gopls: fix out of bounds bug in extract

There was a bug where the code would try to access an out of bounds
index when trimming extra space and comments in extract. This change
also adds a regtest for extract.

Fixes golang/go#55271

Change-Id: I7da716a6a68a9678011abf15def47acdea0b33fe
Reviewed-on: https://go-review.googlesource.com/c/tools/+/432135
Run-TryBot: Suzy Mueller <suzmue@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
gopls-CI: kokoro <noreply+kokoro@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>

gopls/internal/lsp/source/extract.go

@@ -676,7 +676,7 @@ func adjustRangeForCommentsAndWhiteSpace(rng span.Range, tok *token.File, conten
 		prevStart = start
 		// If start is within a comment, move start to the end
 		// of the comment group.
-		if file.Comments[startComment].Pos() <= start && start < file.Comments[startComment].End() {
+		if startComment < len(file.Comments) && file.Comments[startComment].Pos() <= start && start < file.Comments[startComment].End() {
 			start = file.Comments[startComment].End()
 			startComment++
 		}
@@ -697,11 +697,16 @@ func adjustRangeForCommentsAndWhiteSpace(rng span.Range, tok *token.File, conten
 		// Find the index for the first comment that ends after the range end.
 		return file.Comments[i].End() >= rng.End
 	})
+	// Search will return n if not found, so we need to adjust if there are no
+	// comments that would match.
+	if endComment == len(file.Comments) {
+		endComment = -1
+	}
 	for prevEnd != end {
 		prevEnd = end
 		// If end is within a comment, move end to the start
 		// of the comment group.
-		if file.Comments[endComment].Pos() < end && end <= file.Comments[endComment].End() {
+		if endComment >= 0 && file.Comments[endComment].Pos() < end && end <= file.Comments[endComment].End() {
 			end = file.Comments[endComment].Pos()
 			endComment--
 		}

gopls/internal/regtest/misc/extract_test.go

@@ -0,0 +1,69 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+package misc
+
+import (
+	"testing"
+
+	. "golang.org/x/tools/gopls/internal/lsp/regtest"
+	"golang.org/x/tools/gopls/internal/lsp/tests/compare"
+
+	"golang.org/x/tools/gopls/internal/lsp/protocol"
+)
+
+func TestExtractFunction(t *testing.T) {
+	const files = `
+-- go.mod --
+module mod.com
+
+go 1.12
+-- main.go --
+package main
+
+func Foo() int {
+	a := 5
+	return a
+}
+`
+	Run(t, files, func(t *testing.T, env *Env) {
+		env.OpenFile("main.go")
+
+		start := env.RegexpSearch("main.go", "a := 5").ToProtocolPosition()
+		end := env.RegexpSearch("main.go", "return a").ToProtocolPosition()
+
+		actions, err := env.Editor.CodeAction(env.Ctx, "main.go", &protocol.Range{Start: start, End: end}, nil)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		// Find the extract function code action.
+		var extractFunc *protocol.CodeAction
+		for _, action := range actions {
+			if action.Kind == protocol.RefactorExtract && action.Title == "Extract function" {
+				extractFunc = &action
+				break
+			}
+		}
+		if extractFunc == nil {
+			t.Fatal("could not find extract function action")
+		}
+
+		env.ApplyCodeAction(*extractFunc)
+		want := `package main
+
+func Foo() int {
+	a := newFunction()
+	return a
+}
+
+func newFunction() int {
+	a := 5
+	return a
+}
+`
+		if got := env.Editor.BufferText("main.go"); got != want {
+			t.Fatalf("TestFillStruct failed:\n%s", compare.Text(want, got))
+		}
+	})
+}