don't panic on invalid primval types, report an error instead

src/error.rs

 use std::error::Error;
 use std::fmt;
 use rustc::mir;
-use rustc::ty::BareFnTy;
+use rustc::ty::{BareFnTy, Ty};
 use memory::Pointer;
 use rustc_const_math::ConstMathErr;
 use syntax::codemap::Span;
@@ -46,6 +46,7 @@ pub enum EvalError<'tcx> {
     ModifiedConstantMemory,
     AssumptionNotHeld,
     InlineAsm,
+    TypeNotPrimitive(Ty<'tcx>),
 }
 
 pub type EvalResult<'tcx, T> = Result<T, EvalError<'tcx>>;
@@ -106,6 +107,8 @@ fn description(&self) -> &str {
                 "`assume` argument was false",
             EvalError::InlineAsm =>
                 "cannot evaluate inline assembly",
+            EvalError::TypeNotPrimitive(_) =>
+                "expected primitive type, got nonprimitive",
         }
     }
 
@@ -134,6 +137,8 @@ fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
             EvalError::AlignmentCheckFailed { required, has } =>
                write!(f, "tried to access memory with alignment {}, but alignment {} is required",
                       has, required),
+            EvalError::TypeNotPrimitive(ref ty) =>
+                write!(f, "expected primitive type, got {}", ty),
             _ => write!(f, "{}", self.description()),
         }
     }

src/interpreter/mod.rs

@@ -1325,11 +1325,11 @@ fn ty_to_primval_kind(&self, ty: Ty<'tcx>) -> EvalResult<'tcx, PrimValKind> {
                         PrimValKind::from_uint_size(size)
                     }
                 } else {
-                    bug!("primitive read of non-clike enum: {:?}", ty);
+                    return Err(EvalError::TypeNotPrimitive(ty));
                 }
             },
 
-            _ => bug!("primitive read of non-primitive type: {:?}", ty),
+            _ => return Err(EvalError::TypeNotPrimitive(ty)),
         };
 
         Ok(kind)