Add structopt dependency in version 0.3.7

This patch series replaces argparse with structopt in the argument
handling code. As a first step, we need structopt as a dependency.

Import subrepo structopt/:structopt at efbdda4753592e27bc430fb01f7b9650b2f3174d
Import subrepo bitflags/:bitflags at 30668016aca6bd3b02c766e8347e0b4080d4c296
Import subrepo clap/:clap at 784524f7eb193e35f81082cc69454c8c21b948f7
Import subrepo heck/:heck at 093d56fbf001e1506e56dbfa38631d99b1066df1
Import subrepo proc-macro-error/:proc-macro-error at 6c4cfe79a622c5de8ae68557993542be46eacae2
Import subrepo proc-macro2/:proc-macro2 at d5d48eddca4566e5438e8a2cbed4a74e049544de
Import subrepo quote/:quote at 727436c6c137b20f0f34dde5d8fda2679b9747ad
Import subrepo rustversion/:rustversion at 0c5663313516263059ce9059ef81fc7a1cf655ca
Import subrepo syn-mid/:syn-mid at 5d3d85414a9e6674e1857ec22a87b96e04a6851a
Import subrepo syn/:syn at e87c27e87f6f4ef8919d0372bdb056d53ef0d8f3
Import subrepo textwrap/:textwrap at abcd618beae3f74841032aa5b53c1086b0a57ca2
Import subrepo unicode-segmentation/:unicode-segmentation at 637c9874c4fe0c205ff27787faf150a40295c6c3
Import subrepo unicode-width/:unicode-width at 3033826f8bf05e82724140a981d5941e48fce393
Import subrepo unicode-xid/:unicode-xid at 4baae9fffb156ba229665b972a9cd5991787ceb7

14 files changed, 1285 insertions, 0 deletions

diff --git a/rustversion/.gitignore b/rustversion/.gitignore
new file mode 100644
index 0000000..6936990
--- /dev/null
+++ b/rustversion/.gitignore
@@ -0,0 +1,3 @@
+/target
+**/*.rs.bk
+Cargo.lock
diff --git a/rustversion/.travis.yml b/rustversion/.travis.yml
new file mode 100644
index 0000000..1a615ba
--- /dev/null
+++ b/rustversion/.travis.yml
@@ -0,0 +1,17 @@
+language: rust
+
+rust:
+  - nightly
+  - beta
+  - stable
+  - 1.31.0
+
+script:
+  - cargo test
+
+matrix:
+  include:
+    - rust: nightly
+      name: Minimal versions
+      before_script:
+        - cargo update -Z minimal-versions
diff --git a/rustversion/Cargo.toml b/rustversion/Cargo.toml
new file mode 100644
index 0000000..789c5f4
--- /dev/null
+++ b/rustversion/Cargo.toml
@@ -0,0 +1,21 @@
+[package]
+name = "rustversion"
+version = "1.0.1"
+authors = ["David Tolnay <dtolnay@gmail.com>"]
+edition = "2018"
+license = "MIT OR Apache-2.0"
+description = "Conditional compilation according to rustc compiler version"
+repository = "https://github.com/dtolnay/rustversion"
+documentation = "https://docs.rs/rustversion"
+readme = "README.md"
+
+[lib]
+proc-macro = true
+
+[badges]
+travis-ci = { repository = "dtolnay/rustversion" }
+
+[dependencies]
+proc-macro2 = "1.0"
+quote = "1.0"
+syn = { version = "1.0.1", features = ["full"] }
diff --git a/rustversion/LICENSE-APACHE b/rustversion/LICENSE-APACHE
new file mode 100644
index 0000000..16fe87b
--- /dev/null
+++ b/rustversion/LICENSE-APACHE
@@ -0,0 +1,201 @@
+                              Apache License
+                        Version 2.0, January 2004
+                     http://www.apache.org/licenses/
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+TERMS AND CONDITIONS FOR USE, REPRODUCTION, AND DISTRIBUTION
+
+1. Definitions.
