Address comments

src/eval_context.rs

@@ -169,6 +169,20 @@ pub fn stack(&self) -> &[Frame<'tcx>] {
         &self.stack
     }
 
+    /// Returns true if the current frame or any parent frame is part of a ctfe.
+    ///
+    /// Used to disable features in const eval, which do not have a rfc enabling
+    /// them or which can't be written in a way that they produce the same output
+    /// that evaluating the code at runtime would produce.
+    pub fn const_env(&self) -> bool {
+        for frame in self.stack.iter().rev() {
+            if let StackPopCleanup::MarkStatic(_) = frame.return_to_block {
+                return true;
+            }
+        }
+        false
+    }
+
     pub(crate) fn str_to_value(&mut self, s: &str) -> EvalResult<'tcx, Value> {
         let ptr = self.memory.allocate_cached(s.as_bytes())?;
         Ok(Value::ByValPair(PrimVal::Ptr(ptr), PrimVal::from_u128(s.len() as u128)))
@@ -655,7 +669,7 @@ pub(super) fn eval_rvalue_into_lvalue(
             }
 
             Len(ref lvalue) => {
-                if self.frame().const_env() {
+                if self.const_env() {
                     return Err(EvalError::NeedsRfc("computing the length of arrays".to_string()));
                 }
                 let src = self.eval_lvalue(lvalue)?;
@@ -704,7 +718,7 @@ pub(super) fn eval_rvalue_into_lvalue(
             }
 
             NullaryOp(mir::NullOp::Box, ty) => {
-                if self.frame().const_env() {
+                if self.const_env() {
                     return Err(EvalError::NeedsRfc("\"heap\" allocations".to_string()));
                 }
                 // FIXME: call the `exchange_malloc` lang item if available
@@ -718,7 +732,7 @@ pub(super) fn eval_rvalue_into_lvalue(
             }
 
             NullaryOp(mir::NullOp::SizeOf, ty) => {
-                if self.frame().const_env() {
+                if self.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");
@@ -1592,12 +1606,6 @@ 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) -> EvalResult<'tcx, Value> {
         // Subtract 1 because we don't store a value for the ReturnPointer, the local with index 0.
         self.locals[local.index() - 1].ok_or(EvalError::DeadLocal)

src/operator.rs

@@ -151,12 +151,8 @@ 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()));
-                }
+            if (!left.is_bytes() && !right.is_bytes()) && self.const_env() {
+                return Err(EvalError::NeedsRfc("Pointer arithmetic or comparison".to_string()));
             }
             match bin_op {
                 Offset if left_kind == Ptr && right_kind == usize => {

src/terminator/mod.rs

@@ -37,7 +37,7 @@ pub(super) fn eval_terminator(
             Goto { target } => self.goto_block(target),
 
             SwitchInt { ref discr, ref values, ref targets, .. } => {
-                if self.frame().const_env() {
+                if self.const_env() {
                     return Err(EvalError::NeedsRfc("branching (if, match, loop, ...)".to_string()));
                 }
                 let discr_val = self.eval_operand(discr)?;
@@ -95,7 +95,7 @@ pub(super) fn eval_terminator(
 
             Drop { ref location, target, .. } => {
                 trace!("TerminatorKind::drop: {:?}, {:?}", location, self.substs());
-                if self.frame().const_env() {
+                if self.const_env() {
                     return Err(EvalError::NeedsRfc("invoking `Drop::drop`".to_string()));
                 }
                 let lval = self.eval_lvalue(location)?;
@@ -430,7 +430,7 @@ 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() {
+                if self.const_env() {
                     return Err(EvalError::NeedsRfc(format!("calling extern function `{}`", path)));
                 }
                 self.call_missing_fn(instance, destination, arg_operands, sig, path)?;
@@ -439,7 +439,7 @@ fn eval_fn_call_inner(
             Err(other) => return Err(other),
         };
 
-        if self.frame().const_env() && !self.tcx.is_const_fn(instance.def_id()) {
+        if self.const_env() && !self.tcx.is_const_fn(instance.def_id()) {
             return Err(EvalError::NotConst(format!("calling non-const fn `{}`", instance)));
         }