Macro setters

macro_rules! setters {
    (@single $(#[$meta:meta])* $name:ident : $typ:ty => $transform:expr) => { ... };
    (@recurse) => { ... };
    (@recurse $(#[$meta:meta])* $name:ident : $typ:ty, $($tokens:tt)*) => { ... };
    (@recurse $(#[$meta:meta])* $name:ident : $typ:ty => $transform:expr, $($tokens:tt)*) => { ... };
    ($($tokens:tt)*) => { ... };
}

Creates setter methods

The methods created are of the form $name that takes an argument of type $typ and sets the field $name to result of calling $transform with the value of the argument.

In other words. The following macro call:

struct MyStruct<'a> { foo: Option<&'a str> };
impl <'a> MyStruct<'a> {
    setters! { foo: &'a str => Some(foo), }
}

Roughly expands to:

struct MyStruct<'a> { foo: Option<&'a str> };
impl <'a> MyStruct<'a> {
    fn foo(self, foo: &'a str) -> Self {
        Self {
            foo: Some(foo),
            ..self
        }
    }
}