Issue

In the vec! macro implementation there is this rule:

($($x:expr),+ $(,)?) => (
    $crate::__rust_force_expr!(<[_]>::into_vec(box [$($x),+]))
);

What exactly is that <[_]> in it?

Solution

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?

Answered By - kmdreko

Answer Checked By - Senaida (PHPFixing Volunteer)