internal/diff: fix LineEdits bug in fast path

The fast-path "optimization" that skips the main
algorithm when the input is already line-aligned
failed to check that the replacement text consisted
of complete lines.

(Scare quotes because removing the "optimization" causes
tests to fail. See CL 499377 next in stack for why.)

Thanks to pjw for diagnosing the root cause and
providing the test case in CL 498975.

Fixes golang/go#60379

Change-Id: I2ff92de4550754691442362b8a8932ee42971461
Reviewed-on: https://go-review.googlesource.com/c/tools/+/499376
gopls-CI: kokoro <[email protected]>
Run-TryBot: Alan Donovan <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Peter Weinberger <[email protected]>

Author: adonovan

internal/diff/diff.go

@@ -114,13 +114,15 @@ func lineEdits(src string, edits []Edit) ([]Edit, error) {
 		return nil, err
 	}
 
-	// Do all edits begin and end at the start of a line?
+	// Do all deletions begin and end at the start of a line,
+	// and all insertions end with a newline?
 	// TODO(adonovan, pjw): why does omitting this 'optimization'
 	// cause tests to fail? (TestDiff/insert-line,extra_newline)
 	for _, edit := range edits {
 		if edit.Start >= len(src) || // insertion at EOF
 			edit.Start > 0 && src[edit.Start-1] != '\n' || // not at line start
-			edit.End > 0 && src[edit.End-1] != '\n' { // not at line start
+			edit.End > 0 && src[edit.End-1] != '\n' || // not at line start
+			edit.New != "" && edit.New[len(edit.New)-1] != '\n' { // partial insert
 			goto expand
 		}
 	}

internal/diff/diff_test.go

@@ -107,6 +107,14 @@ func TestLineEdits(t *testing.T) {
 			if !reflect.DeepEqual(got, edits) {
 				t.Errorf("LineEdits got\n%q, want\n%q\n%#v", got, edits, tc)
 			}
+			// make sure that applying the edits gives the expected result
+			fixed, err := diff.Apply(tc.In, got)
+			if err != nil {
+				t.Error(err)
+			}
+			if fixed != tc.Out {
+				t.Errorf("Apply(LineEdits): got %q, want %q", fixed, tc.Out)
+			}
 		})
 	}
 }

internal/diff/difftest/difftest.go

@@ -279,6 +279,23 @@ var TestCases = []struct {
 		Edits:     []diff.Edit{{Start: 3, End: 3, New: "\nbbb"}},
 		LineEdits: []diff.Edit{{Start: 0, End: 4, New: "aaa\nbbb\n"}},
 		Unified:   UnifiedPrefix + "@@ -1,2 +1,3 @@\n aaa\n+bbb\n ccc\n",
+	}, {
+		Name: "60379",
+		In: `package a
+
+type S struct {
+s fmt.Stringer
+}
+`,
+		Out: `package a
+
+type S struct {
+	s fmt.Stringer
+}
+`,
+		Edits:     []diff.Edit{{Start: 27, End: 27, New: "\t"}},
+		LineEdits: []diff.Edit{{Start: 27, End: 42, New: "\ts fmt.Stringer\n"}},
+		Unified:   UnifiedPrefix + "@@ -1,5 +1,5 @@\n package a\n \n type S struct {\n-s fmt.Stringer\n+\ts fmt.Stringer\n }\n",
 	},
 }