Introduce MaybeLazy(Cow) for 3-way lazy-less

Author: Urgau

compiler/rustc_target/src/spec/maybe_lazy.rs

@@ -0,0 +1,125 @@
+//! A custom LazyLock+Cow suitable for holding borrowed, owned or lazy data.
+
+use std::borrow::{Borrow, Cow};
+use std::fmt::{Debug, Display};
+use std::ops::Deref;
+use std::sync::LazyLock;
+
+enum MaybeLazyInner<T: 'static + ToOwned + ?Sized> {
+    Lazy(LazyLock<T::Owned>),
+    Cow(Cow<'static, T>),
+}
+
+/// A custom LazyLock+Cow suitable for holding borrowed, owned or lazy data.
+///
+/// Technically this structure has 3 states: borrowed, owned and lazy
+/// They can all be constructed from the [`MaybeLazy::borrowed`], [`MaybeLazy::owned`] and
+/// [`MaybeLazy::lazy`] methods.
+#[repr(transparent)]
+pub struct MaybeLazy<T: 'static + ToOwned + ?Sized> {
+    // Inner state.
+    //
+    // Not to be inlined since we may want in the future to
+    // make this struct usable to statics and we might need to
+    // workaround const-eval limitation (particulary around drop).
+    inner: MaybeLazyInner<T>,
+}
+
+impl<T: 'static + ?Sized + ToOwned> MaybeLazy<T> {
+    /// Create a [`MaybeLazy`] from an borrowed `T`.
+    #[inline]
+    pub const fn borrowed(a: &'static T) -> Self {
+        MaybeLazy { inner: MaybeLazyInner::Cow(Cow::Borrowed(a)) }
+    }
+
+    /// Create a [`MaybeLazy`] from an borrowed `T`.
+    #[inline]
+    pub const fn owned(a: T::Owned) -> Self {
+        MaybeLazy { inner: MaybeLazyInner::Cow(Cow::Owned(a)) }
+    }
+
+    /// Create a [`MaybeLazy`] that is lazy by taking a function pointer.
+    ///
+    /// This function pointer cannot *ever* take a closure. User can potentially
+    /// workaround that by using closure-to-fnptr or `const` items.
+    #[inline]
+    pub const fn lazy(a: fn() -> T::Owned) -> Self {
+        MaybeLazy { inner: MaybeLazyInner::Lazy(LazyLock::new(a)) }
+    }
+}
+
+impl<T: 'static + ?Sized + ToOwned<Owned: Clone>> Clone for MaybeLazy<T> {
+    #[inline]
+    fn clone(&self) -> Self {
+        MaybeLazy {
+            inner: MaybeLazyInner::Cow(match &self.inner {
+                MaybeLazyInner::Lazy(f) => Cow::Owned((*f).to_owned()),
+                MaybeLazyInner::Cow(c) => c.clone(),
+            }),
+        }
+    }
+}
+
+impl<T: 'static + ?Sized + ToOwned<Owned: Default>> Default for MaybeLazy<T> {
+    #[inline]
+    fn default() -> MaybeLazy<T> {
+        MaybeLazy::lazy(T::Owned::default)
+    }
+}
+
+// `Debug`, `Display` and other traits below are implemented in terms of this `Deref`
+impl<T: 'static + ?Sized + ToOwned<Owned: Borrow<T>>> Deref for MaybeLazy<T> {
+    type Target = T;
+
+    #[inline]
+    fn deref(&self) -> &T {
+        match &self.inner {
+            MaybeLazyInner::Lazy(f) => (&**f).borrow(),
+            MaybeLazyInner::Cow(c) => &*c,
+        }
+    }
+}
+
+impl<T: 'static + ?Sized + ToOwned<Owned: Debug> + Debug> Debug for MaybeLazy<T> {
+    #[inline]
+    fn fmt(&self, fmt: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        Debug::fmt(&**self, fmt)
+    }
+}
+
+impl<T: 'static + ?Sized + ToOwned<Owned: Display> + Display> Display for MaybeLazy<T> {
+    #[inline]
+    fn fmt(&self, fmt: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        Display::fmt(&**self, fmt)
+    }
+}
+
+impl<T: 'static + ?Sized + ToOwned> AsRef<T> for MaybeLazy<T> {
+    #[inline]
+    fn as_ref(&self) -> &T {
+        &**self
+    }
+}
+
+impl<B: ?Sized + PartialEq<C> + ToOwned, C: ?Sized + ToOwned> PartialEq<MaybeLazy<C>>
+    for MaybeLazy<B>
+{
+    #[inline]
+    fn eq(&self, other: &MaybeLazy<C>) -> bool {
+        PartialEq::eq(&**self, &**other)
+    }
+}
+
+impl PartialEq<&str> for MaybeLazy<str> {
+    #[inline]
+    fn eq(&self, other: &&str) -> bool {
+        &**self == *other
+    }
+}
+
+impl From<&'static str> for MaybeLazy<str> {
+    #[inline]
+    fn from(s: &'static str) -> MaybeLazy<str> {
+        MaybeLazy::borrowed(s)
+    }
+}

compiler/rustc_target/src/spec/mod.rs

@@ -55,13 +55,16 @@ use tracing::debug;
 
 pub mod abi;
 pub mod crt_objects;
+pub mod maybe_lazy;
 
 mod base;
 pub use base::apple::deployment_target as current_apple_deployment_target;
 pub use base::apple::platform as current_apple_platform;
 pub use base::apple::sdk_version as current_apple_sdk_version;
 pub use base::avr_gnu::ef_avr_arch;
 
+use maybe_lazy::MaybeLazy;
+
 /// Linker is called through a C/C++ compiler.
 #[derive(Clone, Copy, Debug, Eq, Ord, PartialEq, PartialOrd)]
 pub enum Cc {