13 files changed, 967 insertions, 0 deletions
diff --git a/third_party/rust/rustversion/src/attr.rs b/third_party/rust/rustversion/src/attr.rs
new file mode 100644
index 0000000000..6023d1eaf1
--- /dev/null
+++ b/third_party/rust/rustversion/src/attr.rs
@@ -0,0 +1,35 @@
+use crate::error::{Error, Result};
+use crate::expr::{self, Expr};
+use crate::{iter, token};
+use proc_macro::{Span, TokenStream};
+
+pub struct Args {
+    pub condition: Expr,
+    pub then: Then,
+}
+
+pub enum Then {
+    Const(Span),
+    Attribute(TokenStream),
+}
+
+pub fn parse(input: TokenStream) -> Result<Args> {
+    let ref mut input = iter::new(input);
+    let condition = expr::parse(input)?;
+
+    token::parse_punct(input, ',')?;
+    if input.peek().is_none() {
+        return Err(Error::new(Span::call_site(), "expected one or more attrs"));
+    }
+
+    let const_span = token::parse_optional_keyword(input, "const");
+    let then = if let Some(const_span) = const_span {
+        token::parse_optional_punct(input, ',');
+        token::parse_end(input)?;
+        Then::Const(const_span)
+    } else {
+        Then::Attribute(input.collect())
+    };
+
+    Ok(Args { condition, then })
+}
diff --git a/third_party/rust/rustversion/src/bound.rs b/third_party/rust/rustversion/src/bound.rs
new file mode 100644
index 0000000000..a667a5b0dc
--- /dev/null
+++ b/third_party/rust/rustversion/src/bound.rs
@@ -0,0 +1,63 @@
+use crate::date::{self, Date};
+use crate::error::{Error, Result};
+use crate::iter::Iter;
+use crate::release::{self, Release};
+use crate::time;
+use crate::version::{Channel::*, Version};
+use proc_macro::{Group, TokenTree};
+use std::cmp::Ordering;
+
+pub enum Bound {
+    Nightly(Date),
+    Stable(Release),
+}
+
+pub fn parse(paren: Group, iter: Iter) -> Result<Bound> {
+    if let Some(TokenTree::Literal(literal)) = iter.peek() {
+        let repr = literal.to_string();
+        if repr.starts_with(|ch: char| ch.is_ascii_digit()) {
+            if repr.contains('.') {
+                return release::parse(paren, iter).map(Bound::Stable);
+            } else {
+                return date::parse(paren, iter).map(Bound::Nightly);
+            }
+        }
+    }
+    let msg = format!(
+        "expected rustc release number like 1.31, or nightly date like {}",
+        time::today(),
+    );
+    Err(Error::group(paren, msg))
+}
+
+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/third_party/rust/rustversion/src/constfn.rs b/third_party/rust/rustversion/src/constfn.rs
new file mode 100644
index 0000000000..0e50c03969
--- /dev/null
+++ b/third_party/rust/rustversion/src/constfn.rs
@@ -0,0 +1,58 @@
+use crate::error::{Error, Result};
+use proc_macro::{Ident, Span, TokenStream, TokenTree};
+use std::iter;
+
+#[derive(PartialOrd, PartialEq)]
+enum Qualifiers {
+    None,
+    Async,
+    Unsafe,
+    Extern,
+    Abi,
+}
+
+impl Qualifiers {
+    fn from_ident(ident: &Ident) -> Self {
+        match ident.to_string().as_str() {
+            "async" => Qualifiers::Async,
+            "unsafe" => Qualifiers::Unsafe,
+            "extern" => Qualifiers::Extern,
+            _ => Qualifiers::None,
+        }
+    }
+}
+
+pub(crate) fn insert_const(input: TokenStream, const_span: Span) -> Result<TokenStream> {
+    let ref mut input = crate::iter::new(input);
+    let mut out = TokenStream::new();
+    let mut qualifiers = Qualifiers::None;
+    let mut pending = Vec::new();
+
+    while let Some(token) = input.next() {
