Auto merge of #109035 - scottmcm:ptr-read-should-know-undef, r=WaffleLapkin,JakobDegen

Ensure `ptr::read` gets all the same LLVM `load` metadata that dereferencing does

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>`.  Trying to narrow it down, it seems that was because `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.

The root cause is that `ptr::read` is currently implemented via the *untyped* `copy_nonoverlapping`, and thus the `load` doesn't get any type-aware metadata: no `noundef`, no `!range`.  This PR solves that by lowering `ptr::read(p)` to `copy *p` in MIR, for which the backends already do the right thing.

Fortuitiously, this also improves the IR we give to LLVM for things like `mem::replace`, and fixes a couple of long-standing bugs where `ptr::read` on `Copy` types was worse than `*`ing them.

Zulip conversation: <https://rust-lang.zulipchat.com/#narrow/stream/219381-t-libs/topic/Move.20array.3A.3AIntoIter.20to.20ManuallyDrop/near/341189936>

cc `@erikdesjardins` `@JakobDegen` `@workingjubilee` `@the8472`

Fixes #106369
Fixes #73258

0 files changed, 0 insertions, 0 deletions