Watcher performance improvements

[johnfav03]

Added single-file program reuse via UpdateProgram and skips other unnecessary operations. Also keeps a watchSetDirty flag to track when a full rebuild is required.

[Copilot]

Pull request overview

This PR improves watch-mode performance in the internal/execute watcher by reusing an existing single-file Program via UpdateProgram when safe, and introducing a watchSetDirty flag to avoid unnecessary work unless a full watch set rebuild is required.

Changes:

• Add watchSetDirty state to force full rebuilds only when directory/wildcard inclusion changes are suspected.
• Introduce a fast path that attempts Program.UpdateProgram reuse when exactly one file changed and config/watch set are unchanged.
• Reduce unnecessary wildcard file-name reload operations during rebuilds.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

[jakebailey]

Do you have any data about how this affects things?

[johnfav03]

Do you have any data about how this affects things?

In my testing single-file edits show the biggest performance gain, and the gain scales with project size. I had copilot run a benchmark in vscode, and single-file edits see a 50% decrease in rebuild time on average. Higher-volume edits also see an improvement, but it trends closer to 10%.

[jakebailey]

I think this has two correctness regressions in the watcher fast path. I was able to reduce both to focused tests:

func TestWatcherStartsFromExistingBuildInfo(t *testing.T) {
	input := &tscInput{
		files: FileMap{
			"/home/src/workspaces/project/index.ts":      `export const x: number = 1;`,
			"/home/src/workspaces/project/tsconfig.json": `{"compilerOptions":{"composite":true},"files":["index.ts"]}`,
		},
	}
	sys := newTestSys(input, false)

	result := execute.CommandLine(context.Background(), sys, []string{"-p", "tsconfig.json", "--pretty", "false"}, sys)
	assert.Equal(t, result.Status, tsc.ExitStatusSuccess)
	assert.Assert(t, sys.fsFromFileMap().FileExists("/home/src/workspaces/project/tsconfig.tsbuildinfo"))

	sys.clearOutput()
	defer func() {
		if r := recover(); r != nil {
			t.Fatalf("watch startup with existing build info panicked: %v", r)
		}
	}()
	result = execute.CommandLine(context.Background(), sys, []string{"--watch", "--noEmit", "--pretty", "false"}, sys)
	assert.Equal(t, result.Status, tsc.ExitStatusSuccess)
	assert.Assert(t, result.Watcher != nil)
}

func TestWatcherRebuildsWhenJsxImportSourcePragmaChanges(t *testing.T) {
	input := &tscInput{
		files: FileMap{
			"/home/src/workspaces/project/index.tsx": `/** @jsxImportSource foo */
export const x = <div />;`,
			"/home/src/workspaces/project/tsconfig.json": `{
				"compilerOptions":{"jsx":"react-jsx","module":"esnext","moduleResolution":"bundler","noEmit":true},
				"files":["index.tsx"]
			}`,
		},
		commandLineArgs: []string{"--watch"},
	}
	sys := newTestSys(input, false)
	result := execute.CommandLine(context.Background(), sys, []string{"--watch", "--pretty", "false"}, sys)
	if result.Watcher == nil {
		t.Fatal("expected Watcher to be non-nil in watch mode")
	}
	w := result.Watcher.(*execute.Watcher)

	sys.currentWrite.Reset()
	_ = sys.fsFromFileMap().WriteFile("/home/src/workspaces/project/index.tsx", `/** @jsxImportSource bar */
export const x = <div />;`)
	sys.mockWatchBackend.SendEvents([]fswatch.Event{
		{Kind: fswatch.EventUpdate, Path: "/home/src/workspaces/project/index.tsx"},
	})
	w.DoCycle()

	out := sys.currentWrite.String()
	assert.Assert(t, strings.Contains(out, "bar/jsx-runtime"), "expected updated JSX runtime diagnostic, got: %s", out)
	assert.Assert(t, !strings.Contains(out, "foo/jsx-runtime"), "expected stale JSX runtime diagnostic to be gone, got: %s", out)
}

On this branch:

go test -run 'TestWatcher(StartsFromExistingBuildInfo|RebuildsWhenJsxImportSourcePragmaChanges)$' ./internal/execute/tsctests

fails with:

watch startup with existing build info panicked: GetProgram: should not be called without program

and the JSX test continues reporting foo/jsx-runtime after the file was changed to @jsxImportSource bar.

The first one looks like the new watcher fast path is trying tryUpdateProgram when w.program came from ReadBuildInfoProgram, which can be snapshot-only and has no underlying compiler program. The second looks like UpdateProgram is reusing processedFiles / jsxRuntimeImportSpecifiers, but canReplaceFileInProgram doesn’t compare pragmas, so a pragma-only JSX runtime change can reuse stale module resolution state.