ices/58355.rs: fixed with errors

[github-actions]

Issue: rust-lang/rust#58355

#![crate_type = "lib"]

pub fn foo(callback: fn() -> ToString) {
    let mut x: Option<Box<Fn() -> ToString>> = None;
    x = Some(Box::new(callback));
}

=== stdout ===
=== stderr ===
warning: trait objects without an explicit `dyn` are deprecated
 --> /home/runner/work/glacier/glacier/ices/58355.rs:3:30
  |
3 | pub fn foo(callback: fn() -> ToString) {
  |                              ^^^^^^^^
  |
  = note: `#[warn(bare_trait_objects)]` on by default
  = warning: this is accepted in the current edition (Rust 2018) but is a hard error in Rust 2021!
  = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/warnings-promoted-to-error.html>
help: use `dyn`
  |
3 | pub fn foo(callback: fn() -> dyn ToString) {
  |                              +++

warning: trait objects without an explicit `dyn` are deprecated
 --> /home/runner/work/glacier/glacier/ices/58355.rs:4:27
  |
4 |     let mut x: Option<Box<Fn() -> ToString>> = None;
  |                           ^^^^^^^^^^^^^^^^
  |
  = warning: this is accepted in the current edition (Rust 2018) but is a hard error in Rust 2021!
  = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/warnings-promoted-to-error.html>
help: use `dyn`
  |
4 |     let mut x: Option<Box<dyn Fn() -> ToString>> = None;
  |                           +++

warning: trait objects without an explicit `dyn` are deprecated
 --> /home/runner/work/glacier/glacier/ices/58355.rs:4:35
  |
4 |     let mut x: Option<Box<Fn() -> ToString>> = None;
  |                                   ^^^^^^^^
  |
  = warning: this is accepted in the current edition (Rust 2018) but is a hard error in Rust 2021!
  = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/warnings-promoted-to-error.html>
help: use `dyn`
  |
4 |     let mut x: Option<Box<Fn() -> dyn ToString>> = None;
  |                                   +++

error[E0277]: the size for values of type `dyn ToString` cannot be known at compilation time
 --> /home/runner/work/glacier/glacier/ices/58355.rs:5:14
  |
5 |     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; 3 warnings emitted

For more information about this error, try `rustc --explain E0277`.
==============