Add flag to modify the working directory assumed by the diagnostics

[Manishearth]

See rust-lang/cargo#9887

Currently rustc prints diagnostics relative to the current working directory of its invocation. However, cargo has a habit of switching the CWD to the workspace root before invoking rustc, which means that the user's CWD ends up being something different.

It would be nice to have a flag (--diagnostic-cwd?) that lets you specify a path relative to which all diagnostics should be printed.

[camelid]

I wonder if --remap-path-prefix could be helpful for this?

[Manishearth]

Existing discussions: #73740 #87745

[Manishearth]

@camelid No, because that impacts the binary, and that would mean that cargo build would have to rebuild the crate every time you cd to a different folder in the workspace

[Urgau]

We now have --remap-path-scope from trim-paths RFC 3127 that applies scopes to the remap path prefixes such as --remap-path-scope=diagnostics which applies the remapping only for the diagnostics.

Closing in favour of the tracking issue.