Auto merge of #142837 - saethlin:maybeuninit-codegen, r=<try>

MaybeImprove MaybeUninit

I'm investigating possible solutions to #139355.

Author: bors

compiler/rustc_codegen_ssa/src/mir/rvalue.rs

@@ -95,14 +95,6 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                 if let mir::Operand::Constant(const_op) = elem {
                     let val = self.eval_mir_constant(const_op);
                     if val.all_bytes_uninit(self.cx.tcx()) {
-                        let size = bx.const_usize(dest.layout.size.bytes());
-                        bx.memset(
-                            dest.val.llval,
-                            bx.const_undef(bx.type_i8()),
-                            size,
-                            dest.val.align,
-                            MemFlags::empty(),
-                        );
                         return;
                     }
                 }

library/core/src/mem/maybe_uninit.rs

@@ -328,7 +328,7 @@ impl<T> MaybeUninit<T> {
     #[inline(always)]
     #[rustc_diagnostic_item = "maybe_uninit_uninit"]
     pub const fn uninit() -> MaybeUninit<T> {
-        MaybeUninit { uninit: () }
+        const { MaybeUninit { uninit: () } }
     }
 
     /// Creates a new `MaybeUninit<T>` in an uninitialized state, with the memory being