Simplify const_prop logic

Author: syvb

compiler/rustc_mir/src/transform/const_prop.rs

@@ -385,19 +385,17 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
             (),
         );
 
-        let ret = if let Ok(layout) = ecx.layout_of(body.return_ty().subst(tcx, substs)) {
+        let ret = ecx
+            .layout_of(body.return_ty().subst(tcx, substs))
+            .ok()
             // Don't bother allocating memory for ZST types which have no values
             // or for large values.
-            if !layout.is_zst() && layout.size < Size::from_bytes(MAX_ALLOC_LIMIT) {
-                // hopefully all types will allocate, since large types have already been removed,
-                // but check anyways
-                ecx.allocate(layout, MemoryKind::Stack).ok().map(Into::into)
-            } else {
-                None
-            }
-        } else {
-            None
-        };
+            .filter(|ret_layout| {
+                !ret_layout.is_zst() && ret_layout.size < Size::from_bytes(MAX_ALLOC_LIMIT)
+            })
+            // hopefully all types will allocate, since large types have already been removed
+            .and_then(|ret_layout| ecx.allocate(ret_layout, MemoryKind::Stack).ok())
+            .map(Into::into);
 
         ecx.push_stack_frame(
             Instance::new(def_id, substs),