Auto merge of rust-lang#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

tests/fail/unaligned_pointers/unaligned_ptr1.rs

@@ -3,7 +3,7 @@
 
 fn main() {
     // Try many times as this might work by chance.
-    for _ in 0..10 {
+    for _ in 0..20 {
         let x = [2u16, 3, 4]; // Make it big enough so we don't get an out-of-bounds error.
         let x = &x[0] as *const _ as *const u32;
         // This must fail because alignment is violated: the allocation's base is not sufficiently aligned.

tests/fail/unaligned_pointers/unaligned_ptr_addr_of.rs

@@ -4,7 +4,7 @@ use std::ptr;
 
 fn main() {
     // Try many times as this might work by chance.
-    for _ in 0..10 {
+    for _ in 0..20 {
         let x = [2u16, 3, 4]; // Make it big enough so we don't get an out-of-bounds error.
         let x = &x[0] as *const _ as *const u32;
         // This must fail because alignment is violated: the allocation's base is not sufficiently aligned.