+        match token {
+            TokenTree::Ident(ref ident) if ident.to_string() == "fn" => {
+                let const_ident = Ident::new("const", const_span);
+                out.extend(iter::once(TokenTree::Ident(const_ident)));
+                out.extend(pending);
+                out.extend(iter::once(token));
+                out.extend(input);
+                return Ok(out);
+            }
+            TokenTree::Ident(ref ident) if Qualifiers::from_ident(ident) > qualifiers => {
+                qualifiers = Qualifiers::from_ident(ident);
+                pending.push(token);
+            }
+            TokenTree::Literal(_) if qualifiers == Qualifiers::Extern => {
+                qualifiers = Qualifiers::Abi;
+                pending.push(token);
+            }
+            _ => {
+                qualifiers = Qualifiers::None;
+                out.extend(pending.drain(..));
+                out.extend(iter::once(token));
+            }
+        }
+    }
+
+    Err(Error::new(const_span, "only allowed on a fn item"))
+}
diff --git a/third_party/rust/rustversion/src/date.rs b/third_party/rust/rustversion/src/date.rs
new file mode 100644
index 0000000000..5cdb691da6
--- /dev/null
+++ b/third_party/rust/rustversion/src/date.rs
@@ -0,0 +1,50 @@
+use crate::error::{Error, Result};
+use crate::iter::Iter;
+use crate::{time, token};
+use proc_macro::Group;
+use std::fmt::{self, Display};
+
+#[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 fn parse(paren: Group, iter: Iter) -> Result<Date> {
+    try_parse(iter).map_err(|_| {
+        let msg = format!("expected nightly date, like {}", time::today());
+        Error::group(paren, msg)
+    })
+}
+
+fn try_parse(iter: Iter) -> Result<Date, ()> {
+    let year = token::parse_literal(iter).map_err(drop)?;
+    token::parse_punct(iter, '-').map_err(drop)?;
+    let month = token::parse_literal(iter).map_err(drop)?;
+    token::parse_punct(iter, '-').map_err(drop)?;
+    let day = token::parse_literal(iter).map_err(drop)?;
+
+    let year = year.to_string().parse::<u64>().map_err(drop)?;
+    let month = month.to_string().parse::<u64>().map_err(drop)?;
+    let day = day.to_string().parse::<u64>().map_err(drop)?;
+    if year >= 3000 || month > 12 || day > 31 {
+        return Err(());
+    }
+
+    Ok(Date {
+        year: year as u16,
+        month: month as u8,
+        day: day as u8,
+    })
+}
diff --git a/third_party/rust/rustversion/src/error.rs b/third_party/rust/rustversion/src/error.rs
new file mode 100644
index 0000000000..a99625b4aa
--- /dev/null
+++ b/third_party/rust/rustversion/src/error.rs
@@ -0,0 +1,56 @@
+use proc_macro::{Delimiter, Group, Ident, Literal, Punct, Spacing, Span, TokenStream, TokenTree};
+use std::fmt::Display;
+use std::iter::FromIterator;
+
+pub type Result<T, E = Error> = std::result::Result<T, E>;
+
+pub struct Error {
+    begin: Span,
+    end: Span,
+    msg: String,
+}
+
+impl Error {
+    pub fn new(span: Span, msg: impl Display) -> Self {
+        Self::new2(span, span, msg)
+    }
+
+    pub fn new2(begin: Span, end: Span, msg: impl Display) -> Self {
+        Error {
+            begin,
+            end,
+            msg: msg.to_string(),
+        }
+    }
+
+    pub fn group(group: Group, msg: impl Display) -> Self {
+        let mut iter = group.stream().into_iter();
+        let delimiter = group.span();
+        let begin = iter.next().map_or(delimiter, |t| t.span());
