Auto merge of #154327 - WaffleLapkin:drop_in_place_ref, r=RalfJung,scottmcm,saethlin

change the type of the argument of `drop_in_place` lang item to `&mut _`



We used to special case `core::ptr::drop_in_place` when computing LLVM argument attributes with this hack:

https://github.com/rust-lang/rust/blob/db5e2dc248fe5bb26f70d7baec46a3bca9fa3e1d/compiler/rustc_ty_utils/src/abi.rs#L383-L392

This is because even though `drop_in_place` takes a `*mut T` it is semantically a `&mut T` (remember how `&mut Self` is passed to `Drop::drop`). This is apparently relevant for perf.

This PR replaces this hack with a simpler solution -- it makes `drop_in_place` a thin wrapper around newly added `core::ptr::drop_glue`, which is the actual lang item and takes a `&mut T`:

https://github.com/rust-lang/rust/blob/d2563d5003bbecff1efc40c1f5673ceec603825b/library/core/src/ptr/mod.rs#L810-L833

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The rest of the PR is blessing tests and cleaning up things which are not necessary after this change.

One thing that is a bit awkward is that now that `drop_glue` is the actual lang item, a lot of the comments referring to `drop_in_place` are outdated. Should I try fixing that?

I've also changed `async_drop_in_place` to take a `&mut T`, and it simplified the code handling it a bit. (since it's unstable we don't need to introduce a wrapper)

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cc @RalfJung 
Closes rust-lang/rust#154274

Author: bors

example/mini_core.rs

@@ -577,12 +577,10 @@ fn eh_personality() -> ! {
     loop {}
 }
 
-#[lang = "drop_in_place"]
-#[allow(unconditional_recursion)]
-pub unsafe fn drop_in_place<T: ?Sized>(to_drop: *mut T) {
+#[lang = "drop_glue"]
+pub unsafe fn drop_glue<T: ?Sized>(_to_drop: &mut T) {
     // Code here does not matter - this is replaced by the
     // real drop glue by the compiler.
-    drop_in_place(to_drop);
 }
 
 #[lang = "unpin"]