negative-impl

Negative trait implementations on stable Rust.

This crate emulates the unstable negative_impls feature by generating a trait implementation with a condition that will never be true.

Usage

Add this to your Cargo.toml:

toml [dependencies] negative-impl = "0.1"

Compiler support: requires rustc 1.37+

Examples

```rust use negativeimpl::negativeimpl;

pub struct Type {}

[negative_impl]

impl !Send for Type {}

[negative_impl]

impl !Sync for Type {} ```

Supported traits

Currently this crate only supports auto traits.

• Send
• Sync
• Unpin
• UnwindSafe
• RefUnwindSafe

Limitations

Conflicting implementations

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

```rust use negativeimpl::negativeimpl;

pub struct Type {}

[negative_impl]

impl !Send for Type {}

trait Trait {}

impl 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 Trait for T {} impl Trait for Type {} ```

Conditional negative impls

The current behavior of this crate with conditional negative impls is the same as the unstable negative_impls feature, but the unstable negative_impls feature plans to forbid it.

It's a great plan, but when it's implemented, this crate is unlikely to provide the same behavior due to the macro's limitation.

License

Licensed under either of Apache License, Version 2.0 or MIT license 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.