Auto merge of #141560 - compiler-errors:incr-more-characteristic, r=<try>

[PERF] partition shims into CGU of their characteristic types, if possible

We throw all non-`InstanceKind::Item` mono items into the same fallback CGU. What happens to incremental codegen if we didn't do that, but tried a bit harder to partition shims into the same CGU as their characteristic types?

r? `@ghost`

Author: bors

compiler/rustc_monomorphize/src/partitioning.rs

@@ -633,16 +633,16 @@ fn characteristic_def_id_of_mono_item<'tcx>(
     match mono_item {
         MonoItem::Fn(instance) => {
             let def_id = match instance.def {
-                ty::InstanceKind::Item(def) => def,
-                ty::InstanceKind::VTableShim(..)
+                ty::InstanceKind::Item(..)
+                | ty::InstanceKind::VTableShim(..)
                 | ty::InstanceKind::ReifyShim(..)
-                | ty::InstanceKind::FnPtrShim(..)
+                | ty::InstanceKind::Virtual(..)
                 | ty::InstanceKind::ClosureOnceShim { .. }
                 | ty::InstanceKind::ConstructCoroutineInClosureShim { .. }
-                | ty::InstanceKind::Intrinsic(..)
                 | ty::InstanceKind::DropGlue(..)
-                | ty::InstanceKind::Virtual(..)
-                | ty::InstanceKind::CloneShim(..)
+                | ty::InstanceKind::CloneShim(..) => instance.def_id(),
+                ty::InstanceKind::Intrinsic(..)
+                | ty::InstanceKind::FnPtrShim(..)
                 | ty::InstanceKind::ThreadLocalShim(..)
                 | ty::InstanceKind::FnPtrAddrShim(..)
                 | ty::InstanceKind::FutureDropPollShim(..)