gopls/internal/protocol: Mapper: reject start > end

+ test

Fixes golang/go#74394

Change-Id: I1539501bb1b8e0f538e73a1a0c1073d6d47aa6f4
Reviewed-on: https://go-review.googlesource.com/c/tools/+/684255
Auto-Submit: Alan Donovan <[email protected]>
Reviewed-by: Robert Findley <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: adonovan

gopls/internal/protocol/mapper.go

@@ -256,6 +256,9 @@ func (m *Mapper) RangeOffsets(r Range) (int, int, error) {
 	if err != nil {
 		return 0, 0, err
 	}
+	if start > end {
+		return 0, 0, fmt.Errorf("start (offset %d) > end (offset %d)", start, end)
+	}
 	return start, end, nil
 }
 

gopls/internal/test/integration/misc/codeactions_test.go

@@ -42,6 +42,7 @@ func Offset(f *token.File, pos token.Pos) (int, error) {
 }
 
 // Offsets returns Offset(start) and Offset(end).
+// It returns an error if either failed, or if start > end.
 func Offsets(f *token.File, start, end token.Pos) (int, int, error) {
 	startOffset, err := Offset(f, start)
 	if err != nil {
@@ -51,6 +52,9 @@ func Offsets(f *token.File, start, end token.Pos) (int, int, error) {
 	if err != nil {
 		return 0, 0, fmt.Errorf("end: %v", err)
 	}
+	if start > end {
+		return 0, 0, fmt.Errorf("start (offset %d) > end (offset %d)", start, end)
+	}
 	return startOffset, endOffset, nil
 }