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diff --git a/rustversion/LICENSE-MIT b/rustversion/LICENSE-MIT
new file mode 100644
index 0000000..31aa793
--- /dev/null
+++ b/rustversion/LICENSE-MIT
@@ -0,0 +1,23 @@
+Permission is hereby granted, free of charge, to any
+person obtaining a copy of this software and associated
+documentation files (the "Software"), to deal in the
+Software without restriction, including without
+limitation the rights to use, copy, modify, merge,
+publish, distribute, sublicense, and/or sell copies of
+the Software, and to permit persons to whom the Software
+is furnished to do so, subject to the following
+conditions:
+
+The above copyright notice and this permission notice
+shall be included in all copies or substantial portions
+of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF
+ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED
+TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A
+PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT
+SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
+CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR
+IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+DEALINGS IN THE SOFTWARE.
diff --git a/rustversion/README.md b/rustversion/README.md
new file mode 100644
index 0000000..f7fdab1
--- /dev/null
+++ b/rustversion/README.md
@@ -0,0 +1,138 @@
+Compiler version cfg
+====================
+
+[![Build Status](https://api.travis-ci.com/dtolnay/rustversion.svg?branch=master)](https://travis-ci.com/dtolnay/rustversion)
+[![Latest Version](https://img.shields.io/crates/v/rustversion.svg)](https://crates.io/crates/rustversion)
+[![Rust Documentation](https://img.shields.io/badge/api-rustdoc-blue.svg)](https://docs.rs/rustversion)
+
+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>
diff --git a/rustversion/src/attr.rs b/rustversion/src/attr.rs
new file mode 100644
index 0000000..591b2c0
--- /dev/null
+++ b/rustversion/src/attr.rs
@@ -0,0 +1,35 @@
+use crate::expr::Expr;
+use proc_macro2::TokenStream;
+use syn::parse::{Parse, ParseStream, Result};
+use syn::Token;
+
+pub struct Args {
+    pub condition: Expr,
+    pub then: Then,
+}
+
+pub enum Then {
+    Const(Token![const]),
+    Attribute(TokenStream),
+}
+
+impl Parse for Args {
+    fn parse(input: ParseStream) -> Result<Self> {
+        let condition: Expr = input.parse()?;
+
+        input.parse::<Token![,]>()?;
+        if input.is_empty() {
+            return Err(input.error("expected one or more attrs"));
+        }
+
+        let const_token: Option<Token![const]> = input.parse()?;
+        let then = if let Some(const_token) = const_token {
+            input.parse::<Option<Token![,]>>()?;
+            Then::Const(const_token)
+        } else {
+            input.parse().map(Then::Attribute)?
+        };
+
+        Ok(Args { condition, then })
+    }
+}
diff --git a/rustversion/src/bound.rs b/rustversion/src/bound.rs
new file mode 100644
index 0000000..2546637
--- /dev/null
+++ b/rustversion/src/bound.rs
@@ -0,0 +1,84 @@
+use crate::date::Date;
+use crate::version::{Channel::*, Version};
+use quote::quote;
+use std::cmp::Ordering;
+use syn::parse::{Error, Parse, ParseStream, Result};
+use syn::{LitFloat, LitInt, Token};
+
+pub enum Bound {
+    Nightly(Date),
+    Stable(Release),
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
+pub struct Release {
+    pub minor: u16,
+    pub patch: Option<u16>,
+}
+
+impl Parse for Bound {
+    fn parse(input: ParseStream) -> Result<Self> {
+        if input.peek2(Token![-]) {
+            input.parse().map(Bound::Nightly)
+        } else {
+            input.parse().map(Bound::Stable)
+        }
+    }
+}
+
+impl Parse for Release {
+    fn parse(input: ParseStream) -> Result<Self> {
+        let span = input.cursor().token_stream();
+        let error = || Error::new_spanned(&span, "expected rustc release number, like 1.31");
+
+        let major_minor: LitFloat = input.parse().map_err(|_| error())?;
+        let string = quote!(#major_minor).to_string();
+
+        if !string.starts_with("1.") {
+            return Err(error());
+        }
+
+        let minor: u16 = string[2..].parse().map_err(|_| error())?;
+
+        let patch = if input.parse::<Option<Token![.]>>()?.is_some() {
+            let int: LitInt = input.parse().map_err(|_| error())?;
+            Some(int.base10_parse().map_err(|_| error())?)
