Implement mem::{zeroed,uninitialized} in terms of MaybeUninit.

Refs #62061

r?oli-obk

Author: alex

src/libcore/intrinsics.rs

@@ -702,15 +702,6 @@ extern "rust-intrinsic" {
     /// state for the type in question.
     pub fn init<T>() -> T;
 
-    /// Creates an uninitialized value.
-    ///
-    /// `uninit` is unsafe because there is no guarantee of what its
-    /// contents are. In particular its drop-flag may be set to any
-    /// state, which means it may claim either dropped or
-    /// undropped. In the general case one must use `ptr::write` to
-    /// initialize memory previous set to the result of `uninit`.
-    pub fn uninit<T>() -> T;
-
     /// Moves a value out of scope without running drop glue.
     pub fn forget<T: ?Sized>(_: T);
 

src/libcore/mem/mod.rs

@@ -450,8 +450,7 @@ pub const fn needs_drop<T>() -> bool {
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub unsafe fn zeroed<T>() -> T {
-    intrinsics::panic_if_uninhabited::<T>();
-    intrinsics::init()
+    MaybeUninit::zeroed().assume_init()
 }
 
 /// Bypasses Rust's normal memory-initialization checks by pretending to
@@ -476,8 +475,7 @@ pub unsafe fn zeroed<T>() -> T {
 #[rustc_deprecated(since = "1.38.0", reason = "use `mem::MaybeUninit` instead")]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub unsafe fn uninitialized<T>() -> T {
-    intrinsics::panic_if_uninhabited::<T>();
-    intrinsics::uninit()
+    MaybeUninit::uninit().assume_init()
 }
 
 /// Swaps the values at two mutable locations, without deinitializing either one.

src/librustc_codegen_llvm/intrinsic.rs

@@ -234,7 +234,7 @@ impl IntrinsicCallMethods<'tcx> for Builder<'a, 'll, 'tcx> {
                 return;
             }
             // Effectively no-ops
-            "uninit" | "forget" => {
+            "forget" => {
                 return;
             }
             "needs_drop" => {

src/librustc_typeck/check/intrinsic.rs

@@ -145,7 +145,6 @@ pub fn check_intrinsic_type<'tcx>(tcx: TyCtxt<'tcx>, it: &hir::ForeignItem) {
             "rustc_peek" => (1, vec![param(0)], param(0)),
             "panic_if_uninhabited" => (1, Vec::new(), tcx.mk_unit()),
             "init" => (1, Vec::new(), param(0)),
-            "uninit" => (1, Vec::new(), param(0)),
             "forget" => (1, vec![param(0)], tcx.mk_unit()),
             "transmute" => (2, vec![ param(0) ], param(1)),
             "move_val_init" => {