+        let end = iter.last().map_or(begin, |t| t.span());
+        Self::new2(begin, end, msg)
+    }
+
+    pub fn into_compile_error(self) -> TokenStream {
+        // compile_error! { $msg }
+        TokenStream::from_iter(vec![
+            TokenTree::Ident(Ident::new("compile_error", self.begin)),
+            TokenTree::Punct({
+                let mut punct = Punct::new('!', Spacing::Alone);
+                punct.set_span(self.begin);
+                punct
+            }),
+            TokenTree::Group({
+                let mut group = Group::new(Delimiter::Brace, {
+                    TokenStream::from_iter(vec![TokenTree::Literal({
+                        let mut string = Literal::string(&self.msg);
+                        string.set_span(self.end);
+                        string
+                    })])
+                });
+                group.set_span(self.end);
+                group
+            }),
+        ])
+    }
+}
diff --git a/third_party/rust/rustversion/src/expand.rs b/third_party/rust/rustversion/src/expand.rs
new file mode 100644
index 0000000000..813ba85d7d
--- /dev/null
+++ b/third_party/rust/rustversion/src/expand.rs
@@ -0,0 +1,72 @@
+use crate::attr::{self, Then};
+use crate::error::{Error, Result};
+use crate::{constfn, expr, iter, token};
+use proc_macro::{Delimiter, Group, Ident, Punct, Spacing, Span, TokenStream, TokenTree};
+use std::iter::FromIterator;
+
+pub fn cfg(introducer: &str, args: TokenStream, input: TokenStream) -> TokenStream {
+    try_cfg(introducer, args, input).unwrap_or_else(Error::into_compile_error)
+}
+
+fn try_cfg(introducer: &str, args: TokenStream, input: TokenStream) -> Result<TokenStream> {
+    let introducer = Ident::new(introducer, Span::call_site());
+
+    let mut full_args = TokenStream::from(TokenTree::Ident(introducer));
+    if !args.is_empty() {
+        full_args.extend(std::iter::once(TokenTree::Group(Group::new(
+            Delimiter::Parenthesis,
+            args,
+        ))));
+    }
+
+    let ref mut full_args = iter::new(full_args);
+    let expr = expr::parse(full_args)?;
+    token::parse_end(full_args)?;
+
+    if expr.eval(crate::RUSTVERSION) {
+        Ok(input)
+    } else {
+        Ok(TokenStream::new())
+    }
+}
+
+pub fn try_attr(args: attr::Args, input: TokenStream) -> Result<TokenStream> {
+    if !args.condition.eval(crate::RUSTVERSION) {
+        return Ok(input);
+    }
+
+    match args.then {
+        Then::Const(const_token) => constfn::insert_const(input, const_token),
+        Then::Attribute(then) => {
+            // #[cfg_attr(all(), #then)]
+            Ok(TokenStream::from_iter(
+                vec![
+                    TokenTree::Punct(Punct::new('#', Spacing::Alone)),
+                    TokenTree::Group(Group::new(
+                        Delimiter::Bracket,
+                        TokenStream::from_iter(vec![
+                            TokenTree::Ident(Ident::new("cfg_attr", Span::call_site())),
+                            TokenTree::Group(Group::new(
+                                Delimiter::Parenthesis,
+                                TokenStream::from_iter(
+                                    vec![
+                                        TokenTree::Ident(Ident::new("all", Span::call_site())),
+                                        TokenTree::Group(Group::new(
+                                            Delimiter::Parenthesis,
+                                            TokenStream::new(),
+                                        )),
+                                        TokenTree::Punct(Punct::new(',', Spacing::Alone)),
+                                    ]
+                                    .into_iter()
+                                    .chain(then),
+                                ),
+                            )),
+                        ]),
+                    )),
+                ]
+                .into_iter()
+                .chain(input),
+            ))
+        }
+    }
+}
diff --git a/third_party/rust/rustversion/src/expr.rs b/third_party/rust/rustversion/src/expr.rs
new file mode 100644
index 0000000000..45ed7d2156
--- /dev/null
+++ b/third_party/rust/rustversion/src/expr.rs
@@ -0,0 +1,163 @@
+use crate::bound::{self, Bound};
+use crate::date::{self, Date};
+use crate::error::{Error, Result};
+use crate::iter::{self, Iter};
+use crate::release::{self, Release};
+use crate::token;
+use crate::version::{Channel, Version};
+use proc_macro::{Ident, Span, TokenTree};
+
+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)),
+        }
+    }
+}
+
+pub fn parse(iter: Iter) -> Result<Expr> {
+    match &iter.next() {
+        Some(TokenTree::Ident(i)) if i.to_string() == "stable" => parse_stable(iter),
+        Some(TokenTree::Ident(i)) if i.to_string() == "beta" => Ok(Expr::Beta),
+        Some(TokenTree::Ident(i)) if i.to_string() == "nightly" => parse_nightly(iter),
+        Some(TokenTree::Ident(i)) if i.to_string() == "since" => parse_since(i, iter),
+        Some(TokenTree::Ident(i)) if i.to_string() == "before" => parse_before(i, iter),
+        Some(TokenTree::Ident(i)) if i.to_string() == "not" => parse_not(i, iter),
+        Some(TokenTree::Ident(i)) if i.to_string() == "any" => parse_any(i, iter),
+        Some(TokenTree::Ident(i)) if i.to_string() == "all" => parse_all(i, iter),
+        unexpected => {
+            let span = unexpected
+                .as_ref()
+                .map_or_else(Span::call_site, TokenTree::span);
+            Err(Error::new(span, "expected one of `stable`, `beta`, `nightly`, `since`, `before`, `not`, `any`, `all`"))
+        }
+    }
+}
+
+fn parse_nightly(iter: Iter) -> Result<Expr> {
+    let paren = match token::parse_optional_paren(iter) {
+        Some(group) => group,
+        None => return Ok(Expr::Nightly),
+    };
+
+    let ref mut inner = iter::new(paren.stream());
+    let date = date::parse(paren, inner)?;
+    token::parse_optional_punct(inner, ',');
+    token::parse_end(inner)?;
+
+    Ok(Expr::Date(date))
+}
+
+fn parse_stable(iter: Iter) -> Result<Expr> {
+    let paren = match token::parse_optional_paren(iter) {
+        Some(group) => group,
+        None => return Ok(Expr::Stable),
+    };
+
+    let ref mut inner = iter::new(paren.stream());
+    let release = release::parse(paren, inner)?;
+    token::parse_optional_punct(inner, ',');
+    token::parse_end(inner)?;
+
+    Ok(Expr::Release(release))
+}
+
+fn parse_since(introducer: &Ident, iter: Iter) -> Result<Expr> {
+    let paren = token::parse_paren(introducer, iter)?;
+
+    let ref mut inner = iter::new(paren.stream());
+    let bound = bound::parse(paren, inner)?;
+    token::parse_optional_punct(inner, ',');
+    token::parse_end(inner)?;
+
+    Ok(Expr::Since(bound))
+}
+
+fn parse_before(introducer: &Ident, iter: Iter) -> Result<Expr> {
+    let paren = token::parse_paren(introducer, iter)?;
+
+    let ref mut inner = iter::new(paren.stream());
