Add regression test for #58355

src/test/ui/type/issue-58355.rs

@@ -0,0 +1,7 @@
+#![crate_type = "lib"]
+
+pub fn foo(callback: fn() -> dyn ToString) {
+    let mut x: Option<Box<dyn Fn() -> dyn ToString>> = None;
+    x = Some(Box::new(callback));
+    //~^ ERROR: the size for values of type `dyn ToString` cannot be known at compilation time
+}

src/test/ui/type/issue-58355.stderr

@@ -0,0 +1,13 @@
+error[E0277]: the size for values of type `dyn ToString` cannot be known at compilation time
+  --> $DIR/issue-58355.rs:5:14
+   |
+LL |     x = Some(Box::new(callback));
+   |              ^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
+   |
+   = help: within `fn() -> dyn ToString`, the trait `Sized` is not implemented for `dyn ToString`
+   = note: required because it appears within the type `fn() -> dyn ToString`
+   = note: required for the cast from `fn() -> dyn ToString` to the object type `dyn Fn() -> (dyn ToString + 'static)`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.