Macro gen_mut

macro_rules! gen_mut {
    ($m:ty => $code:expr) => { ... };
}

This macro simplifies the implementation of generically mutable APIs, where the mutable and the shared code paths are (mostly) identical. It has the following syntax:

gen_mut!{ $M => {
    /* code */
}}

where $M is the name of the generic parameter you want to unwrap. The code inside has access to three macros, which allow it to emit different code depending on the value of M:

• from_gen!($genref) / from_gen!()

  Calls GenRef::gen_into_shared and GenRef::gen_into_mut on the $genref passed as an argument. The type of return value is different in the shared vs the mutable case, so it is not possible to move the return value outside of the macro call (attempting to do so would run into a type checker error on trying to assign &mut T to &T or vice versa). The return value can be converted back into a GenRef using the into_gen! macro. If no arguments are passed, it returns a closure Fn(GenRef<'_, M, T>) -> &T / Fn(GenRef<'_, M, T>) -> &mut T.

• into_mut!($reference) / into_mut!(&gen $place) / into_mut!()

  Calls GenRef::gen_from_shared and GenRef::gen_from_mut on the reference passed as an argument, and returns the resulting GenRef. The type of the input is different in the shared vs the mutable case, so it is not possible to call this with a reference that was not created via from_gen! or switch_mut_shared!. To allow accessing fields, you can use the into_mut!(&gen $place) syntax, which references the $place expression with the appropriate kind of reference. If no arguments are passed, it returns a closure Fn(&T) -> GenRef<'_, M, T> / Fn(&mut T) -> GenRef<'_, M, T>.

• switch_shared_mut!($shared_expr, $mutable_expr) / switch_shared_mut!({ $shared_tts } { $mutable_tts })

  Expands to shared_expr in the shared case and mutable_expr in the mutable case. The switch_shared_mut!({ $shared_tts } { $mutable_tts }) syntax allows you to expand to arbitrary token trees, not just expressions. This requires you to wrap them in brackets, which will not appear in the expansion. Also note that in this syntax there is no comma separating the two cases.