Stabilize the `ignore` configuration

[sdroege]

This one does not affect how the code is formatted but allows specific files or directories to be excluded from formatting.

What is needed for stabilizing this for one of the next releases?

[nrc]

The question I guess is do we think that we really want this config option (which I think the answer is 'yes') and if it has the right options, or at least, are they forward compatible with the right options.

[sdroege]

What's the right way to move forward with this then?

IMHO it would be nicer to rename the configuration to ignore_files or similar to make it more explicit that this is for ignoring whole files and directories (and not modules for example).

The only way how I can currently imagine how this could be extended is to make it possible to provide module names, or generally paths to items in the code. But then this would basically duplicate the #[rustfmt::skip] attribute, so maybe that's not a good idea anyway.

[nrc]

So, one bigger thought is whether we want to control non-formatting things using rustfmt.toml, or whether we should have a separate configuration file. I think changing the name to ignore_files is a good idea (and could be a small, separate PR). I'm not even sure what syntax we accept, but we might think about if its the right one (is it comma-separated, is it whitespace sensitive, does it have the correct behaviour with respect to all OSs when it comes to unicode edge cases, punctuation, etc., should we accept globs?).

What is your use case for this?

[sdroege]

What is your use case for this?

I have a crate that contains quite some autogenerated code, and I want to run rustfmt only over all the other code. The autogenerated code is in the repo because a) generation is non-trivial and b) having the code in the repo allows tracking changes better.

[sdroege]

#3279 would be another solution for me

[otavio]

If this is inside a lib or a module, it would be better to control inside the source code itself and not on the rustfmt.toml as #3279 suggests. This makes what controls the code style more obvious as we need to import it anyway (using mod).

[nrc]

Yeah, I think having the code generator also generate #[rustfmt::skip] would be best

[jbit]

Having an ignore option stabilised and in rustfmt.toml (or another config file) would be really useful when a project pulls in external projects as submodules.
Right now cargo fmt will reformat code in external submodules with the same rustfmt.toml as the main project. This is rarely a good thing, and with a CI system that runs --check, this can cause failing builds.

[richard-uk1]

Two things, both related to ignoring everything.

1. currently ignore = ["."] doesn't work
2. there is no ignore_all directive

Basically I got stung while working on Rocket (which doesn't use rustfmt) because my vim automatically runs cargo fmt on save. This caused me a lot of pain. However generally I want to run cargo fmt so I'd rather keep it built in.

A way to solve this problem is for packages that can't/don't want to use rustfmt to add a rustfmt.toml to their project with ignore = ["."] or ignore_all = true (if it exists).

[topecongiro]

@derekdreery ignore = ["/"] should work. We recently changed the pattern of ignore to mimic .gitignore.

[richard-uk1]

Can confirm that works - could it be made more discoverable?