proc-macro2 0.2.3

# proc-macro2

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A small shim over the `proc_macro` crate in the compiler intended to multiplex
the current stable interface (as of 2017-07-05) and the [upcoming richer
interface][upcoming].

[upcoming]: https://github.com/rust-lang/rust/pull/40939

The upcoming support has features like:

* Span information on tokens
* No need to go in/out through strings
* Structured input/output

The hope is that libraries ported to `proc_macro2` will be trivial to port to
the real `proc_macro` crate once the support on nightly is stabilize.

## Usage

This crate by default compiles on the stable version of the compiler. It only
uses the stable surface area of the `proc_macro` crate upstream in the compiler
itself. Usage is done via:

```toml
[dependencies]
proc-macro2 = "0.2"
```

followed by

```rust
extern crate proc_macro;
extern crate proc_macro2;

#[proc_macro_derive(MyDerive)]
pub fn my_derive(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
    let input: proc_macro2::TokenStream = input.into();

    let output: proc_macro2::TokenStream = {
        /* transform input */
    };

    output.into()
}
```

If you'd like you can enable the `nightly` feature in this crate. This will
cause it to compile against the **unstable and nightly-only** features of the
`proc_macro` crate. This in turn requires a nightly compiler. This should help
preserve span information, however, coming in from the compiler itself.

You can enable this feature via:

```toml
[dependencies]
proc-macro2 = { version = "0.2", features = ["nightly"] }
```


## Unstable Features

`proc-macro2` supports exporting some methods from `proc_macro` which are
currently highly unstable, and may not be stabilized in the first pass of
`proc_macro` stabilizations. These features are not exported by default. Minor
versions of `proc-macro2` may make breaking changes to them at any time.

To enable these features, the `procmacro2_semver_exempt` config flag must be
passed to rustc.

```
RUSTFLAGS='--cfg procmacro2_semver_exempt' cargo build
```

Note that this must not only be done for your crate, but for any crate that
depends on your crate. This infectious nature is intentional, as it serves as a
reminder that you are outside of the normal semver guarantees.


# License

This project is licensed under either of

 * Apache License, Version 2.0, ([LICENSE-APACHE](LICENSE-APACHE) or
   http://www.apache.org/licenses/LICENSE-2.0)
 * MIT license ([LICENSE-MIT](LICENSE-MIT) or
   http://opensource.org/licenses/MIT)

at your option.

### Contribution

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