Macro hashmapc 

macro_rules! hashmapc {
    (@__ $acc:ident, $key:expr => $val:expr; for $item:pat in $iter:expr; if $cond:expr) => { ... };
    (@__ $acc:ident, $key:expr => $val:expr; for $item:pat in $iter:expr) => { ... };
    (@__ $acc:ident, $key:expr => $val:expr; for $item:pat in $iter:expr; if $cond:expr; $($tail:tt)+) => { ... };
    (@__ $acc:ident, $key:expr => $val:expr; for $item:pat in $iter:expr; $($tail:tt)+) => { ... };
    ($key:expr => $val:expr; $($tail:tt)+) => { ... };
}

Hash Map Comprehension

Creates a HashMap from the contents of the comprehension. It is analogous to Python’s dictionary comprehension except that it uses the => token instead of a colon.

Python code:

numbers = [6, 4, 18]

halves = {str(x): x / 2 for x in numbers}

Rust equivalent code:

let numbers = [6, 4, 18];

let halves = hashmapc!{x.to_string() => x / 2; for x in numbers.iter()};

for &(k, v) in &[("6", 3), ("4", 2), ("18", 9)] {
    assert_eq!(halves[k], v);
}