Auto merge of #140145 - Zoxc:job-server-proxy, r=SparrowLii

Add a jobserver proxy to ensure at least one token is always held

This adds a jobserver proxy to ensure at least one token is always held by `rustc`. Currently with `-Z threads` `rustc` can temporarily give up all its tokens, causing `cargo` to spawn additional `rustc` instances beyond the job limit.

The current behavior causes an issue with `cargo fix` which has a global lock preventing concurrent `rustc` instances, but it also holds a jobserver token, causing a deadlock when `rustc` gives up its token. That is fixed by this PR.

Fixes https://github.com/rust-lang/rust/issues/67385.
Fixes https://github.com/rust-lang/rust/issues/133873.
Fixes https://github.com/rust-lang/rust/issues/140093.

compiler/rustc_query_impl/src/plumbing.rs

@@ -4,6 +4,7 @@
 
 use std::num::NonZero;
 
+use rustc_data_structures::jobserver::Proxy;
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
 use rustc_data_structures::sync::{DynSend, DynSync};
 use rustc_data_structures::unord::UnordMap;
@@ -69,6 +70,11 @@ impl<'tcx> HasDepContext for QueryCtxt<'tcx> {
 impl<'tcx> QueryContext for QueryCtxt<'tcx> {
     type QueryInfo = QueryStackDeferred<'tcx>;
 
+    #[inline]
+    fn jobserver_proxy(&self) -> &Proxy {
+        &*self.jobserver_proxy
+    }
+
     #[inline]
     fn next_job_id(self) -> QueryJobId {
         QueryJobId(