Auto merge of #98112 - saethlin:mir-alignment-checks, r=oli-obk

Insert alignment checks for pointer dereferences when debug assertions are enabled

Closes rust-lang/rust#54915

- [x] Jake tells me this sounds like a place to use `MirPatch`, but I can't figure out how to insert a new basic block with a new terminator in the middle of an existing basic block, using `MirPatch`. (if nobody else backs up this point I'm checking this as "not actually a good idea" because the code looks pretty clean to me after rearranging it a bit)
- [x] Using `CastKind::PointerExposeAddress` is definitely wrong, we don't want to expose. Calling a function to get the pointer address seems quite excessive. ~I'll see if I can add a new `CastKind`.~ `CastKind::Transmute` to the rescue!
- [x] Implement a more helpful panic message like slice bounds checking.

r? `@oli-obk`

Author: bors

core/src/panicking.rs

@@ -162,6 +162,20 @@ fn panic_bounds_check(index: usize, len: usize) -> ! {
     panic!("index out of bounds: the len is {len} but the index is {index}")
 }
 
+#[cold]
+#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
+#[track_caller]
+#[cfg_attr(not(bootstrap), lang = "panic_misaligned_pointer_dereference")] // needed by codegen for panic on misaligned pointer deref
+fn panic_misaligned_pointer_dereference(required: usize, found: usize) -> ! {
+    if cfg!(feature = "panic_immediate_abort") {
+        super::intrinsics::abort()
+    }
+
+    panic!(
+        "misaligned pointer dereference: address must be a multiple of {required:#x} but is {found:#x}"
+    )
+}
+
 /// Panic because we cannot unwind out of a function.
 ///
 /// This function is called directly by the codegen backend, and must not have