Make child module formatting optional

[constancebello]

Some projects (such as uutils/coreutils) have a non-traditional building scheme where some common files are used by multiple crates. In this case, it is a waste of effort to format child modules (and can lead to confusion, see #399). It would be useful to have a rustfmt.toml option to disable this functionality, while keeping the default behavior.

[nrc]

Sounds like a nice option to have. Not as easy as you might expect (or the label might lead you to expect).

[marcusklaas]

How would one do this? Is there a way to prevent the parser from walking the modules?

[nrc]

You can't stop parsing, but you can stop reformatting by simply not continuing to walk in the Visitor implementation.