Function-like macro that "unconsts" trait implementations

const_trait_impl::unconst_trait_impl turns the Nightly syntax for constant trait implementations into analogous non-const syntax that is accepted on stable toolchain.

The list of features taken into account: * consttraitimpl * constfntrait_bound

In a vaccum, unconst_trait_impl procedural function-like macro is fairly useless because its call on constant trait implementation yields the same result as writing the non-const implementation in the first place.

However, with [cfg_attr] and remove_macro_call attributes, unconst_trait_impl macro allows one to conditionally remove the macro call thus providing support for stable toolchain while also providing functionality relying on Nightly features.

Example

```rust, ignore

![cfgattr(feature = "consttraitimpl", feature(consttrait_impl))]

![cfgattr(feature = "constdefaultimpls", feature(constdefault_impls))]

![cfgattr(feature = "constfntraitbound", feature(constfntrait_bound))]

[cfg(not(all(

feature = "const_trait_impl",
feature = "const_default_impls",
feature = "const_fn_trait_bound"

)))] use consttraitimpl::unconsttraitimpl; use core::{default::Default, marker::PhantomData};

[cfg(all(

feature = "const_trait_impl",
feature = "const_default_impls",
feature = "const_fn_trait_bound"

))] use removemacrocall::removemacrocall;

// Since ZST is both Eq and and PartialEq, it has structural match // https://github.com/rust-lang/rust/issues/63438

[derive(Clone, Debug, Hash, Eq, Ord, PartialEq, PartialOrd, Copy)]

pub struct ZST(PhantomData);

pub trait TraitName {}

[cfg_attr(

all(
    feature = "const_trait_impl",
    feature = "const_default_impls",
    feature = "const_fn_trait_bound"
),
remove_macro_call

)] unconsttraitimpl! { impl const TraitName for ZST {} }

// With cargo build --features const_trait_impl, const_default_impls, const_fn_trait_bound // or with `cargo build --all-features, the code below is expanded as is. Otherwise, // it gets "unconsted" to be supported by stable toolchain.

[cfg_attr(

all(
    feature = "const_trait_impl",
    feature = "const_default_impls",
    feature = "const_fn_trait_bound"
),
remove_macro_call

)] unconsttraitimpl! { impl const Default for ZST { fn default() -> Self { ZST(Default::default()) } } } ```

Note: In the real code, the example above could be replaced with a simpler version relying on cfg_aliases crate.

You can learn more about remove_macro_call here: * GitHub * crates.io

License

<sup>Licensed under either of Apache License, Version 2.0 or MIT license at your option.</sup>

<sub>Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in this crate by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.</sub>