in range [%d:%d] of file %s",
+			pos, f.Base(), f.Base()+f.Size(), f.Name())
 	}
-	return tf.Offset(pos), nil
+	return int(pos) - f.Base(), nil
 }
 
-// Pos returns tok.Pos(offset), but first checks that the offset is valid for
+// Pos returns f.Pos(offset), but first checks that the offset is valid for
 // the given file.
-func Pos(tf *token.File, offset int) (token.Pos, error) {
-	if offset < 0 || offset > tf.Size() {
-		return token.NoPos, fmt.Errorf("offset %v is not in range for file of size %v", offset, tf.Size())
+func Pos(f *token.File, offset int) (token.Pos, error) {
+	if !(0 <= offset && offset <= f.Size()) {
+		return token.NoPos, fmt.Errorf("offset %d is not in range for file %s of size %d", offset, f.Name(), f.Size())
 	}
-	return tf.Pos(offset), nil
+	return token.Pos(f.Base() + offset), nil
 }
 
-// InRange reports whether the given position is in the given token.File.
-func InRange(tf *token.File, pos token.Pos) bool {
-	size := tf.Pos(tf.Size())
-	return int(pos) >= tf.Base() && pos <= size
+// InRange reports whether file f contains position pos.
+func InRange(f *token.File, pos token.Pos) bool {
+	return token.Pos(f.Base()) <= pos && pos <= token.Pos(f.Base()+f.Size())
 }