Macro define_writer_expr 

macro_rules! define_writer_expr {
    ( unsafe fn $func:ident($ty1:ty) $( -> $ret:ty)? ) => { ... };
    ( unsafe fn ($func:path)($ty1:ty) $( -> $ret:ty)? ) => { ... };
    ( unsafe fn $func:ident($ty1:ty, $ty2:ty) $( -> $ret:ty)? ) => { ... };
    ( unsafe fn ($func:path)($ty1:ty, $ty2:ty) $( -> $ret:ty)? ) => { ... };
    ( $(#[$extern_meta:meta])* extern $($abi:literal)?
        $(#[$meta:meta])* fn $func:ident($arg1:tt: $ty1:ty) $( -> $ret:ty)?
    ) => { ... };
    ( $(#[$extern_meta:meta])* extern $($abi:literal)?
        $(#[$meta:meta])* fn $func:ident($arg1:tt: $ty1:ty, $arg2:tt: $ty2:ty) $( -> $ret:ty)?
    ) => { ... };
    ( $expr:expr ) => { ... };
}

Defines a writer expression from another expression, unsafe function or extern function.

If unsafe function specified as argument it just creates an FnMut wrapper that calls unsafe fn in unsafe block. The function can be specified by identifier or by braced full path. See Safety section for important details about safety.

If extern function specified as argument it creates extern block with this function and creates an FnMut wrapper that calls unsafe fn in unsafe block. See Safety section for important details about safety.

If an expression given as argument, the macro just returns it as a result.

This macro is used by define_writer, define_try_writer macros.

Safety

Note that writing using writer expression defined with unsafe fn or extern fn` do not require unsafe, so defining writer expression itself should be treated as an unsafe operation.