gopls/internal/lsp/mod: remove TestModfileRemainsUnchanged

Testdata for this test includes a go.mod file, which is problematic for
a number of reasons (e.g. running from the mod cache; go work use -r).

Replace it with a regtest. But really we should just delete the
"tempModfile" setting (golang/go#61970).

Fixes golang/go#57784

Change-Id: I79726c6106f3118d021a8f9ef52f385f1393d4a8
Reviewed-on: https://go-review.googlesource.com/c/tools/+/518976
gopls-CI: kokoro <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Peter Weinberger <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>
Run-TryBot: Robert Findley <[email protected]>

Author: findleyr

gopls/internal/lsp/mod/mod_test.go

@@ -0,0 +1,41 @@
+// Copyright 2023 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 modfile
+
+import (
+	"testing"
+
+	. "golang.org/x/tools/gopls/internal/lsp/regtest"
+)
+
+// This test replaces an older, problematic test (golang/go#57784). But it has
+// been a long time since the go command would mutate go.mod files.
+//
+// TODO(golang/go#61970): the tempModfile setting should be removed entirely.
+func TestTempModfileUnchanged(t *testing.T) {
+	// badMod has a go.mod file that is missing a go directive.
+	const badMod = `
+-- go.mod --
+module badmod.test/p
+-- p.go --
+package p
+`
+
+	WithOptions(
+		Modes(Default), // no reason to test this with a remote gopls
+		ProxyFiles(workspaceProxy),
+		Settings{
+			"tempModfile": true,
+		},
+	).Run(t, badMod, func(t *testing.T, env *Env) {
+		env.OpenFile("p.go")
+		env.AfterChange()
+		want := "module badmod.test/p\n"
+		got := env.ReadWorkspaceFile("go.mod")
+		if got != want {
+			t.Errorf("go.mod content:\n%s\nwant:\n%s", got, want)
+		}
+	})
+}