[steveklabnik]

If `to_string` was a function that was imported into the local namespace, you could. But since it's a method, you can't; you need to provide the trait name.

[dan00]

If there's a function and a method named to_string the user would've to be explicit about which one he uses by adding a namespace, like ToString.

If there's only one to_string function or method, the compiler could just take this one.

[steveklabnik]

To follow up on this slightly, it's not really that this is special syntax. It's that map_err takes a function as an argument, and this is how you refer to this method by name.