Allow codegen backends to opt-out of parallel codegen

Author: WaffleLapkin

compiler/rustc_codegen_ssa/src/back/write.rs

@@ -383,6 +383,12 @@ impl<B: WriteBackendMethods> CodegenContext<B> {
     }
 }
 
+impl<B: CodegenBackend + WriteBackendMethods> CodegenContext<B> {
+    pub fn parallel(&self) -> bool {
+        self.backend.supports_parallel() && !self.opts.unstable_opts.no_parallel_llvm
+    }
+}
+
 fn generate_lto_work<B: ExtraBackendMethods>(
     cgcx: &CodegenContext<B>,
     needs_fat_lto: Vec<FatLtoInput<B>>,
@@ -1400,7 +1406,7 @@ fn start_executing_work<B: ExtraBackendMethods>(
                             .binary_search_by_key(&cost, |&(_, cost)| cost)
                             .unwrap_or_else(|e| e);
                         work_items.insert(insertion_index, (work, cost));
-                        if !cgcx.opts.unstable_opts.no_parallel_llvm {
+                        if cgcx.parallel() {
                             helper.request_token();
                         }
                     }
@@ -1523,7 +1529,7 @@ fn start_executing_work<B: ExtraBackendMethods>(
                     };
                     work_items.insert(insertion_index, (llvm_work_item, cost));
 
-                    if !cgcx.opts.unstable_opts.no_parallel_llvm {
+                    if cgcx.parallel() {
                         helper.request_token();
                     }
                     assert_eq!(main_thread_state, MainThreadState::Codegenning);

compiler/rustc_codegen_ssa/src/traits/backend.rs

@@ -115,6 +115,13 @@ pub trait CodegenBackend {
         codegen_results: CodegenResults,
         outputs: &OutputFilenames,
     ) -> Result<(), ErrorGuaranteed>;
+
+    /// Returns `true` if this backend can be safely called from multiple threads.
+    ///
+    /// Defaults to `true`.
+    fn supports_parallel(&self) -> bool {
+        true
+    }
 }
 
 pub trait ExtraBackendMethods: