[RFC] support panic and target_has_atomic predicates

There are a couple of new predicates in Rust 1.60: `target_has_atomic`
and `panic`. Here's a first cut at supporting them.

The panic predicate depends both on native platform support (default
behavior) and on a potential override through RUSTFLAGS. We should
somehow expose this I guess, but I'm not sure how.

src/error.rs

@@ -36,6 +36,8 @@ pub enum Reason {
     InvalidInteger,
     /// The root cfg() may only contain a single predicate
     MultipleRootPredicates,
+    /// A `target_has_atomic` predicate didn't correctly parse.
+    InvalidHasAtomic,
     /// An element was not part of the builtin information in rustc
     UnknownBuiltin,
 }
@@ -72,9 +74,9 @@ impl fmt::Display for ParseError {
 impl fmt::Display for Reason {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         use Reason::{
-            Empty, InvalidCharacters, InvalidInteger, InvalidNot, MultipleRootPredicates,
-            UnclosedParens, UnclosedQuotes, Unexpected, UnknownBuiltin, UnopenedParens,
-            UnopenedQuotes,
+            Empty, InvalidCharacters, InvalidHasAtomic, InvalidInteger, InvalidNot,
+            MultipleRootPredicates, UnclosedParens, UnclosedQuotes, Unexpected, UnknownBuiltin,
+            UnopenedParens, UnopenedQuotes,
         };
 
         match self {
@@ -101,6 +103,7 @@ impl fmt::Display for Reason {
             InvalidNot(np) => f.write_fmt(format_args!("not() takes 1 predicate, found {}", np)),
             InvalidInteger => f.write_str("invalid integer"),
             MultipleRootPredicates => f.write_str("multiple root predicates"),
+            InvalidHasAtomic => f.write_str("expected integer or \"ptr\""),
             UnknownBuiltin => f.write_str("unknown built-in"),
         }
     }
@@ -109,9 +112,9 @@ impl fmt::Display for Reason {
 impl Error for ParseError {
     fn description(&self) -> &str {
         use Reason::{
-            Empty, InvalidCharacters, InvalidInteger, InvalidNot, MultipleRootPredicates,
-            UnclosedParens, UnclosedQuotes, Unexpected, UnknownBuiltin, UnopenedParens,
-            UnopenedQuotes,
+            Empty, InvalidCharacters, InvalidHasAtomic, InvalidInteger, InvalidNot,
+            MultipleRootPredicates, UnclosedParens, UnclosedQuotes, Unexpected, UnknownBuiltin,
+            UnopenedParens, UnopenedQuotes,
         };
 
         match self.reason {
@@ -125,7 +128,22 @@ impl Error for ParseError {
             InvalidNot(_) => "not() takes 1 predicate",
             InvalidInteger => "invalid integer",
             MultipleRootPredicates => "multiple root predicates",
+            InvalidHasAtomic => "expected integer or \"ptr\"",
             UnknownBuiltin => "unknown built-in",
         }
     }
 }
+
+/// Error parsing a `target_has_atomic` predicate.
+#[derive(Clone, Debug, Eq, PartialEq)]
+pub struct HasAtomicParseError {
+    pub(crate) input: String,
+}
+
+impl fmt::Display for HasAtomicParseError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "expected integer or \"ptr\", found {}", self.input)
+    }
+}
+
+impl Error for HasAtomicParseError {}

src/expr.rs

@@ -40,8 +40,12 @@ pub enum TargetPredicate {
     /// This also applies to the bare [`unix` and `windows`](https://doc.rust-lang.org/reference/conditional-compilation.html#unix-and-windows)
     /// predicates.
     Family(targ::Family),
+    /// [target_has_atomic](https://doc.rust-lang.org/reference/conditional-compilation.html#target_has_atomic).
+    HasAtomic(targ::HasAtomic),
     /// [target_os](https://doc.rust-lang.org/reference/conditional-compilation.html#target_os)
     Os(targ::Os),
+    /// [panic](https://doc.rust-lang.org/reference/conditional-compilation.html#panic)
+    Panic(targ::Panic),
     /// [target_pointer_width](https://doc.rust-lang.org/reference/conditional-compilation.html#target_pointer_width)
     PointerWidth(u8),
     /// [target_vendor](https://doc.rust-lang.org/reference/conditional-compilation.html#target_vendor)
@@ -54,7 +58,9 @@ pub trait TargetMatcher {
 
 impl TargetMatcher for targ::TargetInfo {
     fn matches(&self, tp: &TargetPredicate) -> bool {
-        use TargetPredicate::{Arch, Endian, Env, Family, Os, PointerWidth, Vendor};
+        use TargetPredicate::{
+            Arch, Endian, Env, Family, HasAtomic, Os, Panic, PointerWidth, Vendor,
+        };
 
