Configuring YCM and `rust-analyzer` to work with standalone files

[flu0r1ne]

I was surprised to find that, unlike gopls or other code completion engines I've worked with, rust-analyzer does not work with standalone files. (For example, it fails unceremoniously with a 'rust-analyzer failed to discover workspace' error if there's no Cargo project and Cargo.toml file.) I managed to configure this by integrating information from both the rust-analyzer and YCM documentation. I wanted to share this information; if there's a suitable place in the documentation for it, please let me know. Otherwise, feel free to close this issue, in the hope that it will assist others facing the same problem.

# Configuration script for rust-analyzer to work with standalone files
import subprocess

def GetRustSysroot():
    result = subprocess.check_output( [ 'rustc', '--print', 'sysroot' ], encoding='utf-8' )
    return result.strip()

def IsCargoPackage() -> bool:
    result = subprocess.run( [ 'cargo', 'pkgid' ] )
    return result.returncode == 0

def Settings( **kwargs ):

    if kwargs[ 'language' ] == 'rust':

        if IsCargoPackage():
            return {} # use Cargo.toml autodiscovery

        # Treat current file as a standalone package
        try:
            sysroot = GetRustSysroot()
        except subprocess.CalledProcessError:
            return {}

        file = kwargs['filename']

        return {
            'ls': {
                'linkedProjects': [
                    {
                        "sysroot": sysroot,
                        "crates": [
                            {
                                "root_module": file,
                                "edition": "2021",
                                "deps": []
                            },
                        ],
                    },
                ]
            },
        }

[bstaletic]

Thank you for sharing the info. The easiest way to contr8bute this kind of info is to create a new page on the wiki, which is community maintained.

I am h9nestly not sure how often peopoe need a rust file without a cargo toml. It might also be worth adding a sentence or two to the readme. Maybe just pointing to the documentation of linkedProjects?

Lastly, if we want to catch RA regressions regarding working with standalone foles, we wpuld need tests. That wau, the next time someone tries to update thenightly ycmd uses for rust support, a test might fail in case of RA regression. Probably an overkill.

unlike gopls

Huh... I remember a time where gopls only kind of supported standalone files. It would try, but not very hard. Maybe things have changed.

[flu0r1ne]

Thanks. I added this to the Wiki:

https://github.com/ycm-core/YouCompleteMe/wiki/Rust-Completion-with-Standalone-Files

If other people encounter this and find it useful, I'll add a link to the README.