Auto merge of rust-lang#122744 - RalfJung:check-mono-items, r=<try>

experiment: run mono-item collection in check build

r? `@ghost`
This is just to get an upper bound to the perf overhead associated with fixing rust-lang#99682. We could be a bit more clever and only perform collection, not partitioning, or even more clever and do "mentioned items" collection rather than full collection (when rust-lang#122568 lands) -- but for now let's just get a ballpark number.

compiler/rustc_codegen_ssa/src/base.rs

@@ -573,6 +573,10 @@ pub fn codegen_crate<B: ExtraBackendMethods>(
     metadata: EncodedMetadata,
     need_metadata_module: bool,
 ) -> OngoingCodegen<B> {
+    // Run the monomorphization collector and partition the collected items into
+    // codegen units.
+    let codegen_units = tcx.collect_and_partition_mono_items(()).1;
+
     // Skip crate items and just output metadata in -Z no-codegen mode.
     if tcx.sess.opts.unstable_opts.no_codegen || !tcx.sess.opts.output_types.should_codegen() {
         let ongoing_codegen = start_async_codegen(backend, tcx, target_cpu, metadata, None);
@@ -586,10 +590,6 @@ pub fn codegen_crate<B: ExtraBackendMethods>(
 
     let cgu_name_builder = &mut CodegenUnitNameBuilder::new(tcx);
 
-    // Run the monomorphization collector and partition the collected items into
-    // codegen units.
-    let codegen_units = tcx.collect_and_partition_mono_items(()).1;
-
     // Force all codegen_unit queries so they are already either red or green
     // when compile_codegen_unit accesses them. We are not able to re-execute
     // the codegen_unit query from just the DepNode, so an unknown color would