+    let bound = bound::parse(paren, inner)?;
+    token::parse_optional_punct(inner, ',');
+    token::parse_end(inner)?;
+
+    Ok(Expr::Before(bound))
+}
+
+fn parse_not(introducer: &Ident, iter: Iter) -> Result<Expr> {
+    let paren = token::parse_paren(introducer, iter)?;
+
+    let ref mut inner = iter::new(paren.stream());
+    let expr = self::parse(inner)?;
+    token::parse_optional_punct(inner, ',');
+    token::parse_end(inner)?;
+
+    Ok(Expr::Not(Box::new(expr)))
+}
+
+fn parse_any(introducer: &Ident, iter: Iter) -> Result<Expr> {
+    let paren = token::parse_paren(introducer, iter)?;
+
+    let ref mut inner = iter::new(paren.stream());
+    let exprs = parse_comma_separated(inner)?;
+
+    Ok(Expr::Any(exprs.into_iter().collect()))
+}
+
+fn parse_all(introducer: &Ident, iter: Iter) -> Result<Expr> {
+    let paren = token::parse_paren(introducer, iter)?;
+
+    let ref mut inner = iter::new(paren.stream());
+    let exprs = parse_comma_separated(inner)?;
+
+    Ok(Expr::All(exprs.into_iter().collect()))
+}
+
+fn parse_comma_separated(iter: Iter) -> Result<Vec<Expr>> {
+    let mut exprs = Vec::new();
+
+    while iter.peek().is_some() {
+        let expr = self::parse(iter)?;
+        exprs.push(expr);
+        if iter.peek().is_none() {
+            break;
+        }
+        token::parse_punct(iter, ',')?;
+    }
+
+    Ok(exprs)
+}
diff --git a/third_party/rust/rustversion/src/iter.rs b/third_party/rust/rustversion/src/iter.rs
new file mode 100644
index 0000000000..722013c7d6
--- /dev/null
+++ b/third_party/rust/rustversion/src/iter.rs
@@ -0,0 +1,42 @@
+use proc_macro::{token_stream, Delimiter, TokenStream, TokenTree};
+
+pub type Iter<'a> = &'a mut IterImpl;
+
+pub struct IterImpl {
+    stack: Vec<token_stream::IntoIter>,
+    peeked: Option<TokenTree>,
+}
+
+pub fn new(tokens: TokenStream) -> IterImpl {
+    IterImpl {
+        stack: vec![tokens.into_iter()],
+        peeked: None,
+    }
+}
+
+impl IterImpl {
+    pub fn peek(&mut self) -> Option<&TokenTree> {
+        self.peeked = self.next();
+        self.peeked.as_ref()
+    }
+}
+
+impl Iterator for IterImpl {
+    type Item = TokenTree;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        if let Some(tt) = self.peeked.take() {
+            return Some(tt);
+        }
+        loop {
+            let top = self.stack.last_mut()?;
+            match top.next() {
+                None => drop(self.stack.pop()),
+                Some(TokenTree::Group(ref group)) if group.delimiter() == Delimiter::None => {
+                    self.stack.push(group.stream().into_iter());
+                }
+                Some(tt) => return Some(tt),
+            }
+        }
+    }
+}
diff --git a/third_party/rust/rustversion/src/lib.rs b/third_party/rust/rustversion/src/lib.rs
new file mode 100644
index 0000000000..2e8f1b94d9
--- /dev/null
+++ b/third_party/rust/rustversion/src/lib.rs
@@ -0,0 +1,247 @@
+//! [![github]](https://github.com/dtolnay/rustversion)&ensp;[![crates-io]](https://crates.io/crates/rustversion)&ensp;[![docs-rs]](https://docs.rs/rustversion)
+//!
+//! [github]: https://img.shields.io/badge/github-8da0cb?style=for-the-badge&labelColor=555555&logo=github
+//! [crates-io]: https://img.shields.io/badge/crates.io-fc8d62?style=for-the-badge&labelColor=555555&logo=rust
+//! [docs-rs]: https://img.shields.io/badge/docs.rs-66c2a5?style=for-the-badge&labelColor=555555&logo=docs.rs
+//!
+//! <br>
+//!
