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_middle/src/ty/mod.rs

@@ -3010,7 +3010,16 @@ impl<'tcx> TyCtxt<'tcx> {
     /// Returns the possibly-auto-generated MIR of a `(DefId, Subst)` pair.
     pub fn instance_mir(self, instance: ty::InstanceDef<'tcx>) -> &'tcx Body<'tcx> {
         match instance {
-            ty::InstanceDef::Item(def) => self.optimized_mir_opt_const_arg(def),
+            ty::InstanceDef::Item(def) => match self.def_kind(def.did) {
+                DefKind::Const
+                | DefKind::Static
+                | DefKind::AssocConst
+                | DefKind::Ctor(..)
+                | DefKind::AnonConst => self.mir_for_ctfe_opt_const_arg(def),
+                // If the caller wants `mir_for_ctfe` of a function they should not be using
+                // `instance_mir`, so we'll assume const fn also wants the optimized version.
+                _ => self.optimized_mir_or_const_arg_mir(def),
+            },
             ty::InstanceDef::VtableShim(..)
             | ty::InstanceDef::ReifyShim(..)
             | ty::InstanceDef::Intrinsic(..)