Yeet owning_ref

Turns out
- `owning_ref` is unsound due to `Box` aliasing stuff
- `rustc` doesn't need 99% of the `owning_ref` API
- `rustc` can use a far simpler abstraction that is `OwnedSlice`

compiler/rustc_data_structures/src/memmap.rs

@@ -2,9 +2,7 @@ use std::fs::File;
 use std::io;
 use std::ops::{Deref, DerefMut};
 
-use crate::owning_ref::StableAddress;
-
-/// A trivial wrapper for [`memmap2::Mmap`] that implements [`StableAddress`].
+/// A trivial wrapper for [`memmap2::Mmap`] (or `Vec<u8>` on WASM).
 #[cfg(not(target_arch = "wasm32"))]
 pub struct Mmap(memmap2::Mmap);
 
@@ -46,12 +44,6 @@ impl AsRef<[u8]> for Mmap {
     }
 }
 
-// SAFETY: On architectures other than WASM, mmap is used as backing storage. The address of this
-// memory map is stable. On WASM, `Vec<u8>` is used as backing storage. The `Mmap` type doesn't
-// export any function that can cause the `Vec` to be re-allocated. As such the address of the
-// bytes inside this `Vec` is stable.
-unsafe impl StableAddress for Mmap {}
-
 #[cfg(not(target_arch = "wasm32"))]
 pub struct MmapMut(memmap2::MmapMut);