+        } else {
+            None
+        };
+
+        Ok(Release { minor, patch })
+    }
+}
+
+impl PartialEq<Bound> for Version {
+    fn eq(&self, rhs: &Bound) -> bool {
+        match rhs {
+            Bound::Nightly(date) => match self.channel {
+                Stable | Beta | Dev => false,
+                Nightly(nightly) => nightly == *date,
+            },
+            Bound::Stable(release) => {
+                self.minor == release.minor
+                    && release.patch.map_or(true, |patch| self.patch == patch)
+            }
+        }
+    }
+}
+
+impl PartialOrd<Bound> for Version {
+    fn partial_cmp(&self, rhs: &Bound) -> Option<Ordering> {
+        match rhs {
+            Bound::Nightly(date) => match self.channel {
+                Stable | Beta => Some(Ordering::Less),
+                Nightly(nightly) => Some(nightly.cmp(date)),
+                Dev => Some(Ordering::Greater),
+            },
+            Bound::Stable(release) => {
+                let version = (self.minor, self.patch);
+                let bound = (release.minor, release.patch.unwrap_or(0));
+                Some(version.cmp(&bound))
+            }
+        }
+    }
+}
diff --git a/rustversion/src/date.rs b/rustversion/src/date.rs
new file mode 100644
index 0000000..631b762
--- /dev/null
+++ b/rustversion/src/date.rs
@@ -0,0 +1,77 @@
+use crate::time;
+use std::fmt::{self, Display};
+use std::num::ParseIntError;
+use std::str::FromStr;
+use syn::parse::{Error, Parse, ParseStream};
+use syn::{LitInt, Token};
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
+pub struct Date {
+    pub year: u16,
+    pub month: u8,
+    pub day: u8,
+}
+
+impl Display for Date {
+    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        write!(
+            formatter,
+            "{:04}-{:02}-{:02}",
+            self.year, self.month, self.day,
+        )
+    }
+}
+
+pub struct ParseDateError;
+
+impl From<ParseIntError> for ParseDateError {
+    fn from(_err: ParseIntError) -> Self {
+        ParseDateError
+    }
+}
+
+impl FromStr for Date {
+    type Err = ParseDateError;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        let mut date = s.split('-');
+        let year = date.next().ok_or(ParseDateError)?.parse()?;
+        let month = date.next().ok_or(ParseDateError)?.parse()?;
+        let day = date.next().ok_or(ParseDateError)?.parse()?;
+        match date.next() {
+            None => Ok(Date { year, month, day }),
+            Some(_) => Err(ParseDateError),
+        }
+    }
+}
+
+impl Parse for Date {
+    fn parse(input: ParseStream) -> syn::Result<Self> {
+        let span = input.cursor().token_stream();
+        let error = || {
+            Error::new_spanned(
+                &span,
+                format!("expected nightly date, like {}", time::today()),
+            )
+        };
+
+        let year: LitInt = input.parse().map_err(|_| error())?;
+        input.parse::<Token![-]>()?;
+        let month: LitInt = input.parse().map_err(|_| error())?;
+        input.parse::<Token![-]>()?;
+        let day: LitInt = input.parse().map_err(|_| error())?;
+
+        let year = year.base10_parse::<u64>().map_err(|_| error())?;
+        let month = month.base10_parse::<u64>().map_err(|_| error())?;
+        let day = day.base10_parse::<u64>().map_err(|_| error())?;
+        if year >= 3000 || month > 12 || day > 31 {
+            return Err(error());
+        }
+
+        Ok(Date {
+            year: year as u16,
+            month: month as u8,
+            day: day as u8,
+        })
+    }
+}
diff --git a/rustversion/src/expr.rs b/rustversion/src/expr.rs
new file mode 100644
index 0000000..2ea91af
--- /dev/null
+++ b/rustversion/src/expr.rs
@@ -0,0 +1,177 @@
