VS Code problem matcher are restricted to be static "regexes". You can't
create a problem matcher dynamically, and you can't use custom code in
lieu of problem matcher.
This creates a problem for rust/cargo compiler errors. They use paths
relative to the root of the Cargo workspace, but VS Code doesn't
necessary know where that root is.
Luckily, there's a way out: our current problem matcher is defined like
this:
"fileLocation": [ "autoDetect", "${workspaceRoot}" ],
That means that relative pahts would be resoleved relative to workspace
root. VS Code allows to specify a command inside `${}`. So we can plug
custom logic there to fetch Cargo's workspace root!
And that's exactly what this PR is doing!
As per matklad, we now pass the responsibility for finding the binary to the frontend.
Also, added caching for finding the binary path to reduce
the amount of filesystem interactions.
Move the task provider anonymous class into a real class, as this seems
to be how Microsoft do this in their documentation.
resolveTask is now implemented, which is used by VSCode to determine how
to execute tasks that the user has defined in tasks.json.
