Rollup merge of #116399 - WaffleLapkin:erase_small_things, r=cjgillot

Small changes w/ `query::Erase<_>`

r? `@cjgillot`
cc `@Zoxc`

Author: matthiaskrgr

compiler/rustc_middle/src/query/erase.rs

@@ -2,7 +2,8 @@ use crate::mir;
 use crate::query::CyclePlaceholder;
 use crate::traits;
 use crate::ty::{self, Ty};
-use std::mem::{size_of, transmute_copy, MaybeUninit};
+use std::intrinsics::transmute_unchecked;
+use std::mem::{size_of, MaybeUninit};
 
 #[derive(Copy, Clone)]
 pub struct Erased<T: Copy> {
@@ -29,31 +30,40 @@ pub fn erase<T: EraseType>(src: T) -> Erase<T> {
     };
 
     Erased::<<T as EraseType>::Result> {
+        // `transmute_unchecked` is needed here because it does not have `transmute`'s size check
+        // (and thus allows to transmute between `T` and `MaybeUninit<T::Result>`) (we do the size
+        // check ourselves in the `const` block above).
+        //
+        // `transmute_copy` is also commonly used for this (and it would work here since
+        // `EraseType: Copy`), but `transmute_unchecked` better explains the intent.
+        //
         // SAFETY: It is safe to transmute to MaybeUninit for types with the same sizes.
-        data: unsafe { transmute_copy(&src) },
+        data: unsafe { transmute_unchecked::<T, MaybeUninit<T::Result>>(src) },
     }
 }
 
 /// Restores an erased value.
 #[inline(always)]
 pub fn restore<T: EraseType>(value: Erase<T>) -> T {
     let value: Erased<<T as EraseType>::Result> = value;
+    // See comment in `erase` for why we use `transmute_unchecked`.
+    //
     // SAFETY: Due to the use of impl Trait in `Erase` the only way to safely create an instance
     // of `Erase` is to call `erase`, so we know that `value.data` is a valid instance of `T` of
     // the right size.
-    unsafe { transmute_copy(&value.data) }
+    unsafe { transmute_unchecked::<MaybeUninit<T::Result>, T>(value.data) }
 }
 
 impl<T> EraseType for &'_ T {
-    type Result = [u8; size_of::<*const ()>()];
+    type Result = [u8; size_of::<&'static ()>()];
 }
 
 impl<T> EraseType for &'_ [T] {
-    type Result = [u8; size_of::<*const [()]>()];
+    type Result = [u8; size_of::<&'static [()]>()];
 }
 
 impl<T> EraseType for &'_ ty::List<T> {
-    type Result = [u8; size_of::<*const ()>()];
+    type Result = [u8; size_of::<&'static ty::List<()>>()];
 }
 
 impl<I: rustc_index::Idx, T> EraseType for &'_ rustc_index::IndexSlice<I, T> {