Use cargo-metadata command to learn about workspace root

[matklad]

Hi! recently Cargo changed the way it reports errors (they are now realative to the workspace root), and this commits fixes cargo.el to support it.

The idea is to ask Cargo itself about where's the workspace root, and fallback to old Cargo.toml discovery, if that fails.

[Dushistov]

Great, at now with rustc 1.24.0-beta.12 (ed2c0f084 2018-02-12) all features of cargo.el have started work again.

[Dushistov]

@matklad

After some further testing looks there is issue with this PR.
With this patch cargo-process-check and all other commands run
againts workspace, not current project.
And this is break build. Because of some of my workspace crates require different --target=.

For example consider such structure:

a
b
ios
android

before this PR, I can run cargo-process-check against a and b without problems.
For ios and android (that depends on a and b) I need to run cargo check --target=X.

But at now I can not run cargo-process-check for any of those crates.

[davemilter]

@Dushistov @matklad

There is a much simpler example. See attachment.
With current beta and this patch you can not run cargo-process-check inside
crate without errors, if another crate in workspace depends on this crate and you breaks API.

So before that PR and rustc 1.24 you can do some refactoring inside one crate,
run cargo-process-check and when all ready starts adopt all it's dependencies inside workspace,
but now this ability is broken.
See example workspace in attachment:

1.zip

But anyway, thanks @matklad for your efforts to fix #51

[matklad]

So, it seems to me that we should be aware of two directories here:

1. package-root -- the directory with Cargo.toml for the current package.
2. worksapce-root -- the directory with current workspace.

I believe you can get the directories using the following functions:

(defun cargo-process--package-root ()
  "Find the root of the current Cargo package."
  (let* ((root (locate-dominating-file (or buffer-file-name default-directory) "Cargo.toml")))
    (and root (file-truename root))))

(defun cargo-process--workspace-root ()
  "Find the worksapce root using `cargo metadata`."
  (let* ((metadata-text (shell-command-to-string
			 "cargo metadata --format-version 1 --no-deps"))
	 (metadata-json (json-read-from-string metadata-text))
	 (workspace-root (alist-get 'workspace_root metadata-json))
	 (fallback (cargo-process--package-root))
	 (root (or workspace-root fallback)))
    (and root (file-truename root))))

Next, we should run cargo using package-dir as CWD, and process errors using workspace-dir. The problem is, I don't quite understand how exactly the two directories are set in Emacs :)

I believe deafult-directory here is for process cwd:

cargo.el/cargo-process.el

Line 227 in ba652e4

(default-directory (or project-root default-directory)))

But where's the dir for error links?

[matklad]

FWIW, the correct way to handle compiling a crate would be to look at cargo metada for workspace-members key and pass a --package argument to cargo, but looking for Cargo.toml from CWD could be a good approximation.

[Dushistov]

@matklad

But where's the dir for error links?

I belive default directory (cwd of compilation command) is root directory for errors links.
And this is really hard to modify because of compilation-mode is part of emacs.

[matklad]

Hm, interesting!

I think one possible solution here is always launch cargo inside workspace-dir, but pass it an additional --manifest-path $package_dir/Cargo.toml argument.

[kwrooijen]

This should be fixed with #56