factor getting the discriminant layout to a new method

src/librustc/ty/layout.rs

@@ -127,6 +127,7 @@ impl IntegerExt for Integer {
 
 pub trait PrimitiveExt {
     fn to_ty<'tcx>(&self, tcx: TyCtxt<'tcx>) -> Ty<'tcx>;
+    fn to_int_ty<'tcx>(&self, tcx: TyCtxt<'tcx>) -> Ty<'tcx>;
 }
 
 impl PrimitiveExt for Primitive {
@@ -138,6 +139,16 @@ impl PrimitiveExt for Primitive {
             Pointer => tcx.mk_mut_ptr(tcx.mk_unit()),
         }
     }
+
+    /// Return an *integer* type matching this primitive.
+    /// Useful in particular when dealing with enum discriminants.
+    fn to_int_ty(&self, tcx: TyCtxt<'tcx>) -> Ty<'tcx> {
+        match *self {
+            Int(i, signed) => i.to_ty(tcx, signed),
+            Pointer => tcx.types.usize,
+            Float(..) => bug!("floats do not have an int type"),
+        }
+    }
 }
 
 /// The first half of a fat pointer.

src/librustc_mir/interpret/operand.rs

@@ -5,7 +5,7 @@ use std::convert::TryInto;
 
 use rustc::{mir, ty};
 use rustc::ty::layout::{
-    self, Size, LayoutOf, TyLayout, HasDataLayout, IntegerExt, VariantIdx,
+    self, Size, LayoutOf, TyLayout, HasDataLayout, IntegerExt, PrimitiveExt, VariantIdx,
 };
 
 use rustc::mir::interpret::{
@@ -687,13 +687,7 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                         (dataful_variant.as_u32() as u128, dataful_variant)
                     },
                     Ok(raw_discr) => {
-                        // FIXME: WTF, some discriminants don't have integer type.
-                        use layout::Primitive;
-                        let discr_layout = self.layout_of(match discr_layout.value {
-                            Primitive::Int(int, signed) => int.to_ty(*self.tcx, signed),
-                            Primitive::Pointer => self.tcx.types.usize,
-                            Primitive::Float(..) => bug!("there are no float discriminants"),
-                        })?;
+                        let discr_layout = self.layout_of(discr_layout.value.to_int_ty(*self.tcx))?;
                         let discr_val = ImmTy::from_uint(raw_discr, discr_layout);
                         // We need to use machine arithmetic.
                         let niche_start_val = ImmTy::from_uint(niche_start, discr_layout);

src/librustc_mir/interpret/place.rs

@@ -9,7 +9,7 @@ use rustc::mir;
 use rustc::mir::interpret::truncate;
 use rustc::ty::{self, Ty};
 use rustc::ty::layout::{
-    self, Size, Align, LayoutOf, TyLayout, HasDataLayout, VariantIdx, IntegerExt
+    self, Size, Align, LayoutOf, TyLayout, HasDataLayout, VariantIdx, PrimitiveExt
 };
 use rustc::ty::TypeFoldable;
 
@@ -1060,14 +1060,7 @@ where
                     variant_index.as_usize() < dest.layout.ty.ty_adt_def().unwrap().variants.len(),
                 );
                 if variant_index != dataful_variant {
-                    // FIXME: WTF, some discriminants don't have integer type.
-                    use layout::Primitive;
-                    let discr_layout = self.layout_of(match discr_layout.value {
-                        Primitive::Int(int, signed) => int.to_ty(*self.tcx, signed),
-                        Primitive::Pointer => self.tcx.types.usize,
-                        Primitive::Float(..) => bug!("there are no float discriminants"),
-                    })?;
-
+                    let discr_layout = self.layout_of(discr_layout.value.to_int_ty(*self.tcx))?;
                     // We need to use machine arithmetic.
                     let variants_start = niche_variants.start().as_u32();
                     let variants_start_val = ImmTy::from_uint(variants_start, discr_layout);