feat: Print formatted file names in mix format --verbose

[georgeguimaraes]

mix format is completely silent on success right now, so you have no idea which files it actually changed. This is also useful for LLMs reading the output of mix format to know what was touched. This adds a --verbose flag that prints files as they're formatted, using the same * verb path convention as the rest of mix:

$ mix format --verbose
* formatting lib/my_app/router.ex
* formatting lib/my_app/server.ex

Already-formatted files produce no output. Default behavior (silent) is unchanged.

[Copilot]

Pull request overview

This PR adds informative output to mix format that displays which files were actually formatted, addressing the current silent behavior that provides no feedback about what changed. The feature follows Mix's established convention of using * verb path format for action messages.

Changes:

• Adds formatted file output using * formatting path format with ANSI coloring
• Introduces --quiet flag to restore previous silent behavior
• Refactors internal tracking to distinguish between formatted and unchanged files

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
lib/mix/lib/mix/tasks/format.ex	Core implementation: adds --quiet flag, refactors write_or_print to return formatting status, updates collect_status and check! to track and print formatted files
lib/mix/test/mix/tasks/format_test.exs	Adds three tests verifying formatted file output, --quiet flag suppression, and --dry-run suppression

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[Copilot]

Missing test coverage for --check-formatted flag interaction with the new formatting output feature. While the implementation correctly doesn't print formatting messages when --check-formatted is used (since it doesn't actually format files), this behavior should be explicitly tested to prevent regressions. Consider adding a test similar to the --dry-run and --quiet tests that verifies no formatting messages are printed when using --check-formatted.

Suggested change

	test "does not print formatted file names with --check-formatted", context do
	in_tmp(context.test, fn ->
	File.write!("a.ex", """
	foo bar
	""")
	Mix.Tasks.Format.run(["a.ex, "--check-formatted"])
	refute_received {:mix_shell, :info, ["* formatting a.ex"]}
	end)
	end

[josevalim]

This behaviour needs to be opt-in. We can have a --verbose mode.

[georgeguimaraes]

This behaviour needs to be opt-in. We can have a --verbose mode.

:( ok

[georgeguimaraes]

@josevalim updated

[josevalim]

💚 💙 💜 💛 ❤️

[georgeguimaraes]

😘