`clear_env_rust_log` not working for in-process `rustc`/`cargo`

[schomatis]

To reproduce:

RUST_LOG=cargo=debug rls --cli

Unsetting RUST_LOG doesn't work when the logger has already been initialized (in the first call to rls), rustc/cargo in-process calls will be guided by the original RUST_LOG value. Out of process calls (e.g., dependencies compilation) do benefit from clear_env_rust_log as the new process initializes its log (e.g., rustc_log::run).

Not sure if there's a way to reconfigure an already initialized log.

---

Interestingly (and somewhat related to this but not a responsibility of clear_env_rust_log),

RUSTC_LOG=debug rls --cli

produces the rustc log for the in-process (primary packages) compilation. I would have expected those to happen with RUST_LOG (even after the RUST_ to RUSTC_ transition) because clear_env_rust_log uses the same unset env.var mechanism, but they seem to appear (no matter the filter) only with the RUSTC_ variant.

(edit: actually, the output generated in the RUSTC_ case, which contains mostly MIR and codegen information, might not come particularly from in-process rustc calls and I may be confounding sources)

(edit 2: yes, it seems the output is mainly parallel codegen spawned processes, similar to what's described in the first part of the issue)

[schomatis]

Closing, once #1486 lands this loses much of its initial motivation.