* this sort of works

```toml
[dependencies]
defmt-03 = { package = "defmt", version = "0.3", optional = true }
defmt-04 = { package = "defmt", version = "0.4", optional = true }
```

and then

```rust
#[cfg(feature = "defmt-03")]
use defmt_03 as defmt;
#[cfg(feature = "defmt-04")]
use defmt_04 as defmt;

[cfg_attr(any(feature = "defmt-03", feature = "defmt-04"), derive(defmt::Format))]
pub enum Blah { .. }
```