diff --git a/src/error.rs b/src/error.rs
index 919232d9eef..38b64870f89 100644
--- a/src/error.rs
+++ b/src/error.rs
@@ -63,6 +63,8 @@ pub enum EvalError<'tcx> {
     HeapAllocNonPowerOfTwoAlignment(u64),
     Unreachable,
     Panic,
+    NeedsRfc(String),
+    NotConst(String),
 }
 
 pub type EvalResult<'tcx, T = ()> = Result<T, EvalError<'tcx>>;
@@ -156,6 +158,10 @@ fn description(&self) -> &str {
                 "entered unreachable code",
             EvalError::Panic =>
                 "the evaluated program panicked",
+            EvalError::NeedsRfc(_) =>
+                "this feature needs an rfc before being allowed inside constants",
+            EvalError::NotConst(_) =>
+                "this feature is not compatible with constant evaluation",
         }
     }
 
@@ -191,6 +197,10 @@ fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
                 write!(f, "expected primitive type, got {}", ty),
             EvalError::Layout(ref err) =>
                 write!(f, "rustc layout computation failed: {:?}", err),
+            EvalError::NeedsRfc(ref msg) =>
+                write!(f, "\"{}\" needs an rfc before being allowed inside constants", msg),
+            EvalError::NotConst(ref msg) =>
+                write!(f, "Cannot evaluate within constants: \"{}\"", msg),
             _ => write!(f, "{}", self.description()),
         }
     }
diff --git a/src/eval_context.rs b/src/eval_context.rs
index 2c27dea2843..74b0e6ec068 100644
--- a/src/eval_context.rs
+++ b/src/eval_context.rs
@@ -655,6 +655,9 @@ pub(super) fn eval_rvalue_into_lvalue(
             }
 
             Len(ref lvalue) => {
+                if self.frame().const_env() {
+                    return Err(EvalError::NeedsRfc("computing the length of arrays".to_string()));
+                }
                 let src = self.eval_lvalue(lvalue)?;
                 let ty = self.lvalue_ty(lvalue);
                 let (_, len) = src.elem_ty_and_len(ty);
@@ -701,6 +704,9 @@ pub(super) fn eval_rvalue_into_lvalue(
             }
 
             NullaryOp(mir::NullOp::Box, ty) => {
+                if self.frame().const_env() {
+                    return Err(EvalError::NeedsRfc("\"heap\" allocations".to_string()));
+                }
                 // FIXME: call the `exchange_malloc` lang item if available
                 if self.type_size(ty)?.expect("box only works with sized types") == 0 {
                     let align = self.type_align(ty)?;
@@ -712,6 +718,9 @@ pub(super) fn eval_rvalue_into_lvalue(
             }
 
             NullaryOp(mir::NullOp::SizeOf, ty) => {
+                if self.frame().const_env() {
+                    return Err(EvalError::NeedsRfc("computing the size of types (size_of)".to_string()));
+                }
                 let size = self.type_size(ty)?.expect("SizeOf nullary MIR operator called for unsized type");
                 self.write_primval(dest, PrimVal::from_u128(size as u128), dest_ty)?;
             }
@@ -1583,6 +1592,12 @@ pub fn modify_local<F>(
 }
 
 impl<'tcx> Frame<'tcx> {
+    pub fn const_env(&self) -> bool {
+        match self.return_to_block {
+            StackPopCleanup::MarkStatic(_) => true,
+            _ => false,
+        }
+    }
     pub fn get_local(&self, local: mir::Local, field: Option<usize>) -> EvalResult<'tcx, Value> {
         // Subtract 1 because we don't store a value for the ReturnPointer, the local with index 0.
         if let Some(field) = field {
diff --git a/src/operator.rs b/src/operator.rs
index e5ed99b2434..09058c23886 100644
--- a/src/operator.rs
+++ b/src/operator.rs
@@ -151,6 +151,13 @@ pub fn binary_op(
         let usize = PrimValKind::from_uint_size(self.memory.pointer_size());
         let isize = PrimValKind::from_int_size(self.memory.pointer_size());
         if !left_kind.is_float() && !right_kind.is_float() {
+            if (!left.is_bytes() && !right.is_bytes()) && self.frame().const_env() {
+                if left.is_ptr() && right.is_ptr() {
+                    return Err(EvalError::NotConst("Comparing pointers".to_string()));
+                } else {
+                    return Err(EvalError::NeedsRfc("Comparing Pointers integers with pointers".to_string()));
+                }
+            }
             match bin_op {
                 Offset if left_kind == Ptr && right_kind == usize => {
                     let pointee_ty = left_ty.builtin_deref(true, ty::LvaluePreference::NoPreference).expect("Offset called on non-ptr type").ty;
diff --git a/src/terminator/mod.rs b/src/terminator/mod.rs
index c773620cbb5..049b66c67a4 100644
--- a/src/terminator/mod.rs
+++ b/src/terminator/mod.rs
@@ -37,6 +37,9 @@ pub(super) fn eval_terminator(
             Goto { target } => self.goto_block(target),
 
             SwitchInt { ref discr, ref values, ref targets, .. } => {
+                if self.frame().const_env() {
+                    return Err(EvalError::NeedsRfc("branching (if, match, loop, ...)".to_string()));
+                }
                 let discr_val = self.eval_operand(discr)?;
                 let discr_ty = self.operand_ty(discr);
                 let discr_prim = self.value_to_primval(discr_val, discr_ty)?;
@@ -92,6 +95,9 @@ pub(super) fn eval_terminator(
 
             Drop { ref location, target, .. } => {
                 trace!("TerminatorKind::drop: {:?}, {:?}", location, self.substs());
+                if self.frame().const_env() {
+                    return Err(EvalError::NeedsRfc("invoking `Drop::drop`".to_string()));
+                }
                 let lval = self.eval_lvalue(location)?;
                 let ty = self.lvalue_ty(location);
                 self.goto_block(target);
@@ -424,11 +430,19 @@ fn eval_fn_call_inner(
         let mir = match self.load_mir(instance.def) {
             Ok(mir) => mir,
             Err(EvalError::NoMirFor(path)) => {
+                if self.frame().const_env() {
+                    return Err(EvalError::NeedsRfc(format!("calling extern function `{}`", path)));
+                }
                 self.call_missing_fn(instance, destination, arg_operands, sig, path)?;
                 return Ok(true);
             },
             Err(other) => return Err(other),
         };
+
+        if self.frame().const_env() && !self.tcx.is_const_fn(instance.def_id()) {
+            return Err(EvalError::NotConst(format!("calling non-const fn `{}`", instance)));
+        }
+        
         let (return_lvalue, return_to_block) = match destination {
             Some((lvalue, block)) => (lvalue, StackPopCleanup::Goto(block)),
             None => (Lvalue::undef(), StackPopCleanup::None),