Fix ICE and report a human readable error

src/librustc/ty/cast.rs

@@ -58,6 +58,8 @@ pub enum CastKind {
 }
 
 impl<'tcx> CastTy<'tcx> {
+    /// Returns `Some` for integral/pointer casts.
+    /// casts like unsizing casts will return `None`
     pub fn from_ty(t: Ty<'tcx>) -> Option<CastTy<'tcx>> {
         match t.sty {
             ty::Bool => Some(CastTy::Int(IntTy::Bool)),

src/librustc_mir/transform/qualify_min_const_fn.rs

@@ -148,7 +148,7 @@ fn check_rvalue(
         Rvalue::Len(place) | Rvalue::Discriminant(place) | Rvalue::Ref(_, _, place) => {
             check_place(tcx, mir, place, span, PlaceMode::Read)
         }
-        Rvalue::Cast(_, operand, cast_ty) => {
+        Rvalue::Cast(CastKind::Misc, operand, cast_ty) => {
             use rustc::ty::cast::CastTy;
             let cast_in = CastTy::from_ty(operand.ty(mir, tcx)).expect("bad input type for cast");
             let cast_out = CastTy::from_ty(cast_ty).expect("bad output type for cast");
@@ -163,6 +163,10 @@ fn check_rvalue(
                 _ => check_operand(tcx, mir, operand, span),
             }
         }
+        Rvalue::Cast(_, _, _) => Err((
+            span,
+            "only int casts are allowed in const fn".into(),
+        )),
         // binops are fine on integers
         Rvalue::BinaryOp(_, lhs, rhs) | Rvalue::CheckedBinaryOp(_, lhs, rhs) => {
             check_operand(tcx, mir, lhs, span)?;
@@ -177,8 +181,11 @@ fn check_rvalue(
                 ))
             }
         }
-        // checked by regular const fn checks
-        Rvalue::NullaryOp(..) => Ok(()),
+        Rvalue::NullaryOp(NullOp::SizeOf, _) => Ok(()),
+        Rvalue::NullaryOp(NullOp::Box, _) => Err((
+            span,
+            "heap allocations are not allowed in const fn".into(),
+        )),
         Rvalue::UnaryOp(_, operand) => {
             let ty = operand.ty(mir, tcx);
             if ty.is_integral() || ty.is_bool() {

src/test/ui/consts/min_const_fn/bad_const_fn_body_ice.rs

@@ -0,0 +1,5 @@
+const fn foo(a: i32) -> Vec<i32> {
+    vec![1, 2, 3] //~ ERROR heap allocations are not allowed in const fn
+}
+
+fn main() {}

src/test/ui/consts/min_const_fn/bad_const_fn_body_ice.stderr

@@ -0,0 +1,10 @@
+error: heap allocations are not allowed in const fn
+  --> $DIR/bad_const_fn_body_ice.rs:2:5
+   |
+LL |     vec![1, 2, 3] //~ ERROR heap allocations are not allowed in const fn
+   |     ^^^^^^^^^^^^^
+   |
+   = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
+
+error: aborting due to previous error
+