+//! 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>
+
+#![allow(
+    clippy::cast_lossless,
+    clippy::cast_possible_truncation,
+    clippy::derive_partial_eq_without_eq,
+    clippy::doc_markdown,
+    clippy::enum_glob_use,
+    clippy::from_iter_instead_of_collect,
+    // https://github.com/rust-lang/rust-clippy/issues/8539
+    clippy::iter_with_drain,
+    clippy::module_name_repetitions,
+    clippy::must_use_candidate,
+    clippy::needless_doctest_main,
+    clippy::needless_pass_by_value,
+    clippy::redundant_else,
+    clippy::toplevel_ref_arg,
+    clippy::unreadable_literal
+)]
+
+extern crate proc_macro;
+
+mod attr;
+mod bound;
+mod constfn;
+mod date;
+mod error;
+mod expand;
+mod expr;
+mod iter;
+mod release;
+mod time;
+mod token;
+mod version;
+
+use crate::error::Error;
+use crate::version::Version;
+use proc_macro::TokenStream;
+
+const RUSTVERSION: Version = include!(concat!(env!("OUT_DIR"), "/version.expr"));
+
+#[proc_macro_attribute]
+pub fn stable(args: TokenStream, input: TokenStream) -> TokenStream {
+    expand::cfg("stable", args, input)
+}
+
+#[proc_macro_attribute]
+pub fn beta(args: TokenStream, input: TokenStream) -> TokenStream {
+    expand::cfg("beta", args, input)
+}
+
+#[proc_macro_attribute]
+pub fn nightly(args: TokenStream, input: TokenStream) -> TokenStream {
+    expand::cfg("nightly", args, input)
+}
+
+#[proc_macro_attribute]
+pub fn since(args: TokenStream, input: TokenStream) -> TokenStream {
+    expand::cfg("since", args, input)
+}
+
+#[proc_macro_attribute]
+pub fn before(args: TokenStream, input: TokenStream) -> TokenStream {
+    expand::cfg("before", args, input)
+}
+
+#[proc_macro_attribute]
+pub fn not(args: TokenStream, input: TokenStream) -> TokenStream {
+    expand::cfg("not", args, input)
+}
+
+#[proc_macro_attribute]
+pub fn any(args: TokenStream, input: TokenStream) -> TokenStream {
+    expand::cfg("any", args, input)
+}
+
+#[proc_macro_attribute]
+pub fn all(args: TokenStream, input: TokenStream) -> TokenStream {
+    expand::cfg("all", args, input)
+}
+
+#[proc_macro_attribute]
+pub fn attr(args: TokenStream, input: TokenStream) -> TokenStream {
+    attr::parse(args)
+        .and_then(|args| expand::try_attr(args, input))
+        .unwrap_or_else(Error::into_compile_error)
+}
+
+#[cfg(not(cfg_macro_not_allowed))]
+#[proc_macro]
+pub fn cfg(input: TokenStream) -> TokenStream {
+    use proc_macro::{Ident, Span, TokenTree};
+    (|| {
+        let ref mut args = iter::new(input);
+        let expr = expr::parse(args)?;
+        token::parse_end(args)?;
+        let boolean = expr.eval(RUSTVERSION);
+        let ident = Ident::new(&boolean.to_string(), Span::call_site());
+        Ok(TokenStream::from(TokenTree::Ident(ident)))
+    })()
+    .unwrap_or_else(Error::into_compile_error)
+}
diff --git a/third_party/rust/rustversion/src/release.rs b/third_party/rust/rustversion/src/release.rs
new file mode 100644
index 0000000000..3f421c6579
--- /dev/null
+++ b/third_party/rust/rustversion/src/release.rs
@@ -0,0 +1,34 @@
+use crate::error::{Error, Result};
+use crate::iter::Iter;
+use crate::token;
+use proc_macro::Group;
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
+pub struct Release {
+    pub minor: u16,
+    pub patch: Option<u16>,
+}
+
+pub fn parse(paren: Group, iter: Iter) -> Result<Release> {
+    try_parse(iter).map_err(|_| Error::group(paren, "expected rustc release number, like 1.31"))
+}
+
+fn try_parse(iter: Iter) -> Result<Release, ()> {
+    let major_minor = token::parse_literal(iter).map_err(drop)?;
+    let string = major_minor.to_string();
+
+    if !string.starts_with("1.") {
+        return Err(());
+    }
+
+    let minor: u16 = string[2..].parse().map_err(drop)?;
+
+    let patch = if token::parse_optional_punct(iter, '.').is_some() {
+        let int = token::parse_literal(iter).map_err(drop)?;
+        Some(int.to_string().parse().map_err(drop)?)