         match tp {
             Arch(a) => a == &self.arch,
@@ -65,12 +71,16 @@ impl TargetMatcher for targ::TargetInfo {
                 None => env.0.is_empty(),
             },
             Family(fam) => self.families.contains(fam),
+            HasAtomic(has_atomic) => self.has_atomics.contains(*has_atomic),
             Os(os) => Some(os) == self.os.as_ref(),
             PointerWidth(w) => *w == self.pointer_width,
             Vendor(ven) => match &self.vendor {
                 Some(v) => ven == v,
                 None => ven == &targ::Vendor::unknown,
             },
+            Panic(_panic) => {
+                todo!("probably want to model 'panic' as having a default value + an override")
+            }
         }
     }
 }
@@ -81,7 +91,9 @@ impl TargetMatcher for target_lexicon::Triple {
     #[allow(clippy::match_same_arms)]
     fn matches(&self, tp: &TargetPredicate) -> bool {
         use target_lexicon::*;
-        use TargetPredicate::{Arch, Endian, Env, Family, Os, PointerWidth, Vendor};
+        use TargetPredicate::{
+            Arch, Endian, Env, Family, HasAtomic, Os, Panic, PointerWidth, Vendor,
+        };
 
         match tp {
             Arch(arch) => {
@@ -263,6 +275,11 @@ impl TargetMatcher for target_lexicon::Triple {
                     _ => false,
                 }
             }
+            HasAtomic(_) => {
+                // atomic support depends on both the architecture and the OS. Assume false for
+                // this.
+                false
+            }
             Os(os) => match os.0.parse::<OperatingSystem>() {
                 Ok(o) => match self.environment {
                     Environment::HermitKernel => os == &targ::Os::hermit,
@@ -282,6 +299,10 @@ impl TargetMatcher for target_lexicon::Triple {
                     }
                 }
             },
+            Panic(_) => {
+                // panic support depends on the OS. Assume false for this.
+                false
+            }
             Vendor(ven) => match ven.0.parse::<target_lexicon::Vendor>() {
                 Ok(v) => self.vendor == v,
                 Err(_) => false,
@@ -337,6 +358,8 @@ pub(crate) enum Which {
     Env,
     Family,
     Os,
+    HasAtomic(targ::HasAtomic),
+    Panic,
     PointerWidth(u8),
     Vendor,
 }
@@ -409,6 +432,10 @@ impl InnerPredicate {
                     s[it.span.clone().unwrap()].to_owned(),
                 ))),
                 Which::Endian(end) => Target(TargetPredicate::Endian(*end)),
+                Which::HasAtomic(has_atomic) => Target(TargetPredicate::HasAtomic(*has_atomic)),
+                Which::Panic => Target(TargetPredicate::Panic(targ::Panic::new(
+                    s[it.span.clone().unwrap()].to_owned(),
+                ))),
                 Which::PointerWidth(pw) => Target(TargetPredicate::PointerWidth(*pw)),
             },
             IP::Test => Test,

src/expr/parser.rs

@@ -109,6 +109,10 @@ impl Expression {
                         }
                     }
                 }
+                "panic" => InnerPredicate::Target(InnerTarget {
+                    which: Which::Panic,
+                    span: Some(span),
+                }),
                 target_key if key.starts_with("target_") => {
                     let (val, vspan) = match val {
                         None => {
@@ -154,6 +158,14 @@ impl Expression {
                             })?),
                             span: None,
                         },
+                        "has_atomic" => InnerTarget {
+                            which: Which::HasAtomic(val.parse().map_err(|_err| ParseError {
+                                original: original.to_owned(),
+                                span: vspan,
+                                reason: Reason::InvalidHasAtomic,
+                            })?),
+                            span: None,
+                        },
                         "pointer_width" => InnerTarget {
                             which: Which::PointerWidth(val.parse().map_err(|_err| ParseError {
                                 original: original.to_owned(),
@@ -174,6 +186,7 @@ impl Expression {
                                     "target_family",
                                     "target_env",
                                     "target_endian",
+                                    "target_has_atomic",
                                     "target_pointer_width",
                                     "target_vendor",
                                 ]),

src/targets.rs

@@ -1,4 +1,4 @@
-use crate::error::Reason;
+use crate::error::{HasAtomicParseError, Reason};
 use std::{borrow::Cow, ops::Deref};
 
 mod builtins;
@@ -35,6 +35,10 @@ pub struct Family(pub Cow<'static, str>);
 #[derive(Clone, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
 pub struct Env(pub Cow<'static, str>);
 
