Auto format generated templates

[leandrocp]

While working on #4820 I've realized that manually formatting templates with a default config won't work well because the user's project may have a different config for line length, generating a file that won't be formatted in that project context, forcing the user to run mix format to fix it anyways. Besides that a project may have custom plugins that is expected to modify those files.

So why not format on demand? The test shows how it could work. WDYT?

[josevalim]

@leandrocp in this case, it is a new app, so the user most likely won't have existing formatting rules. But I agree this removes the need for us to manually fix our templates (which is annoying) and this will be more useful for phx.gen.* templates.

Also note that both generators also inject content and we should not format files on injected content. So we still need to do some manual fixes.

Finally, formatting may fail, so I recommend wrapping the formatter call in a rescue and returning the original in such cases.

TL;DR: 👍

• We need to apply this to phx.gen.* too
• We need to rescue errors
• Fix whatever is left manually

[josevalim]

Actually, there are complications:

1. On phx.new, we don't have the HEEx formatter, so we can't format them. I still think the above would be helpful for Elixir. So I suggest checking the file extension for .ex and .exs and apply automatic formatting then.

2. For phx.gen.*, if there are plugins and the plugins are not available, then we will call compile, but the odds are low because we need phoenix compiled to invoke the generators. So it should be ok with formatting any .ex, .exs, .heex file.

[josevalim]

Actually, there is no guarantee the user will have a formatter for .heex files, so we can truly only format .ex and .exs for now in both cases. It is a great help, but the rest has to be manually. :)

[chrismccord]

Closing this for now as it has gone stale. We may pick this back up in the future. Thanks!