Auto merge of #114528 - dtolnay:globalalloccast, r=cjgillot

Delete some useless casts from global_allocator expansion

These casts are from when #\[global_allocator\] needed to translate back and forth between `*mut u8` and `*mut Opaque`. This should have been cleaned up as part of https://github.com/rust-lang/rust/pull/51241/commits/f6ab74b8e7efed01c1045773b6693f23f6ebd93c.

Difference to code generated by `#[global_allocator] static ALLOCATOR: Allocator = Allocator;`:

```diff
 const _: () = {
     #[rustc_std_internal_symbol]
     unsafe fn __rust_alloc(arg0: usize, arg1: usize) -> *mut u8 {
         ::core::alloc::GlobalAlloc::alloc(
             &ALLOCATOR,
             ::core::alloc::Layout::from_size_align_unchecked(arg0, arg1),
-        ) as *mut u8
+        )
     }
     #[rustc_std_internal_symbol]
     unsafe fn __rust_dealloc(arg0: *mut u8, arg1: usize, arg2: usize) -> () {
         ::core::alloc::GlobalAlloc::dealloc(
             &ALLOCATOR,
-            arg0 as *mut u8,
+            arg0,
             ::core::alloc::Layout::from_size_align_unchecked(arg1, arg2),
         )
     }
     #[rustc_std_internal_symbol]
     unsafe fn __rust_realloc(
         arg0: *mut u8,
         arg1: usize,
         arg2: usize,
         arg3: usize,
     ) -> *mut u8 {
         ::core::alloc::GlobalAlloc::realloc(
             &ALLOCATOR,
-            arg0 as *mut u8,
+            arg0,
             ::core::alloc::Layout::from_size_align_unchecked(arg1, arg2),
             arg3,
-        ) as *mut u8
+        )
     }
     #[rustc_std_internal_symbol]
     unsafe fn __rust_alloc_zeroed(arg0: usize, arg1: usize) -> *mut u8 {
         ::core::alloc::GlobalAlloc::alloc_zeroed(
             &ALLOCATOR,
             ::core::alloc::Layout::from_size_align_unchecked(arg0, arg1),
-        ) as *mut u8
+        )
     }
 };
```

compiler/rustc_builtin_macros/src/global_allocator.rs

@@ -78,11 +78,11 @@ impl AllocFnFactory<'_, '_> {
         };
         let args = method.inputs.iter().map(|ty| self.arg_ty(ty, &mut abi_args, &mut mk)).collect();
         let result = self.call_allocator(method.name, args);
-        let (output_ty, output_expr) = self.ret_ty(&method.output, result);
+        let output_ty = self.ret_ty(&method.output);
         let decl = self.cx.fn_decl(abi_args, ast::FnRetTy::Ty(output_ty));
         let header = FnHeader { unsafety: Unsafe::Yes(self.span), ..FnHeader::default() };
         let sig = FnSig { decl, header, span: self.span };
-        let body = Some(self.cx.block_expr(output_expr));
+        let body = Some(self.cx.block_expr(result));
         let kind = ItemKind::Fn(Box::new(Fn {
             defaultness: ast::Defaultness::Final,
             sig,
@@ -140,8 +140,7 @@ impl AllocFnFactory<'_, '_> {
             AllocatorTy::Ptr => {
                 let ident = ident();
                 args.push(self.cx.param(self.span, ident, self.ptr_u8()));
-                let arg = self.cx.expr_ident(self.span, ident);
+                self.cx.expr_ident(self.span, ident)
-                self.cx.expr_cast(self.span, arg, self.ptr_u8())
             }
 
             AllocatorTy::Usize => {
@@ -156,18 +155,11 @@ impl AllocFnFactory<'_, '_> {
         }
     }
 
-    fn ret_ty(&self, ty: &AllocatorTy, expr: P<Expr>) -> (P<Ty>, P<Expr>) {
+    fn ret_ty(&self, ty: &AllocatorTy) -> P<Ty> {
         match *ty {
-            AllocatorTy::ResultPtr => {
+            AllocatorTy::ResultPtr => self.ptr_u8(),
-                // We're creating:
-                //
-                //      #expr as *mut u8
 
-                let expr = self.cx.expr_cast(self.span, expr, self.ptr_u8());
+            AllocatorTy::Unit => self.cx.ty(self.span, TyKind::Tup(ThinVec::new())),
-                (self.ptr_u8(), expr)
-            }
-
-            AllocatorTy::Unit => (self.cx.ty(self.span, TyKind::Tup(ThinVec::new())), expr),
 
             AllocatorTy::Layout | AllocatorTy::Usize | AllocatorTy::Ptr => {
                 panic!("can't convert `AllocatorTy` to an output")