Auto merge of #7848 - ehuss:beta-revert-jobserver, r=alexcrichton

[beta] Revert scalable jobserver.

This reverts #7731, #7829, and #7836.

This should prevent #7840 on beta. I feel more comfortable reverting this than merging #7844, since the impact is still unknown.

src/cargo/core/compiler/context/mod.rs

@@ -69,11 +69,6 @@ pub struct Context<'a, 'cfg> {
     /// metadata files in addition to the rlib itself. This is only filled in
     /// when `pipelining` above is enabled.
     rmeta_required: HashSet<Unit<'a>>,
-
-    /// When we're in jobserver-per-rustc process mode, this keeps those
-    /// jobserver clients for each Unit (which eventually becomes a rustc
-    /// process).
-    pub rustc_clients: HashMap<Unit<'a>, Client>,
 }
 
 impl<'a, 'cfg> Context<'a, 'cfg> {
@@ -117,7 +112,6 @@ impl<'a, 'cfg> Context<'a, 'cfg> {
             unit_dependencies,
             files: None,
             rmeta_required: HashSet::new(),
-            rustc_clients: HashMap::new(),
             pipelining,
         })
     }
@@ -497,21 +491,4 @@ impl<'a, 'cfg> Context<'a, 'cfg> {
     pub fn rmeta_required(&self, unit: &Unit<'a>) -> bool {
         self.rmeta_required.contains(unit) || self.bcx.config.cli_unstable().timings.is_some()
     }
-
-    pub fn new_jobserver(&mut self) -> CargoResult<Client> {
-        let tokens = self.bcx.build_config.jobs as usize;
-        let client = Client::new(tokens).chain_err(|| "failed to create jobserver")?;
-
-        // Drain the client fully
-        for i in 0..tokens {
-            client.acquire_raw().chain_err(|| {
-                format!(
-                    "failed to fully drain {}/{} token from jobserver at startup",
-                    i, tokens,
-                )
-            })?;
-        }
-
-        Ok(client)
-    }
 }