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+Compiler version cfg
+====================
+
+[<img alt="github" src="https://img.shields.io/badge/github-dtolnay/rustversion-8da0cb?style=for-the-badge&labelColor=555555&logo=github" height="20">](https://github.com/dtolnay/rustversion)
+[<img alt="crates.io" src="https://img.shields.io/crates/v/rustversion.svg?style=for-the-badge&color=fc8d62&logo=rust" height="20">](https://crates.io/crates/rustversion)
+[<img alt="docs.rs" src="https://img.shields.io/badge/docs.rs-rustversion-66c2a5?style=for-the-badge&labelColor=555555&logo=docs.rs" height="20">](https://docs.rs/rustversion)
+[<img alt="build status" src="https://img.shields.io/github/actions/workflow/status/dtolnay/rustversion/ci.yml?branch=master&style=for-the-badge" height="20">](https://github.com/dtolnay/rustversion/actions?query=branch%3Amaster)
+
+This crate provides macros for conditional compilation according to rustc
+compiler version, analogous to [`#[cfg(...)]`][cfg] and
+[`#[cfg_attr(...)]`][cfg_attr].
+
+[cfg]: https://doc.rust-lang.org/reference/conditional-compilation.html#the-cfg-attribute
+[cfg_attr]: https://doc.rust-lang.org/reference/conditional-compilation.html#the-cfg_attr-attribute
+
+```toml
+[dependencies]
+rustversion = "1.0"
+```
+
+<br>
+
+## Selectors
+
+- <b>`#[rustversion::stable]`</b>
+  —<br>
+  True on any stable compiler.
+
+- <b>`#[rustversion::stable(1.34)]`</b>
+  —<br>
+  True on exactly the specified stable compiler.
+
+- <b>`#[rustversion::beta]`</b>
+  —<br>
+  True on any beta compiler.
+
+- <b>`#[rustversion::nightly]`</b>
+  —<br>
+  True on any nightly compiler or dev build.
+
+- <b>`#[rustversion::nightly(2019-01-01)]`</b>
+  —<br>
+  True on exactly one nightly.
+
+- <b>`#[rustversion::since(1.34)]`</b>
+  —<br>
+  True on that stable release and any later compiler, including beta and
+  nightly.
+
+- <b>`#[rustversion::since(2019-01-01)]`</b>
+  —<br>
+  True on that nightly and all newer ones.
+
+- <b>`#[rustversion::before(`</b><i>version or date</i><b>`)]`</b>
+  —<br>
+  Negative of *#[rustversion::since(...)]*.
+
+- <b>`#[rustversion::not(`</b><i>selector</i><b>`)]`</b>
+  —<br>
+  Negative of any selector; for example *#[rustversion::not(nightly)]*.
+
+- <b>`#[rustversion::any(`</b><i>selectors...</i><b>`)]`</b>
+  —<br>
+  True if any of the comma-separated selectors is true; for example
+  *#[rustversion::any(stable, beta)]*.
+
+- <b>`#[rustversion::all(`</b><i>selectors...</i><b>`)]`</b>
+  —<br>
+  True if all of the comma-separated selectors are true; for example
+  *#[rustversion::all(since(1.31), before(1.34))]*.
+
+- <b>`#[rustversion::attr(`</b><i>selector</i><b>`, `</b><i>attribute</i><b>`)]`</b>
+  —<br>
+  For conditional inclusion of attributes; analogous to `cfg_attr`.
+
+<br>
+
+## Use cases
+
+Providing additional trait impls as types are stabilized in the standard library
+without breaking compatibility with older compilers; in this case Pin\<P\>
+stabilized in [Rust 1.33][pin]:
+
+[pin]: https://blog.rust-lang.org/2019/02/28/Rust-1.33.0.html#pinning
+
+```rust
+#[rustversion::since(1.33)]
+use std::pin::Pin;
+
+#[rustversion::since(1.33)]
+impl<P: MyTrait> MyTrait for Pin<P> {
+    /* ... */
+}
+```
+
+Similar but for language features; the ability to control alignment greater than
+1 of packed structs was stabilized in [Rust 1.33][packed].
+
+[packed]: https://github.com/rust-lang/rust/blob/master/RELEASES.md#version-1330-2019-02-28
+
+```rust
+#[rustversion::attr(before(1.33), repr(packed))]
+#[rustversion::attr(since(1.33), repr(packed(2)))]
+struct Six(i16, i32);
+
+fn main() {
+    println!("{}", std::mem::align_of::<Six>());
+}
+```
+
+Augmenting code with `const` as const impls are stabilized in the standard
+library. This use of `const` as an attribute is recognized as a special case by
+the rustversion::attr macro.
+
+```rust
+use std::time::Duration;
+
+#[rustversion::attr(since(1.32), const)]
+fn duration_as_days(dur: Duration) -> u64 {
+    dur.as_secs() / 60 / 60 / 24
+}
+```
+
+<br>
+
+#### License
+
+<sup>
+Licensed under either of <a href="LICENSE-APACHE">Apache License, Version
+2.0</a> or <a href="LICENSE-MIT">MIT license</a> at your option.
+</sup>
+
+<br>
+
+<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>