Rollup merge of rust-lang#81412 - hyd-dev:array-assume-init-wrong-assertion, r=m-ou-se

Fix assertion in `MaybeUninit::array_assume_init()` for zero-length arrays

That assertion has a false positive ([playground](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=63922b8c897b04112adcdf346deb1d0e)):
```rust
#![feature(maybe_uninit_array_assume_init)]

use std::mem::MaybeUninit;

enum Uninhabited {}

fn main() {
    unsafe {
        // thread 'main' panicked at 'attempted to instantiate uninhabited type `Uninhabited`'
        MaybeUninit::<Uninhabited>::array_assume_init([]);
    }
}
```
*Previously reported in rust-lang#80600 (comment)

This PR makes it ignore zero-length arrays.

cc rust-lang#80908

Author: JohnTitor

core/src/mem/maybe_uninit.rs

@@ -839,7 +839,7 @@ impl<T> MaybeUninit<T> {
         // * MaybeUnint does not drop, so there are no double-frees
         // And thus the conversion is safe
         unsafe {
-            intrinsics::assert_inhabited::<T>();
+            intrinsics::assert_inhabited::<[T; N]>();
             (&array as *const _ as *const [T; N]).read()
         }
     }

core/tests/mem.rs

@@ -152,6 +152,8 @@ fn uninit_array_assume_init() {
     let array = unsafe { MaybeUninit::array_assume_init(array) };
 
     assert_eq!(array, [3, 1, 4, 1, 5]);
+
+    let [] = unsafe { MaybeUninit::<!>::array_assume_init([]) };
 }
 
 #[test]