Auto merge of #102315 - RalfJung:assert_unsafe_precondition, r=thomcc

add a few more assert_unsafe_precondition

Add debug-assertion checking for `ptr.read()`, `ptr.write(_)`, and `unreachable_unchecked.`

This is quite useful for [cargo-careful](https://github.com/RalfJung/cargo-careful).

Author: bors

core/src/hint.rs

@@ -100,7 +100,10 @@ use crate::intrinsics;
 pub const unsafe fn unreachable_unchecked() -> ! {
     // SAFETY: the safety contract for `intrinsics::unreachable` must
     // be upheld by the caller.
-    unsafe { intrinsics::unreachable() }
+    unsafe {
+        intrinsics::assert_unsafe_precondition!(() => false);
+        intrinsics::unreachable()
+    }
 }
 
 /// Emits a machine instruction to signal the processor that it is running in

core/src/ptr/mod.rs

@@ -1114,6 +1114,7 @@ pub const unsafe fn read<T>(src: *const T) -> T {
     // Also, since we just wrote a valid value into `tmp`, it is guaranteed
     // to be properly initialized.
     unsafe {
+        assert_unsafe_precondition!([T](src: *const T) => is_aligned_and_not_null(src));
         copy_nonoverlapping(src, tmp.as_mut_ptr(), 1);
         tmp.assume_init()
     }
@@ -1307,6 +1308,7 @@ pub const unsafe fn write<T>(dst: *mut T, src: T) {
     // `dst` cannot overlap `src` because the caller has mutable access
     // to `dst` while `src` is owned by this function.
     unsafe {
+        assert_unsafe_precondition!([T](dst: *mut T) => is_aligned_and_not_null(dst));
         copy_nonoverlapping(&src as *const T, dst, 1);
         intrinsics::forget(src);
     }