What is “” in Rust?

Breaking down the specific parts of the syntax:

• <T>::f is the syntax to explicitly call f associated with a type T. Usually just T::f is enough, but pedantically, :: requires a path, which is why it is used here since [_] is not. The <...> allows any type to be used as a path. See Why do I need angle brackets in <$a> when implementing macro based on type?
• [T] is the type denoting a slice of type T.
• _ used as a type is a placeholder or “wildcard”. It is not a type itself, but serves to indicate that the type should be inferred. See What does it mean to instantiate a Rust generic with an underscore?