# negative-impl

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Negative trait implementations on stable Rust.

This crate emulates the [unstable `negative_impls` feature](https://doc.rust-lang.org/nightly/unstable-book/language-features/negative-impls.html)
by [generating a trait implementation with a condition that will never be true](https://github.com/taiki-e/negative-impl/issues/6#issuecomment-1669714453).

## Usage

Add this to your `Cargo.toml`:

```toml
[dependencies]
negative-impl = "0.1"
```

## Examples

```rust
use negative_impl::negative_impl;

pub struct Type {}

#[negative_impl]
impl !Send for Type {}
#[negative_impl]
impl !Sync for Type {}
```

## Supported traits

Currently this crate only supports [auto traits](https://doc.rust-lang.org/reference/special-types-and-traits.html#auto-traits).

- [`Send`](https://doc.rust-lang.org/std/marker/trait.Send.html)
- [`Sync`](https://doc.rust-lang.org/std/marker/trait.Sync.html)
- [`Unpin`](https://doc.rust-lang.org/std/marker/trait.Unpin.html)
- [`UnwindSafe`](https://doc.rust-lang.org/std/panic/trait.UnwindSafe.html)
- [`RefUnwindSafe`](https://doc.rust-lang.org/std/panic/trait.RefUnwindSafe.html)

## Limitations

### Conflicting implementations

The following code cannot compile due to `impl<T: Send> Trait for T` and
`impl Trait for Type` conflict.

```rust
use negative_impl::negative_impl;

pub struct Type {}

#[negative_impl]
impl !Send for Type {}

trait Trait {}

impl<T: Send> Trait for T {}
impl Trait for Type {}
```

```text
error[E0119]: conflicting implementations of trait `Trait` for type `Type`:
  --> src/lib.rs:60:1
   |
14 | impl<T: Send> Trait for T {}
   | ------------------------- first implementation here
15 | impl Trait for Type {}
   | ^^^^^^^^^^^^^^^^^^^ conflicting implementation for `Type`
```

The above code can be compiled using the unstable `negative_impls` feature.

```rust
#![feature(negative_impls)]

pub struct Type {}

impl !Send for Type {}

trait Trait {}

impl<T: Send> Trait for T {}
impl Trait for Type {}
```

## License

Licensed under either of [Apache License, Version 2.0](LICENSE-APACHE) or
[MIT license](LICENSE-MIT) at your option.

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