internal/analysisinternal: ValidateFix: more specific errors

These details help us diagnose errors in gopls especially relating
to synthezized End() positions beyond EOF.

Change-Id: Iff36f5c4e01f2256f2cbf8cc03b27d7b3aa74b11
Reviewed-on: https://go-review.googlesource.com/c/tools/+/651097
Reviewed-by: Robert Findley <rfindley@google.com>
Commit-Queue: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Alan Donovan <adonovan@google.com>

Author: adonovan

go/analysis/internal/checker/fix_test.go

@@ -93,7 +93,7 @@ func TestReportInvalidDiagnostic(t *testing.T) {
 		// TextEdit has invalid Pos.
 		{
 			"bad Pos",
-			`analyzer "a" suggests invalid fix .*: missing file info for pos`,
+			`analyzer "a" suggests invalid fix .*: no token.File for TextEdit.Pos .0.`,
 			func(pos token.Pos) analysis.Diagnostic {
 				return analysis.Diagnostic{
 					Pos:     pos,
@@ -110,7 +110,7 @@ func TestReportInvalidDiagnostic(t *testing.T) {
 		// TextEdit has invalid End.
 		{
 			"End < Pos",
-			`analyzer "a" suggests invalid fix .*: pos .* > end`,
+			`analyzer "a" suggests invalid fix .*: TextEdit.Pos .* > TextEdit.End .*`,
 			func(pos token.Pos) analysis.Diagnostic {
 				return analysis.Diagnostic{
 					Pos:     pos,

internal/analysisinternal/analysis.go

@@ -419,18 +419,19 @@ func validateFix(fset *token.FileSet, fix *analysis.SuggestedFix) error {
 		start := edit.Pos
 		file := fset.File(start)
 		if file == nil {
-			return fmt.Errorf("missing file info for pos (%v)", edit.Pos)
+			return fmt.Errorf("no token.File for TextEdit.Pos (%v)", edit.Pos)
 		}
 		if end := edit.End; end.IsValid() {
 			if end < start {
-				return fmt.Errorf("pos (%v) > end (%v)", edit.Pos, edit.End)
+				return fmt.Errorf("TextEdit.Pos (%v) > TextEdit.End (%v)", edit.Pos, edit.End)
 			}
 			endFile := fset.File(end)
 			if endFile == nil {
-				return fmt.Errorf("malformed end position %v", end)
+				return fmt.Errorf("no token.File for TextEdit.End (%v; File(start).FileEnd is %d)", end, file.Base()+file.Size())
 			}
 			if endFile != file {
-				return fmt.Errorf("edit spans files %v and %v", file.Name(), endFile.Name())
+				return fmt.Errorf("edit #%d spans files (%v and %v)",
+					i, file.Position(edit.Pos), endFile.Position(edit.End))
 			}
 		} else {
 			edit.End = start // update the SuggestedFix