check that required_consts really contains all required consts

Author: RalfJung

src/librustc_mir/interpret/machine.rs

@@ -119,6 +119,11 @@ pub trait Machine<'mir, 'tcx>: Sized {
     /// that is added to the memory so that the work is not done twice.
     const GLOBAL_KIND: Option<Self::MemoryKind>;
 
+    /// Whether this machine allow const evaluation of `mir::Operand` to fail.
+    /// Usually machines do not, as frame pushing already makes sure that all consts evaluated.
+    /// However, ConstProp does not do regular frame pushing.
+    const PERMIT_LATE_CONST_EVAL_FAIL: bool = false;
+
     /// Whether memory accesses should be alignment-checked.
     fn enforce_alignment(memory_extra: &Self::MemoryExtra) -> bool;
 

src/librustc_mir/interpret/operand.rs

@@ -517,7 +517,15 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             Constant(ref constant) => {
                 let val =
                     self.subst_from_current_frame_and_normalize_erasing_regions(constant.literal);
-                self.const_to_op(val, layout)?
+                // This should not raise any errors, except for TooGeneric (during ConstProp).
+                self.const_to_op(val, layout).map_err(|err|
+                    if M::PERMIT_LATE_CONST_EVAL_FAIL {
+                        err
+                    } else {
+                        err.print_backtrace();
+                        span_bug!(self.cur_span(), "const eval should have failed already when evaluating required_consts, so this error is unexpected: {:?}", err)
+                    }
+                )?
             }
         };
         trace!("{:?}: {:?}", mir_op, *op);

src/librustc_mir/transform/const_prop.rs

@@ -179,6 +179,8 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for ConstPropMachine<'mir, 'tcx>
 
     type MemoryExtra = ();
 
+    const PERMIT_LATE_CONST_EVAL_FAIL: bool = true;
+
     fn find_mir_or_eval_fn(
         _ecx: &mut InterpCx<'mir, 'tcx, Self>,
         _instance: ty::Instance<'tcx>,