Require a box expression's type to be Sized

compiler/rustc_middle/src/traits/mod.rs

@@ -225,6 +225,8 @@ pub enum ObligationCauseCode<'tcx> {
     SizedReturnType,
     /// Yield type must be `Sized`.
     SizedYieldType,
+    /// Box expression result type must be `Sized`.
+    SizedBoxType,
     /// Inline asm operand type must be `Sized`.
     InlineAsmSized,
     /// `[T, ..n]` implies that `T` must be `Copy`.

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

@@ -1893,9 +1893,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
 
                 // While this is located in `nll::typeck` this error is not
                 // an NLL error, it's a required check to prevent creation
-                // of unsized rvalues in certain cases:
-                // * operand of a box expression
-                // * callee in a call expression
+                // of unsized rvalues in a call expression.
                 diag.emit();
             }
         }

compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

@@ -2072,6 +2072,9 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
             ObligationCauseCode::SizedYieldType => {
                 err.note("the yield type of a generator must have a statically known size");
             }
+            ObligationCauseCode::SizedBoxType => {
+                err.note("the type of a box expression must have a statically known size");
+            }
             ObligationCauseCode::AssignmentLhsSized => {
                 err.note("the left-hand-side of an assignment must have a statically known size");
             }

compiler/rustc_typeck/src/check/expr.rs

@@ -313,6 +313,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             _ => NoExpectation,
         });
         let referent_ty = self.check_expr_with_expectation(expr, expected_inner);
+        self.require_type_is_sized(referent_ty, expr.span, traits::SizedBoxType);
         self.tcx.mk_box(referent_ty)
     }