+use crate::bound::{Bound, Release};
+use crate::date::Date;
+use crate::version::{Channel, Version};
+use syn::parse::{Parse, ParseStream, Result};
+use syn::punctuated::Punctuated;
+use syn::{parenthesized, token, Token};
+
+pub enum Expr {
+    Stable,
+    Beta,
+    Nightly,
+    Date(Date),
+    Since(Bound),
+    Before(Bound),
+    Release(Release),
+    Not(Box<Expr>),
+    Any(Vec<Expr>),
+    All(Vec<Expr>),
+}
+
+impl Expr {
+    pub fn eval(&self, rustc: Version) -> bool {
+        use self::Expr::*;
+
+        match self {
+            Stable => rustc.channel == Channel::Stable,
+            Beta => rustc.channel == Channel::Beta,
+            Nightly => match rustc.channel {
+                Channel::Nightly(_) | Channel::Dev => true,
+                Channel::Stable | Channel::Beta => false,
+            },
+            Date(date) => match rustc.channel {
+                Channel::Nightly(rustc) => rustc == *date,
+                Channel::Stable | Channel::Beta | Channel::Dev => false,
+            },
+            Since(bound) => rustc >= *bound,
+            Before(bound) => rustc < *bound,
+            Release(release) => {
+                rustc.channel == Channel::Stable
+                    && rustc.minor == release.minor
+                    && release.patch.map_or(true, |patch| rustc.patch == patch)
+            }
+            Not(expr) => !expr.eval(rustc),
+            Any(exprs) => exprs.iter().any(|e| e.eval(rustc)),
+            All(exprs) => exprs.iter().all(|e| e.eval(rustc)),
+        }
+    }
+}
+
+type Exprs = Punctuated<Expr, Token![,]>;
+
+mod keyword {
+    syn::custom_keyword!(stable);
+    syn::custom_keyword!(beta);
+    syn::custom_keyword!(nightly);
+    syn::custom_keyword!(since);
+    syn::custom_keyword!(before);
+    syn::custom_keyword!(not);
+    syn::custom_keyword!(any);
+    syn::custom_keyword!(all);
+}
+
+impl Parse for Expr {
+    fn parse(input: ParseStream) -> Result<Self> {
+        let lookahead = input.lookahead1();
+        if lookahead.peek(keyword::stable) {
+            Self::parse_stable(input)
+        } else if lookahead.peek(keyword::beta) {
+            Self::parse_beta(input)
+        } else if lookahead.peek(keyword::nightly) {
+            Self::parse_nightly(input)
+        } else if lookahead.peek(keyword::since) {
+            Self::parse_since(input)
+        } else if lookahead.peek(keyword::before) {
+            Self::parse_before(input)
+        } else if lookahead.peek(keyword::not) {
+            Self::parse_not(input)
+        } else if lookahead.peek(keyword::any) {
+            Self::parse_any(input)
+        } else if lookahead.peek(keyword::all) {
+            Self::parse_all(input)
+        } else {
+            Err(lookahead.error())
+        }
+    }
+}
+
+impl Expr {
+    fn parse_nightly(input: ParseStream) -> Result<Self> {
+        input.parse::<keyword::nightly>()?;
+
+        if !input.peek(token::Paren) {
+            return Ok(Expr::Nightly);
+        }
+
+        let paren;
+        parenthesized!(paren in input);
+        let date: Date = paren.parse()?;
+        paren.parse::<Option<Token![,]>>()?;
+
+        Ok(Expr::Date(date))
+    }
+
+    fn parse_beta(input: ParseStream) -> Result<Self> {
+        input.parse::<keyword::beta>()?;
+
+        Ok(Expr::Beta)
+    }
+
+    fn parse_stable(input: ParseStream) -> Result<Self> {
+        input.parse::<keyword::stable>()?;
+
+        if !input.peek(token::Paren) {
+            return Ok(Expr::Stable);
+        }
+
+        let paren;
+        parenthesized!(paren in input);
+        let release: Release = paren.parse()?;
+        paren.parse::<Option<Token![,]>>()?;
+
+        Ok(Expr::Release(release))
+    }
+
+    fn parse_since(input: ParseStream) -> Result<Self> {
