`MaybeUninit::assume_init_read` should have `noundef` load metadata

I was looking into `array::IntoIter` optimization, and noticed that it wasn't annotating the loads with `noundef` for simple things like `array::IntoIter<i32, N>`.

Turned out to be a more general problem as `MaybeUninit::assume_init_read` isn't marking the load as initialized (<https://rust.godbolt.org/z/Mxd8TPTnv>), which is unfortunate since that's basically its reason to exist.

This PR lowers `ptr::read(p)` to `copy *p` in MIR, which fortuitiously also improves the IR we give to LLVM for things like `mem::replace`.

1 files changed, 4 insertions, 3 deletions

diff --git a/compiler/rustc_lint_defs/src/builtin.rs b/compiler/rustc_lint_defs/src/builtin.rs
index 46ec1a2dca1..91966e75b5f 100644
--- a/compiler/rustc_lint_defs/src/builtin.rs
+++ b/compiler/rustc_lint_defs/src/builtin.rs
@@ -1026,12 +1026,13 @@ declare_lint! {
     /// ### Example
     ///
     /// ```rust,compile_fail
-    /// #![feature(const_ptr_read)]
+    /// #![feature(const_mut_refs)]
     /// const FOO: () = unsafe {
     ///     let x = &[0_u8; 4];
     ///     let y = x.as_ptr().cast::<u32>();
-    ///     y.read(); // the address of a `u8` array is unknown and thus we don't know if
-    ///     // it is aligned enough for reading a `u32`.
+    ///     let mut z = 123;
+    ///     y.copy_to_nonoverlapping(&mut z, 1); // the address of a `u8` array is unknown
+    ///     // and thus we don't know if it is aligned enough for copying a `u32`.
     /// };
     /// ```
     ///