Lint when t.Cleanup closes over t

[jakebailey]

If t is used in a cleanup, things break because the test has already ended. Add a lint rule to detect this and fix the one case.

[Copilot]

Pull request overview

Adds a new customlint analyzer to detect testing.T/testing.B/testing.TB variables being captured and used inside Cleanup closures (which can break because the test has already finished), and updates the one affected test case accordingly.

Changes:

• Register a new cleanup analyzer in the customlint plugin.
• Implement the analyzer to report identifier uses of captured testing variables within Cleanup function literals.
• Add analyzer testdata + golden output, and adjust an LSP completion test to avoid capturing t in a cleanup closure.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
internal/lsp/server_completion_test.go	Updates cleanup usage to avoid closing over t.
_tools/customlint/plugin.go	Registers the new cleanup analyzer in the plugin.
_tools/customlint/cleanup.go	Implements the cleanup analyzer logic.
_tools/customlint/testdata/cleanup/cleanup.go	Adds test cases that should/shouldn’t be flagged by the new analyzer.
_tools/customlint/testdata/cleanup/cleanup.go.golden	Adds expected diagnostics output for the new analyzer.

[Copilot]

closeClient() returns an error when the LSP server goroutines fail to shut down cleanly; ignoring it here can let test failures slip by silently. Consider handling the error without referencing t (e.g., fail-fast via panic or other non-testing.T reporting mechanism) so cleanup issues still cause the test to fail while satisfying the new lint rule.

Suggested change

	t.Cleanup(func() { _ = closeClient() })
	t.Cleanup(func() {
	if err := closeClient(); err != nil {
	panic(err)
	}
	})