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:

[dependencies]
negative-impl = "0.1"

Compiler support: requires rustc 1.56+

Examples

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.

• 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.

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 {}

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.

#![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 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.