+        input.parse::<keyword::since>()?;
+
+        let paren;
+        parenthesized!(paren in input);
+        let bound: Bound = paren.parse()?;
+        paren.parse::<Option<Token![,]>>()?;
+
+        Ok(Expr::Since(bound))
+    }
+
+    fn parse_before(input: ParseStream) -> Result<Self> {
+        input.parse::<keyword::before>()?;
+
+        let paren;
+        parenthesized!(paren in input);
+        let bound: Bound = paren.parse()?;
+        paren.parse::<Option<Token![,]>>()?;
+
+        Ok(Expr::Before(bound))
+    }
+
+    fn parse_not(input: ParseStream) -> Result<Self> {
+        input.parse::<keyword::not>()?;
+
+        let paren;
+        parenthesized!(paren in input);
+        let expr: Expr = paren.parse()?;
+        paren.parse::<Option<Token![,]>>()?;
+
+        Ok(Expr::Not(Box::new(expr)))
+    }
+
+    fn parse_any(input: ParseStream) -> Result<Self> {
+        input.parse::<keyword::any>()?;
+
+        let paren;
+        parenthesized!(paren in input);
+        let exprs: Exprs = paren.parse_terminated(Expr::parse)?;
+
+        Ok(Expr::Any(exprs.into_iter().collect()))
+    }
+
+    fn parse_all(input: ParseStream) -> Result<Self> {
+        input.parse::<keyword::all>()?;
+
+        let paren;
+        parenthesized!(paren in input);
+        let exprs: Exprs = paren.parse_terminated(Expr::parse)?;
+
+        Ok(Expr::All(exprs.into_iter().collect()))
+    }
+}
diff --git a/rustversion/src/lib.rs b/rustversion/src/lib.rs
new file mode 100644
index 0000000..cf8ed21
--- /dev/null
+++ b/rustversion/src/lib.rs
@@ -0,0 +1,254 @@
+//! 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
+//!
+//! <br>
+//!
+//! # Selectors
+//!
+//! - <p style="margin-left:50px;text-indent:-50px">
+//!   <b><code>#[rustversion::stable]</code></b>
+//!   —<br>
+//!   True on any stable compiler.
+//!   </p>
+//!
+//! - <p style="margin-left:50px;text-indent:-50px">
+//!   <b><code>#[rustversion::stable(1.34)]</code></b>
+//!   —<br>
+//!   True on exactly the specified stable compiler.
+//!   </p>
+//!
+//! - <p style="margin-left:50px;text-indent:-50px">
+//!   <b><code>#[rustversion::beta]</code></b>
+//!   —<br>
+//!   True on any beta compiler.
+//!   </p>
+//!
+//! - <p style="margin-left:50px;text-indent:-50px">
+//!   <b><code>#[rustversion::nightly]</code></b>
+//!   —<br>
+//!   True on any nightly compiler or dev build.
+//!   </p>
+//!
+//! - <p style="margin-left:50px;text-indent:-50px">
+//!   <b><code>#[rustversion::nightly(2019-01-01)]</code></b>
+//!   —<br>
+//!   True on exactly one nightly.
+//!   </p>
+//!
+//! - <p style="margin-left:50px;text-indent:-50px">
+//!   <b><code>#[rustversion::since(1.34)]</code></b>
+//!   —<br>
+//!   True on that stable release and any later compiler, including beta and
+//!   nightly.
+//!   </p>
+//!
+//! - <p style="margin-left:50px;text-indent:-50px">
+//!   <b><code>#[rustversion::since(2019-01-01)]</code></b>
+//!   —<br>
+//!   True on that nightly and all newer ones.
+//!   </p>
+//!
+//! - <p style="margin-left:50px;text-indent:-50px">
+//!   <b><code>#[rustversion::before(</code></b><i>version or date</i><b><code>)]</code></b>
+//!   —<br>
+//!   Negative of <i>#[rustversion::since(...)]</i>.
+//!   </p>
+//!
+//! - <p style="margin-left:50px;text-indent:-50px">
+//!   <b><code>#[rustversion::not(</code></b><i>selector</i><b><code>)]</code></b>
+//!   —<br>
+//!   Negative of any selector; for example <i>#[rustversion::not(nightly)]</i>.
+//!   </p>
+//!
+//! - <p style="margin-left:50px;text-indent:-50px">
+//!   <b><code>#[rustversion::any(</code></b><i>selectors...</i><b><code>)]</code></b>
+//!   —<br>
+//!   True if any of the comma-separated selectors is true; for example
+//!   <i>#[rustversion::any(stable, beta)]</i>.