+/// The panic strategy used on this target by default.
+#[derive(Clone, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
+pub struct Panic(pub Cow<'static, str>);
+
 macro_rules! field_impls {
     ($kind:ident) => {
         impl $kind {
@@ -80,6 +84,44 @@ field_impls!(Vendor);
 field_impls!(Os);
 field_impls!(Family);
 field_impls!(Env);
+field_impls!(Panic);
+
+/// Integer size and pointers for which there's support for atomic functions.
+#[derive(Copy, Clone, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
+#[non_exhaustive]
+pub enum HasAtomic {
+    /// The platform supports atomics for the given integer size in bits (e.g. `AtomicU8` if
+    /// `HasAtomic::IntegerSize(8)`).
+    IntegerSize(u16),
+
+    /// The platform supports atomics for pointers (`AtomicPtr`).
+    Pointer,
+}
+
+impl std::str::FromStr for HasAtomic {
+    type Err = HasAtomicParseError;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        if let Ok(size) = s.parse::<u16>() {
+            Ok(Self::IntegerSize(size))
+        } else if s == "ptr" {
+            Ok(HasAtomic::Pointer)
+        } else {
+            Err(HasAtomicParseError {
+                input: s.to_owned(),
+            })
+        }
+    }
+}
+
+impl std::fmt::Display for HasAtomic {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            Self::IntegerSize(size) => write!(f, "{}", size),
+            Self::Pointer => write!(f, "ptr"),
+        }
+    }
+}
 
 /// A set of families for a target.
 ///
@@ -145,6 +187,66 @@ impl std::fmt::Display for Families {
     }
 }
 
+/// A set of [`HasAtomic`] instances a target.
+#[derive(Clone, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
+pub struct HasAtomics(Cow<'static, [HasAtomic]>);
+
+impl HasAtomics {
+    /// Constructs a new instance.
+    ///
+    /// If you have a `&'static [HasAtomic]`, prefer [`Self::new_const`].
+    #[inline]
+    pub fn new(val: impl IntoIterator<Item = HasAtomic>) -> Self {
+        let mut has_atomics: Vec<_> = val.into_iter().collect();
+        has_atomics.sort_unstable();
+        Self(Cow::Owned(has_atomics))
+    }
+
+    /// Constructs a new instance of this struct from a static slice of [`HasAtomic`].
+    ///
+    /// `val` must be in sorted order: this constructor cannot check for that due to
+    /// limitations in current versions of Rust.
+    #[inline]
+    pub const fn new_const(val: &'static [HasAtomic]) -> Self {
+        // TODO: Check that val is sorted.
+        Self(Cow::Borrowed(val))
+    }
+
+    /// Returns true if this list of families contains a given family.
+    #[inline]
+    pub fn contains(&self, val: HasAtomic) -> bool {
+        self.0.contains(&val)
+    }
+}
+
+impl Deref for HasAtomics {
+    type Target = [HasAtomic];
+    fn deref(&self) -> &Self::Target {
+        &*self.0
+    }
+}
+
+impl AsRef<[HasAtomic]> for HasAtomics {
+    #[inline]
+    fn as_ref(&self) -> &[HasAtomic] {
+        &*self.0
+    }
+}
+
+impl std::fmt::Display for HasAtomics {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "{{")?;
+        let len = self.0.len();
+        for (idx, has_atomic) in self.0.iter().enumerate() {
+            write!(f, "{}", has_atomic)?;
+            if idx + 1 < len {
+                write!(f, ", ")?;
+            }
+        }
+        write!(f, "}}")
+    }
+}
+
 macro_rules! target_enum {
     (
         $(#[$outer:meta])*
@@ -238,6 +340,11 @@ pub struct TargetInfo {
     /// [target_endian](https://doc.rust-lang.org/reference/conditional-compilation.html#target_endian)
     /// predicate.
     pub endian: Endian,
+    /// The target's support for atomics. Used by the has_target_atomics predicate.
+    pub has_atomics: HasAtomics,
+    /// The panic strategy used on this target by default. Used by the
+    /// [panic](https://doc.rust-lang.org/beta/reference/conditional-compilation.html#panic) predicate.
+    pub panic: Panic,
 }
 
 /// Attempts to find the `TargetInfo` for the specified target triple
@@ -257,7 +364,7 @@ pub fn get_builtin_target_by_triple(triple: &str) -> Option<&'static TargetInfo>
 /// versions.
 ///
 /// ```
-/// assert_eq!("1.59.0", cfg_expr::targets::rustc_version());
+/// assert_eq!("1.60.0", cfg_expr::targets::rustc_version());
 /// ```
 pub fn rustc_version() -> &'static str {
     builtins::RUSTC_VERSION