Auto merge of rust-lang#104616 - RalfJung:ctfe-alignment, r=oli-obk,RalfJung

always check alignment during CTFE

We originally disabled alignment checks because they got in the way -- there are some things we do with the interpreter during CTFE which does not correspond to actually running user-written code, but is purely administrative, and we didn't want alignment checks there, so we just disabled them entirely. But with `-Zextra-const-ub-checks` we anyway had to figure out how to disable those alignment checks while doing checks in regular code. So now it is easy to enable CTFE alignment checking by default. Let's see what the perf consequences of that are.

r? `@oli-obk`

Author: bors

src/machine.rs

@@ -22,8 +22,9 @@ use rustc_middle::{
 };
 use rustc_span::def_id::{CrateNum, DefId};
 use rustc_span::Symbol;
-use rustc_target::abi::Size;
+use rustc_target::abi::{Size, Align};
 use rustc_target::spec::abi::Abi;
+use rustc_const_eval::const_eval::CheckAlignment;
 
 use crate::{
     concurrency::{data_race, weak_memory},
@@ -752,15 +753,28 @@ impl<'mir, 'tcx> Machine<'mir, 'tcx> for MiriMachine<'mir, 'tcx> {
     const PANIC_ON_ALLOC_FAIL: bool = false;
 
     #[inline(always)]
-    fn enforce_alignment(ecx: &MiriInterpCx<'mir, 'tcx>) -> bool {
-        ecx.machine.check_alignment != AlignmentCheck::None
+    fn enforce_alignment(ecx: &MiriInterpCx<'mir, 'tcx>) -> CheckAlignment {
+        if ecx.machine.check_alignment == AlignmentCheck::None {
+            CheckAlignment::No
+        } else {
+            CheckAlignment::Error
+        }
     }
 
     #[inline(always)]
     fn use_addr_for_alignment_check(ecx: &MiriInterpCx<'mir, 'tcx>) -> bool {
         ecx.machine.check_alignment == AlignmentCheck::Int
     }
 
+    fn alignment_check_failed(
+        _ecx: &InterpCx<'mir, 'tcx, Self>,
+        has: Align,
+        required: Align,
+        _check: CheckAlignment,
+    ) -> InterpResult<'tcx, ()> {
+        throw_ub!(AlignmentCheckFailed { has, required })
+    }
+
     #[inline(always)]
     fn enforce_validity(ecx: &MiriInterpCx<'mir, 'tcx>) -> bool {
         ecx.machine.validate