Split out a rustfmt-core crate

[nrc]

Motivation is to have a crate which can be used as a library (e.g., by RLS, Rustdoc) with minimal deps.

We would split out a rustfmt-bin crate which would include the current bin directory, anything for parsing config files, doing diffs, output to terminal, etc. The rustfmt-core crate would be left with just the core reformatting stuff. The trickiest bit of this is that the core crate will need to contain the actual in-memory config, but the config file parsing should be in the bin crate. I think that means we either have to define the config in two places (not very DRY, but tolerable) or use a fancy build script.

[mjkillough]

The rustfmt-core crate would be left with just the core reformatting stuff. The trickiest bit of this is that the core crate will need to contain the actual in-memory config, but the config file parsing should be in the bin crate. I think that means we either have to define the config in two places (not very DRY, but tolerable) or use a fancy build script.

Could there be a rustfmt-config create, which contains all the config machinery, with the serialization/deserialization bits (and dependencies) hidden behind a non-default feature? (rustfmt-core could then depend on it without the feature, and rustfmt-bin with the feature).

I have done a very quick prototype and it seemed to be possible.

[nrc]

That sounds like a good idea! We could also have the config stuff in rustfmt-core, and have a feature for that crate.

[karyon]

this seems to be fixed?

[nrc]

Probably. But I do want to check that it has the right API, etc. We also need a way to work with workspaces and the Rust repo before this is done.

[topecongiro]

AFAIK, we cannot use workspace to separate crates as long as rustfmt is registered as a submodule under the rustc repo. We could still have multiple crates under the same repository without using workspace, however (like we currently do with rustfmt-config-proc-macro).

Starting from 2.0, I would like to re-organize this repo into something like the following:

src/
  cargo-fmt.rs
  rustfmt.rs
  ...and other binaries
rustfmt-config/
  rustfmt-config-proc-macro/
rustfmt-core/
  lib.rs
  expr.rs
  ...and other library files

...that is, we will have three crates inside this repo: rustfmt for binaries, rustfmt-core for rustfmt-as-a-library and rustfmt-config for rustfmt configuration. Or we could have rustfmt-config under rustfmt-core and add a feature for it, like @nrc stated in #1583 (comment).