Function pointers. Rust's functions all have unique anonymous types too.
In C or C++ the functions isupper and islower (which are predicates that decide whether a char is an upper or lowercase letter respectively) have the same type.
In Rust such functions always have their own unique anonymous type. char::is_uppercase and char::is_lowercase don't have the same type.
This has a consequence when we want to use a functional-like specialisation. For example "Walter White".starts_with(char::is_uppercase) isn't just passing a function pointer - that can't work, what happens instead is there's an implementation of Pattern on things which match a certain FnMut trait, these two function types match that trait, therefore Pattern is implemented for each type, so there's a monomorphization step, baking a custom implementation of this function for this specific predicate, if you use a different predicate you get a different monomorphization.
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Function pointers. Rust's functions all have unique anonymous types too.
In C or C++ the functions isupper and islower (which are predicates that decide whether a char is an upper or lowercase letter respectively) have the same type.
In Rust such functions always have their own unique anonymous type. char::is_uppercase and char::is_lowercase don't have the same type.
This has a consequence when we want to use a functional-like specialisation. For example "Walter White".starts_with(char::is_uppercase) isn't just passing a function pointer - that can't work, what happens instead is there's an implementation of Pattern on things which match a certain FnMut trait, these two function types match that trait, therefore Pattern is implemented for each type, so there's a monomorphization step, baking a custom implementation of this function for this specific predicate, if you use a different predicate you get a different monomorphization.