Send raw tokens through the LLVM coordination channel

Sending an io::Result of the acquired token is a bit odd as the receiver
can't productively do anything on error (other than panic) and largely
if we've encountered an error when reading from the pipe it's not
something that we expect to happen (i.e., likely indicates something has
gone wrong outside our control or is a rustc programmer error) so a
panic is appropriate.

Author: Mark-Simulacrum

src/librustc_codegen_ssa/back/write.rs

@@ -34,7 +34,6 @@ use rustc_jobserver::{Client, Acquired};
 
 use std::any::Any;
 use std::fs;
-use std::io;
 use std::mem;
 use std::path::{Path, PathBuf};
 use std::str;
@@ -890,7 +889,7 @@ fn execute_lto_work_item<B: ExtraBackendMethods>(
 }
 
 pub enum Message<B: WriteBackendMethods> {
-    Token(io::Result<Acquired>),
+    Token(Acquired),
     NeedsFatLTO {
         result: FatLTOInput<B>,
         worker_id: usize,
@@ -982,6 +981,7 @@ fn start_executing_work<B: ExtraBackendMethods>(
     // get managed in the main loop below.
     let coordinator_send2 = coordinator_send.clone();
     let helper = jobserver.into_helper_thread(move |token| {
+        let token = token.expect("acquired token successfully");
         drop(coordinator_send2.send(Box::new(Message::Token::<B>(token))));
     }).expect("failed to spawn helper thread");
 
@@ -1374,25 +1374,15 @@ fn start_executing_work<B: ExtraBackendMethods>(
                 // this to spawn a new unit of work, or it may get dropped
                 // immediately if we have no more work to spawn.
                 Message::Token(token) => {
-                    match token {
-                        Ok(token) => {
-                            tokens.push(token);
-
-                            if main_thread_worker_state == MainThreadWorkerState::LLVMing {
-                                // If the main thread token is used for LLVM work
-                                // at the moment, we turn that thread into a regular
-                                // LLVM worker thread, so the main thread is free
-                                // to react to codegen demand.
-                                main_thread_worker_state = MainThreadWorkerState::Idle;
-                                running += 1;
-                            }
-                        }
-                        Err(e) => {
-                            let msg = &format!("failed to acquire jobserver token: {}", e);
-                            shared_emitter.fatal(msg);
-                            // Exit the coordinator thread
-                            panic!("{}", msg)
-                        }
+                    tokens.push(token);
+
+                    if main_thread_worker_state == MainThreadWorkerState::LLVMing {
+                        // If the main thread token is used for LLVM work
+                        // at the moment, we turn that thread into a regular
+                        // LLVM worker thread, so the main thread is free
+                        // to react to codegen demand.
+                        main_thread_worker_state = MainThreadWorkerState::Idle;
+                        running += 1;
                     }
                 }