Macro comp

macro_rules! comp {
    (@parse_if
        $item_expr: expr;
        if $condition: expr
    ) => { ... };
    (@parse_if
        $item_expr: expr;
        if let $( $if_let_pattern: pat )|+ = $if_let_expr: expr
    ) => { ... };
    (@parse_if
        $item_expr: expr;
        if $condition: expr;
        for $($rest: tt)*
    ) => { ... };
    (@parse_if
        $item_expr: expr;
        if let $( $if_let_pattern: pat )|+ = $if_let_expr: expr;
        for $($rest: tt)*
    ) => { ... };
    (@parse_if
        $item_expr: expr;
        if $condition: expr;
        if $($rest: tt)*
    ) => { ... };
    (@parse_if
        $item_expr: expr;
        if let $( $if_let_pattern: pat )|+ = $if_let_expr: expr;
        if $($rest: tt)*
    ) => { ... };
    (
        $item_expr: expr;
        for $pattern: pat in $into_iterator: expr $(;)?
    ) => { ... };
    (
        $item_expr: expr;
        for $pattern: pat in $into_iterator: expr
        $(
            ; if $condition: expr
            $( ; if let $( $if_let_pattern: pat )|+ = $if_let_expr: expr )*
        )+
        $(;)?
    ) => { ... };
    (
        $item_expr: expr;
        for $pattern: pat in $into_iterator: expr
        $(
            ; if let $( $if_let_pattern: pat )|+ = $if_let_expr: expr
            $( ; if $condition: expr )*
        )+
        $(;)?
    ) => { ... };
    (
        $item_expr: expr;
        for $pattern: pat in $into_iterator: expr;
        for $($rest: tt)*
    ) => { ... };
    (
        $item_expr: expr;
        for $pattern: pat in $into_iterator: expr;
        $(
            if $condition: expr;
            $( if let $( $if_let_pattern: pat )|+ = $if_let_expr: expr; )*
        )+
        for $($rest: tt)*
    ) => { ... };
    (
        $item_expr: expr;
        for $pattern: pat in $into_iterator: expr;
        $(
            if let $( $if_let_pattern: pat )|+ = $if_let_expr: expr;
            $( if $condition: expr; )*
        )+
        for $($rest: tt)*
    ) => { ... };
}

A Python-like lazy generator-expression

For details see module level documentation