Allow bitflags v1 and v2 to coexist

[Urhengulas]

There is a defmt::bitflags! macro which depends on the bitflags::bitflags! macro from the bitflags v1 crate. Since it's introduction there was a bitflags v2, but unfortunately we cannot just bump the dependency, since the defmt::bitflags! macro leaks implementation details of the bitflags::bitflags! macro and therefore the dependency update could break the code of defmt users.

Therefore we want to add two new cargo features to defmt: bitflags1 and bitflags2. They each gate a defmt::bitflags macro which depends on bitflags v1 and bitflags v2 respectively. To not break any existing code, the bitflags1 feature should be enabled by default. A compile_error when both are enabled sounds useful as well.

Note that there is an existing PR to update to bitflags v2: #746.

[jonathanpallant]

The important note here is co-exist. Any support for bitflags v2 has to use a new name for the macro, and we have to support the old one at the same time.

Imagine Application X depends on Library A and Library B. Library A wants to use the existing bitflags API but Library B is newer and wants to use the newer bitflags v2 API (that we haven't added yet, but that we might add). Both crates need to be compiled using the exact same set of cargo features, because that's how cargo works unfortunately.

[Urhengulas]

True. So I propose the following:

• hide defmt::bitflags behind a feature but enable it by default
• add bitflags v2 as defmt::bitflags2! behind a feature, not enabled by default

@jonathanpallant do you see any problems with that approach?

cc @manushT

[jonathanpallant]

Do we have any existing features and is anyone out there likely to be using defmt with default-features = false? Because that proposal would break them.

The other option is:

1. Leave defmt::bitflags as it is
2. Add a new feature bitflagsv2 which provides defmt::bitflagsv2 using an optional dependency.

It's still not unclear to be if anyone needs us to upgrade to bitflags v2. It's an improvement, but I don't think bitflags v1 is broken for anyone right now.