+//!   </p>
+//!
+//! - <p style="margin-left:50px;text-indent:-50px">
+//!   <b><code>#[rustversion::all(</code></b><i>selectors...</i><b><code>)]</code></b>
+//!   —<br>
+//!   True if all of the comma-separated selectors are true; for example
+//!   <i>#[rustversion::all(since(1.31), before(1.34))]</i>.
+//!   </p>
+//!
+//! - <p style="margin-left:50px;text-indent:-50px">
+//!   <b><code>#[rustversion::attr(</code></b><i>selector</i><b><code>, </code></b><i>attribute</i><b><code>)]</code></b>
+//!   —<br>
+//!   For conditional inclusion of attributes; analogous to
+//!   <code>cfg_attr</code>.
+//!   </p>
+//!
+//! <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
+//!
+//! ```
+//! # trait MyTrait {}
+//! #
+//! #[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
+//!
+//! ```
+//! #[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.
+//!
+//! ```
+//! use std::time::Duration;
+//!
+//! #[rustversion::attr(since(1.32), const)]
+//! fn duration_as_days(dur: Duration) -> u64 {
+//!     dur.as_secs() / 60 / 60 / 24
+//! }
+//! ```
+//!
+//! <br>
+
+extern crate proc_macro;
+
+mod attr;
+mod bound;
+mod date;
+mod expr;
+mod rustc;
+mod time;
+mod version;
+
+use crate::attr::Then;
+use crate::expr::Expr;
+use proc_macro::TokenStream;
+use proc_macro2::{Ident, Span, TokenStream as TokenStream2};
+use quote::quote;
+use syn::{parse_macro_input, ItemFn, Result};
+
+#[proc_macro_attribute]
+pub fn stable(args: TokenStream, input: TokenStream) -> TokenStream {
+    cfg("stable", args, input)
+}
+
+#[proc_macro_attribute]
+pub fn beta(args: TokenStream, input: TokenStream) -> TokenStream {
+    cfg("beta", args, input)
+}
+
+#[proc_macro_attribute]
+pub fn nightly(args: TokenStream, input: TokenStream) -> TokenStream {
+    cfg("nightly", args, input)
+}
+
+#[proc_macro_attribute]
+pub fn since(args: TokenStream, input: TokenStream) -> TokenStream {
+    cfg("since", args, input)
+}
+
+#[proc_macro_attribute]
+pub fn before(args: TokenStream, input: TokenStream) -> TokenStream {
+    cfg("before", args, input)
+}
+
+#[proc_macro_attribute]
+pub fn not(args: TokenStream, input: TokenStream) -> TokenStream {
+    cfg("not", args, input)
+}
+
+#[proc_macro_attribute]
+pub fn any(args: TokenStream, input: TokenStream) -> TokenStream {
+    cfg("any", args, input)
+}
+
+#[proc_macro_attribute]
+pub fn all(args: TokenStream, input: TokenStream) -> TokenStream {
+    cfg("all", args, input)
+}
+
+fn cfg(top: &str, args: TokenStream, input: TokenStream) -> TokenStream {
+    match try_cfg(top, args, input) {
+        Ok(tokens) => tokens,
+        Err(err) => TokenStream::from(err.to_compile_error()),
+    }
+}
+
+fn try_cfg(top: &str, args: TokenStream, input: TokenStream) -> Result<TokenStream> {
+    let args = TokenStream2::from(args);
+    let top = Ident::new(top, Span::call_site());
+
+    let mut full_args = quote!(#top);
+    if !args.is_empty() {
+        full_args.extend(quote!((#args)));
+    }
+
+    let expr: Expr = syn::parse2(full_args)?;
+    let version = rustc::version()?;
+
+    if expr.eval(version) {
+        Ok(input)
+    } else {
+        Ok(TokenStream::new())
+    }
+}
+
+#[proc_macro_attribute]
+pub fn attr(args: TokenStream, input: TokenStream) -> TokenStream {
+    let args = parse_macro_input!(args as attr::Args);
+
+    match try_attr(args, input) {
+        Ok(tokens) => tokens,
+        Err(err) => TokenStream::from(err.to_compile_error()),
+    }
+}
+
+fn try_attr(args: attr::Args, input: TokenStream) -> Result<TokenStream> {
+    let version = rustc::version()?;
+
+    if !args.condition.eval(version) {
+        return Ok(input);
+    }
+
+    match args.then {
+        Then::Const(const_token) => {
+            let mut input: ItemFn = syn::parse(input)?;
