New disable_all_formatting config option

[cmbrandenburg]

This pull request adds a new Boolean configuration option called disable_all_formatting, which causes rustfmt to skip all formatting in all files.

This option could be useful for someone who:

1. Has configured their text editor to automatically run rustfmt when saving a Rust source file, and,
2. Is editing source files in a Rust project that's not set up to use rustfmt.

Currently, in that scenario, the programmer will often inadvertently format lots of Rust code—even code unrelated to what the programmer is working on. The programmer must then undo all the formatting changes while keeping their intended changes.

Whereas, with the new disable_all_formatting option, the programmer can drop this .rustfmt.toml into the project's top-level directory and avoid formatting unrelated sections of code.

disable_all_formatting: true

[brandur]

At the risk of piling onto this one, I just wanted to say that I've run into this problem a few times now, and was mildly astonished that no current resolution exists.

I've enabled Rust.vim with let g:rustfmt_autosave = 1, which works fine for my own projects or any others that have a rustfmt.toml. However, when I come across a project not setup for rustfmt, there doesn't appear to be any way to avoid accidental reformat-the-world situations except for me to reconfigure Rust.vim at a global level.

The proper course of action is probably to convince the project owner to support rustfmt, but it'd be nice to have an escape hatch to temporarily avoid having to shave that yak by just creating a local rustfmt.toml with a disable directive in it. I came here thinking to submit a PR for it, but it turns out that this very timely one was already here.

Onto the color of the shed: the naming of disable_all_formatting seems to be a little redundant given that you're already tweaking an option in rustfmt.toml and we know that rustfmt's raison d'être is formatting. Something more succinct like "disable" might be enough.

[nrc]

Thanks for the PR! Seems like a good idea to add this, although it does seem to be working around a deficiency in the editor, which is a bit of a shame. A couple of comments need to be addressed before landing, but the strategy is good.

[nrc]

This test doesn't do anything because there is not corresponding file in tests/target. However, since you only want to do an idempotent test, you could move it to tests/target and it would be fine.

[cmbrandenburg]

You're right. I misread the Contributing.md instructions and put the file in the wrong directory.

Creating a test is as easy as creating a new file in ./tests/source/ and an equally named one in ./tests/target/. If it is only required that rustfmt leaves a piece of code unformatted, it may suffice to only create a target file.

[nrc]

You probably want this check in the caller, rather than here, since there might be line-based reformating as well as AST-based reformatting

[cmbrandenburg]

Does anyone else have an opinion about the config option name? @brandur recommends shortening it from disable_all_formatting to disable. Either is fine by me.

[nrc]

Thanks for the changes!

I don't think either name for the option is ideal, but I can't think of a better one.

[brandur]

FWIW, I don't feel too strongly about it either. If no one feels too strongly one way or the other, I'd defer to the author of the patch.