Auto merge of #78407 - oli-obk:ub_checkable_ctfe, r=RalfJung,pnkfelix

Make CTFE able to check for UB...

... by not doing any optimizations on the `const fn` MIR used in CTFE. This means we duplicate all `const fn`'s MIR now, once for CTFE, once for runtime. This PR is for checking the perf effect, so we have some data when talking about https://github.com/rust-lang/const-eval/blob/master/rfcs/0000-const-ub.md

To do this, we now have two queries for obtaining mir: `optimized_mir` and `mir_for_ctfe`. It is now illegal to invoke `optimized_mir` to obtain the MIR of a const/static item's initializer, an array length, an inline const expression or an enum discriminant initializer. For `const fn`, both `optimized_mir` and `mir_for_ctfe` work, the former returning the MIR that LLVM should use if the function is called at runtime. Similarly it is illegal to invoke `mir_for_ctfe` on regular functions.

This is all checked via appropriate assertions and I don't think it is easy to get wrong, as there should be no `mir_for_ctfe` calls outside the const evaluator or metadata encoding. Almost all rustc devs should keep using `optimized_mir` (or `instance_mir` for that matter).

compiler/rustc_mir/src/const_eval/machine.rs

@@ -201,6 +201,22 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
 
     type MemoryExtra = MemoryExtra;
 
+    fn load_mir(
+        ecx: &InterpCx<'mir, 'tcx, Self>,
+        instance: ty::InstanceDef<'tcx>,
+    ) -> InterpResult<'tcx, &'tcx mir::Body<'tcx>> {
+        match instance {
+            ty::InstanceDef::Item(def) => {
+                if ecx.tcx.is_ctfe_mir_available(def.did) {
+                    Ok(ecx.tcx.mir_for_ctfe_opt_const_arg(def))
+                } else {
+                    throw_unsup!(NoMirFor(def.did))
+                }
+            }
+            _ => Ok(ecx.tcx.instance_mir(instance)),
+        }
+    }
+
     fn find_mir_or_eval_fn(
         ecx: &mut InterpCx<'mir, 'tcx, Self>,
         instance: ty::Instance<'tcx>,