Using storage_types macro requires defining crate features

[Nemo157]

storage_types! uses #[cfg(feature = "u32")] internally to enable and disable different storage types. Unfortunately this is non-hygenic and depends on the features of the crate calling storage_types!, not the features of uom.

The workaround is to just add a default u32 (or whichever storage type you want) feature to the crate that calls the macro as well.

I don't know if there's a simple solution to this problem currently, hopefully Macros 2.0 might allow hygenic cfg attributes?

[iliekturtles]

Good point! I haven't really followed what changes will be happening in Macros 2.0 so I'm not sure if that will have a solution to the issue. As a first step I'll add additional documentation to the macro to mention this limitation (PRs also welcome).

A possible solution without Macros 2.0 is to move #cfg attributes out of the match bodies and on to match arms. Not sure if this will work for sure but this may make the #cfg attributes apply in the uom crate before the macro is executed by the calling crate.

[Nemo157]

A possible solution without Macros 2.0 is to move #cfg attributes out of the match bodies and on to match arms.

That was my first thought, but it doesn't look like macro_rules! allows attributes on the match arms.

[Nemo157]

I think it's actually the modularization changes for Macros 2.0 that would easily allow fixing this, the helper arms of the storage_types! macro could be split out into separate macros that could then be cfged out, e.g. somewhat like

#[cfg(feature = "u8")]
macro storage_type_u8($(#[$attr:meta])*) @$M:ident ($($tt:tt)*)) {
    storage_type!(@$M $(#[$attr])* u8, u8; $($tt)*);
}

#[cfg(not(feature = "u8"))]
macro storage_type_u8($(#[$attr:meta])*) @$M:ident ($($tt:tt)*)) {
}

which storage_types! could then delegate to hygienically.

Taking another look at the Macros 2.0 tracking issue, it doesn't sound like it's coming anytime soon though. I'm going to have an attempt at coming up with a solution that works with macro_rules!.