error[E0038]: the trait `IFoo` cannot be made into an object
  --> span-no-self-error.rs:9:7
   |
9  | trait IFoo {
   |       ^^^^ `IFoo` cannot be made into an object
   |
note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
  --> span-no-self-error.rs:10:8
   |
9  | trait IFoo {
   |       ---- this trait cannot be made into an object...
10 |     fn arg_type();
   |        ^^^^^^^^ ...because associated function `arg_type` has no `self` parameter
help: consider turning `arg_type` into a method by giving it a `&self` argument
   |
10 |     fn arg_type(&self);
   |                 +++++
help: alternatively, consider constraining `arg_type` so it does not apply to trait objects
   |
10 |     fn arg_type() where Self: Sized;
   |                   +++++++++++++++++

error: aborting due to previous error

For more information about this error, try `rustc --explain E0038`.