+            input.sig.constness = Some(const_token);
+            Ok(TokenStream::from(quote!(#input)))
+        }
+        Then::Attribute(then) => {
+            let input = TokenStream2::from(input);
+            Ok(TokenStream::from(quote! {
+                #[cfg_attr(all(), #then)]
+                #input
+            }))
+        }
+    }
+}
diff --git a/rustversion/src/rustc.rs b/rustversion/src/rustc.rs
new file mode 100644
index 0000000..4e7699d
--- /dev/null
+++ b/rustversion/src/rustc.rs
@@ -0,0 +1,195 @@
+use std::env;
+use std::ffi::OsString;
+use std::fmt::{self, Display};
+use std::io;
+use std::process::Command;
+use std::str::FromStr;
+use std::string::FromUtf8Error;
+
+use crate::date::Date;
+use crate::version::{Channel::*, Version};
+use proc_macro2::Span;
+
+#[derive(Debug)]
+pub enum Error {
+    Exec(io::Error),
+    Utf8(FromUtf8Error),
+    Parse(String),
+}
+
+pub type Result<T> = std::result::Result<T, Error>;
+
+impl Display for Error {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        use self::Error::*;
+
+        match self {
+            Exec(e) => write!(f, "failed to run `rustc --version`: {}", e),
+            Utf8(e) => write!(f, "failed to parse output of `rustc --version`: {}", e),
+            Parse(string) => write!(
+                f,
+                "unexpected output from `rustc --version`, please file an issue: {:?}",
+                string,
+            ),
+        }
+    }
+}
+
+impl From<FromUtf8Error> for Error {
+    fn from(err: FromUtf8Error) -> Self {
+        Error::Utf8(err)
+    }
+}
+
+impl From<Error> for syn::Error {
+    fn from(err: Error) -> Self {
+        syn::Error::new(Span::call_site(), err)
+    }
+}
+
+pub fn version() -> Result<Version> {
+    let rustc = env::var_os("RUSTC").unwrap_or_else(|| OsString::from("rustc"));
+    let output = Command::new(rustc)
+        .arg("--version")
+        .output()
+        .map_err(Error::Exec)?;
+    let string = String::from_utf8(output.stdout)?;
+
+    match parse(&string) {
+        Some(version) => Ok(version),
+        None => Err(Error::Parse(string)),
+    }
+}
+
+fn parse(string: &str) -> Option<Version> {
+    let last_line = string.lines().last().unwrap_or(&string);
+    let mut words = last_line.trim().split(' ');
+
+    if words.next()? != "rustc" {
+        return None;
+    }
+
+    let mut version_channel = words.next()?.split('-');
+    let version = version_channel.next()?;
+    let channel = version_channel.next();
+
+    let mut digits = version.split('.');
+    let major = digits.next()?;
+    if major != "1" {
+        return None;
+    }
+    let minor = digits.next()?.parse().ok()?;
+    let patch = digits.next().unwrap_or("0").parse().ok()?;
+
+    let channel = match channel {
+        None => Stable,
+        Some(channel) if channel == "dev" => Dev,
+        Some(channel) if channel.starts_with("beta") => Beta,
+        Some(channel) if channel == "nightly" => {
+            match words.next() {
+                Some(hash) => {
+                    if !hash.starts_with('(') {
+                        return None;
+                    }
+                    let date = words.next()?;
+                    if !date.ends_with(')') {
+                        return None;
+                    }
+                    let date = Date::from_str(&date[..date.len() - 1]).ok()?;
+                    Nightly(date)
+                }
+                None => Dev,
+            }
+        }
+        Some(_) => return None,
+    };
+
+    Some(Version {
+        minor,
+        patch,
+        channel,
+    })
+}
+
+#[test]
+fn test_parse() {
+    let cases = &[
+        (
+            "rustc 1.0.0 (a59de37e9 2015-05-13) (built 2015-05-14)",
+            Version {
+                minor: 0,
