Macro collect 

macro_rules! collect {
    (@count_tts $($tts:tt)*) => { ... };
    (@replace_expr $_tt:tt $sub:expr) => { ... };
    (@collect
        ty: $col_ty:ty,
        es: [$v0:expr, $($vs:expr),* $(,)*],
        // `cb` is an expression that is inserted after each "step" in constructing the collection.  It largely exists for testing purposes.
        cb: ($col:ident) $cb:expr,
    ) => { ... };
    [] => { ... };
    [as $col_ty:ty] => { ... };
    [as $col_ty:ty:] => { ... };
    [as $col_ty:ty: $v0:expr] => { ... };
    [as $col_ty:ty: $v0:expr, $($vs:expr),* $(,)*] => { ... };
    [as $col_ty:ty: $($ks:expr => $vs:expr),+ $(,)*] => { ... };
    [$($vs:expr),+ $(,)*] => { ... };
    [$($ks:expr => $vs:expr),+ $(,)*] => { ... };
}

This macro can be used to easily construct arbitrary collections, including Vec, String, and HashMap. It also endeavours to construct the collection with a single allocation, where possible.

For more details, see the crate documentation.