Let InstCombine remove Clone shims inside Clone shims

Co-authored-by: scottmcm <scottmcm@users.noreply.github.com>

compiler/rustc_middle/src/ty/sty.rs

@@ -1864,9 +1864,9 @@ impl<'tcx> Ty<'tcx> {
             // Definitely absolutely not copy.
             ty::Ref(_, _, hir::Mutability::Mut) => false,
 
-            // Thin pointers & thin shared references are pure-clone-copy, but for
-            // anything with custom metadata it might be more complicated.
-            ty::Ref(_, _, hir::Mutability::Not) | ty::RawPtr(..) => false,
+            // The standard library has a blanket Copy impl for shared references and raw pointers,
+            // for all unsized types.
+            ty::Ref(_, _, hir::Mutability::Not) | ty::RawPtr(..) => true,
 
             ty::Coroutine(..) | ty::CoroutineWitness(..) => false,
 

compiler/rustc_mir_transform/src/shim.rs

@@ -17,7 +17,7 @@ use std::iter;
 
 use crate::{
     abort_unwinding_calls, add_call_guards, add_moves_for_packed_drops, deref_separator,
-    mentioned_items, pass_manager as pm, remove_noop_landing_pads, simplify,
+    instsimplify, mentioned_items, pass_manager as pm, remove_noop_landing_pads, simplify,
 };
 use rustc_middle::mir::patch::MirPatch;
 use rustc_mir_dataflow::elaborate_drops::{self, DropElaborator, DropFlagMode, DropStyle};
@@ -154,6 +154,7 @@ fn make_shim<'tcx>(tcx: TyCtxt<'tcx>, instance: ty::InstanceKind<'tcx>) -> Body<
             &deref_separator::Derefer,
             &remove_noop_landing_pads::RemoveNoopLandingPads,
             &simplify::SimplifyCfg::MakeShim,
+            &instsimplify::InstSimplify,
             &abort_unwinding_calls::AbortUnwindingCalls,
             &add_call_guards::CriticalCallEdges,
         ],

library/rtstartup/rsbegin.rs

@@ -29,6 +29,8 @@ trait Copy {}
 #[lang = "freeze"]
 auto trait Freeze {}
 
+impl<T: ?Sized> Copy for *mut T {}
+
 #[lang = "drop_in_place"]
 #[inline]
 #[allow(unconditional_recursion)]

library/rtstartup/rsend.rs

@@ -17,6 +17,8 @@ trait Copy {}
 #[lang = "freeze"]
 auto trait Freeze {}
 
+impl<T: ?Sized> Copy for *mut T {}
+
 #[lang = "drop_in_place"]
 #[inline]
 #[allow(unconditional_recursion)]

tests/assembly/simd-intrinsic-mask-load.rs

@@ -18,6 +18,7 @@ pub trait Sized {}
 
 #[lang = "copy"]
 trait Copy {}
+impl<T: ?Sized> Copy for *const T {}
 
 #[repr(simd)]
 pub struct i8x16([i8; 16]);

tests/assembly/simd-intrinsic-mask-store.rs

@@ -18,6 +18,7 @@ pub trait Sized {}
 
 #[lang = "copy"]
 trait Copy {}
+impl<T: ?Sized> Copy for *mut T {}
 
 #[repr(simd)]
 pub struct i8x16([i8; 16]);

tests/codegen/avr/avr-func-addrspace.rs

@@ -17,6 +17,7 @@
 pub trait Sized {}
 #[lang = "copy"]
 pub trait Copy {}
+impl<T: ?Sized> Copy for *const T {}
 #[lang = "receiver"]
 pub trait Receiver {}
 #[lang = "tuple_trait"]

tests/codegen/clone-shims.rs

@@ -0,0 +1,15 @@
+// Clone shims for aggregates are generated by just calling the Clone shims for all their members.
+// Those calls generate a lot of unnecessary IR if the members are Copy. This test ensures that we
+// optimize away those inner calls without needing to inline them.
+
+//@ compile-flags: -Cno-prepopulate-passes -Csymbol-mangling-version=v0 -Zinline-mir=no
+#![crate_type = "lib"]
+
+pub type Test = (i32, i32, *const i32);
+pub static TEST: fn(&Test) -> Test = <Test as core::clone::Clone>::clone;
+
+// CHECK-NOT: call <i32 as core::clone::Clone>::clone
+// CHECK-NOT: call <*const i32 as core::clone::Clone>::clone
+// CHECK:     ; <(i32, i32, *const i32) as core::clone::Clone>::clone
+// CHECK-NOT: call <i32 as core::clone::Clone>::clone
+// CHECK-NOT: call <*const i32 as core::clone::Clone>::clone

tests/codegen/emcripten-catch-unwind.rs

@@ -16,6 +16,8 @@ trait Freeze {}
 #[lang = "copy"]
 trait Copy {}
 
+impl<T> Copy for *mut T {}
+
 #[rustc_intrinsic]
 fn size_of<T>() -> usize {
     loop {}

tests/codegen/riscv-abi/riscv64-lp64-lp64f-lp64d-abi.rs

@@ -18,6 +18,7 @@ impl Copy for i64 {}
 impl Copy for u64 {}
 impl Copy for f32 {}
 impl Copy for f64 {}
+impl<T> Copy for *mut T {}
 
 // CHECK: define void @f_void()
 #[no_mangle]

tests/codegen/sanitizer/kcfi/emit-type-metadata-trait-objects.rs

@@ -15,6 +15,7 @@
 trait Sized {}
 #[lang = "copy"]
 trait Copy {}
+impl<T: ?Sized> Copy for &T {}
 #[lang = "receiver"]
 trait Receiver {}
 #[lang = "dispatch_from_dyn"]