+                patch: 0,
+                channel: Stable,
+            },
+        ),
+        (
+            "rustc 1.18.0",
+            Version {
+                minor: 18,
+                patch: 0,
+                channel: Stable,
+            },
+        ),
+        (
+            "rustc 1.24.1 (d3ae9a9e0 2018-02-27)",
+            Version {
+                minor: 24,
+                patch: 1,
+                channel: Stable,
+            },
+        ),
+        (
+            "rustc 1.35.0-beta.3 (c13114dc8 2019-04-27)",
+            Version {
+                minor: 35,
+                patch: 0,
+                channel: Beta,
+            },
+        ),
+        (
+            "rustc 1.36.0-nightly (938d4ffe1 2019-04-27)",
+            Version {
+                minor: 36,
+                patch: 0,
+                channel: Nightly(Date {
+                    year: 2019,
+                    month: 4,
+                    day: 27,
+                }),
+            },
+        ),
+        (
+            "rustc 1.36.0-dev",
+            Version {
+                minor: 36,
+                patch: 0,
+                channel: Dev,
+            },
+        ),
+        (
+            "rustc 1.36.0-nightly",
+            Version {
+                minor: 36,
+                patch: 0,
+                channel: Dev,
+            },
+        ),
+        (
+            "warning: invalid logging spec 'warning', ignoring it
+             rustc 1.30.0-nightly (3bc2ca7e4 2018-09-20)",
+            Version {
+                minor: 30,
+                patch: 0,
+                channel: Nightly(Date {
+                    year: 2018,
+                    month: 9,
+                    day: 20,
+                }),
+            },
+        ),
+    ];
+
+    for (string, expected) in cases {
+        assert_eq!(parse(string).unwrap(), *expected);
+    }
+}
diff --git a/rustversion/src/time.rs b/rustversion/src/time.rs
new file mode 100644
index 0000000..1e6dd90
--- /dev/null
+++ b/rustversion/src/time.rs
@@ -0,0 +1,44 @@
+use crate::date::Date;
+use std::time::{SystemTime, UNIX_EPOCH};
+
+// Timestamp of 2016-03-01 00:00:00 in UTC.
+const BASE: u64 = 1456790400;
+const BASE_YEAR: u16 = 2016;
+const BASE_MONTH: u8 = 3;
+
+// Days between leap days.
+const CYCLE: u64 = 365 * 4 + 1;
+
+const DAYS_BY_MONTH: [u8; 12] = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];
+
+pub fn today() -> Date {
+    let default = Date {
+        year: 2019,
+        month: 1,
+        day: 1,
+    };
+    try_today().unwrap_or(default)
+}
+
+fn try_today() -> Option<Date> {
+    let now = SystemTime::now();
+    let since_epoch = now.duration_since(UNIX_EPOCH).ok()?;
+    let secs = since_epoch.as_secs();
+
+    let approx_days = secs.checked_sub(BASE)? / 60 / 60 / 24;
+    let cycle = approx_days / CYCLE;
+    let mut rem = approx_days % CYCLE;
+
+    let mut year = BASE_YEAR + cycle as u16 * 4;
+    let mut month = BASE_MONTH;
+    loop {
+        let days_in_month = DAYS_BY_MONTH[month as usize - 1];
+        if rem < days_in_month as u64 {
+            let day = rem as u8 + 1;
+            return Some(Date { year, month, day });
+        }
+        rem -= days_in_month as u64;
+        year += (month == 12) as u16;
+        month = month % 12 + 1;
+    }
+}
diff --git a/rustversion/src/version.rs b/rustversion/src/version.rs
new file mode 100644
index 0000000..ab3992f
--- /dev/null
+++ b/rustversion/src/version.rs
@@ -0,0 +1,16 @@
+use crate::date::Date;
+
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub struct Version {
+    pub minor: u16,
+    pub patch: u16,
+    pub channel: Channel,
+}
+
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub enum Channel {
+    Stable,
+    Beta,
+    Nightly(Date),
+    Dev,
+}