Alternative workaround for unsized box error.

Author: jesyspa

compiler/rustc_mir/src/borrow_check/type_check/mod.rs

@@ -2021,10 +2021,8 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
 
             Rvalue::NullaryOp(_, ty) => {
                 // Even with unsized locals cannot box an unsized value.
-                if self.unsized_feature_enabled() {
-                    let span = body.source_info(location).span;
-                    self.ensure_place_sized(ty, span);
-                }
+                let span = body.source_info(location).span;
+                self.ensure_place_sized(ty, span);
 
                 let trait_ref = ty::TraitRef {
                     def_id: tcx.require_lang_item(LangItem::Sized, Some(self.last_span)),

src/test/ui/typeck/issue-87935-unsized-box-expr.rs

@@ -0,0 +1,10 @@
+#![feature(box_syntax)]
+// Box expression needs to be movable, and hence has to be of a Sized type.
+fn main() {
+    let _x: Box<[u32]> = box { loop {} };
+    //~^ ERROR: cannot move a value of type [u32]
+
+    // Check that a deduced size does not cause issues.
+    let _y: Box<[u32]> = box [];
+    let _z: Box<[u32; 0]> = box { loop {} };
+}

src/test/ui/typeck/issue-87935-unsized-box-expr.stderr

@@ -0,0 +1,9 @@
+error[E0161]: cannot move a value of type [u32]: the size of [u32] cannot be statically determined
+  --> $DIR/issue-87935-unsized-box-expr.rs:4:26
+   |
+LL |     let _x: Box<[u32]> = box { loop {} };
+   |                          ^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0161`.