Auto merge of #85546 - hyd-dev:unwind, r=RalfJung

const-eval: disallow unwinding across functions that `!fn_can_unwind()`

Following https://github.com/rust-lang/miri/pull/1776#discussion_r633074343, so r? `@RalfJung`

This PR turns `unwind` in `StackPopCleanup::Goto` into a new enum `StackPopUnwind`, with a `NotAllowed` variant to indicate that unwinding is not allowed. This variant is chosen based on `rustc_middle::ty::layout::fn_can_unwind()` in `eval_fn_call()` when pushing the frame. A check is added in `unwind_to_block()` to report UB if unwinding happens across a `StackPopUnwind::NotAllowed` frame.

Tested with Miri `HEAD` with [minor changes](https://github.com/rust-lang/miri/compare/HEAD..9cf3c7f0d86325a586fbcbf2acdc9232b861f1d8) and the rust-lang/miri#1776 branch with [these changes](d866c1c52f..626638fbfe).

compiler/rustc_mir/src/transform/const_prop.rs

@@ -33,6 +33,7 @@ use crate::interpret::{
     self, compile_time_machine, AllocId, Allocation, ConstValue, CtfeValidationMode, Frame, ImmTy,
     Immediate, InterpCx, InterpResult, LocalState, LocalValue, MemPlace, Memory, MemoryKind, OpTy,
     Operand as InterpOperand, PlaceTy, Pointer, Scalar, ScalarMaybeUninit, StackPopCleanup,
+    StackPopUnwind,
 };
 use crate::transform::MirPass;
 
@@ -198,7 +199,7 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for ConstPropMachine<'mir, 'tcx>
         _abi: Abi,
         _args: &[OpTy<'tcx>],
         _ret: Option<(&PlaceTy<'tcx>, BasicBlock)>,
-        _unwind: Option<BasicBlock>,
+        _unwind: StackPopUnwind,
     ) -> InterpResult<'tcx, Option<&'mir Body<'tcx>>> {
         Ok(None)
     }
@@ -208,7 +209,7 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for ConstPropMachine<'mir, 'tcx>
         _instance: ty::Instance<'tcx>,
         _args: &[OpTy<'tcx>],
         _ret: Option<(&PlaceTy<'tcx>, BasicBlock)>,
-        _unwind: Option<BasicBlock>,
+        _unwind: StackPopUnwind,
     ) -> InterpResult<'tcx> {
         throw_machine_stop_str!("calling intrinsics isn't supported in ConstProp")
     }