[ra_ap_vfs] Provide fallible API to fetch file path from file ID

[snprajwal]

Currently, the API to retrieve a file path from the VFS with a file ID (https://docs.rs/ra_ap_vfs/latest/ra_ap_vfs/struct.Vfs.html#method.file_path) panics if the file does not exist in the VFS. An exists() boolean method allows a preliminary check, but it would be great to have a fallible API that returns an Option<&VfsPath> and saves the user from having to do an existential check every time. Maybe it can be named file_path_if_exists()? I'm open to ideas, and am happy to contribute this addition!

[lnicola]

How are you getting a FileId that doesn't exist? We seem to use exists in a single place in the entirety of r-a, for an id that's passed back by the client (in a /resolve request).

[snprajwal]

I am writing an analysis engine that builds a Cargo project, and then retrieves individual file IDs while walking the HIR. Building the project returns a (db, vfs): (RootDatabase, Vfs) tuple, but I wasn't sure if the two are guaranteed to be in sync across edits. If that is the case, then a fallible method would not be necessary.

[davidbarsky]

They're not guaranteed to be sync across edits (see #15837), but they will be in sync with the changes planned in #18948.

[snprajwal]

Got it, thank you! Looking forward to seeing the new reworked VFS API. Cheers!