Currently all of our VS Code diagnostics are given the source of
`rustc`. However, if you have something like `cargo-watch.command` set
to `clippy` it will also watch for Clippy lints. The `rustc` source is a
bit misleading in that case.
Fortunately, Rust's tool lints (RFC 2103) line up perfectly with VS
Code's concept of `source`. This checks for lints scoped to a given tool
and then splits them in to a `source` and tool-specific `code`.
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