+    } else {
+        None
+    };
+
+    Ok(Release { minor, patch })
+}
diff --git a/third_party/rust/rustversion/src/time.rs b/third_party/rust/rustversion/src/time.rs
new file mode 100644
index 0000000000..3c21463dd8
--- /dev/null
+++ b/third_party/rust/rustversion/src/time.rs
@@ -0,0 +1,51 @@
+use crate::date::Date;
+use std::env;
+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: 2020,
+        month: 2,
+        day: 25,
+    };
+    try_today().unwrap_or(default)
+}
+
+fn try_today() -> Option<Date> {
+    if let Some(pkg_name) = env::var_os("CARGO_PKG_NAME") {
+        if pkg_name.to_str() == Some("rustversion-tests") {
+            return None; // Stable date for ui testing.
+        }
+    }
+
+    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/third_party/rust/rustversion/src/token.rs b/third_party/rust/rustversion/src/token.rs
new file mode 100644
index 0000000000..ebb6f1c012
--- /dev/null
+++ b/third_party/rust/rustversion/src/token.rs
@@ -0,0 +1,78 @@
+use crate::error::{Error, Result};
+use crate::iter::Iter;
+use proc_macro::{Delimiter, Group, Ident, Literal, Span, TokenTree};
+
+pub fn parse_punct(iter: Iter, ch: char) -> Result<()> {
+    match iter.next() {
+        Some(TokenTree::Punct(ref punct)) if punct.as_char() == ch => Ok(()),
+        unexpected => {
+            let span = unexpected
+                .as_ref()
+                .map_or_else(Span::call_site, TokenTree::span);
+            Err(Error::new(span, format!("expected `{}`", ch)))
+        }
+    }
+}
+
+pub fn parse_optional_punct(iter: Iter, ch: char) -> Option<()> {
+    match iter.peek() {
+        Some(TokenTree::Punct(punct)) if punct.as_char() == ch => iter.next().map(drop),
+        _ => None,
+    }
+}
+
+pub fn parse_optional_keyword(iter: Iter, keyword: &str) -> Option<Span> {
+    match iter.peek() {
+        Some(TokenTree::Ident(ident)) if ident.to_string() == keyword => {
+            Some(iter.next().unwrap().span())
+        }
+        _ => None,
+    }
+}
+
+pub fn parse_literal(iter: Iter) -> Result<Literal> {
+    match iter.next() {
+        Some(TokenTree::Literal(literal)) => Ok(literal),
+        unexpected => {
+            let span = unexpected
+                .as_ref()
+                .map_or_else(Span::call_site, TokenTree::span);
+            Err(Error::new(span, "expected literal"))
+        }
+    }
+}
+
+pub fn parse_paren(introducer: &Ident, iter: Iter) -> Result<Group> {
+    match iter.peek() {
+        Some(TokenTree::Group(group)) if group.delimiter() == Delimiter::Parenthesis => {
+            match iter.next() {
+                Some(TokenTree::Group(group)) => Ok(group),
+                _ => unreachable!(),
+            }
+        }
+        Some(unexpected) => Err(Error::new(unexpected.span(), "expected `(`")),
+        None => Err(Error::new(
+            introducer.span(),
+            format!("expected `(` after `{}`", introducer),
+        )),
+    }
+}
+
+pub fn parse_optional_paren(iter: Iter) -> Option<Group> {
+    match iter.peek() {
+        Some(TokenTree::Group(group)) if group.delimiter() == Delimiter::Parenthesis => {
+            match iter.next() {
+                Some(TokenTree::Group(group)) => Some(group),
+                _ => unreachable!(),
+            }
+        }
+        _ => None,
+    }
+}
+
+pub fn parse_end(iter: Iter) -> Result<()> {
+    match iter.next() {
+        None => Ok(()),
+        Some(unexpected) => Err(Error::new(unexpected.span(), "unexpected token")),
+    }
+}
diff --git a/third_party/rust/rustversion/src/version.rs b/third_party/rust/rustversion/src/version.rs
new file mode 100644
index 0000000000..f4a69f5185
--- /dev/null
+++ b/third_party/rust/rustversion/src/version.rs
@@ -0,0 +1,18 @@
+#![allow(dead_code)]
+
+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,
+}