Implement OwnedSlice

compiler/rustc_data_structures/src/lib.rs

@@ -27,6 +27,8 @@
 #![feature(thread_id_value)]
 #![feature(vec_into_raw_parts)]
 #![feature(get_mut_unchecked)]
+#![feature(lint_reasons)]
+#![feature(unwrap_infallible)]
 #![allow(rustc::default_hash_types)]
 #![allow(rustc::potential_query_instability)]
 #![deny(rustc::untranslatable_diagnostic)]
@@ -82,6 +84,7 @@ pub mod vec_linked_list;
 pub mod work_queue;
 pub use atomic_ref::AtomicRef;
 pub mod frozen;
+pub mod owned_slice;
 pub mod sso;
 pub mod steal;
 pub mod tagged_ptr;

compiler/rustc_data_structures/src/owned_slice.rs

@@ -0,0 +1,113 @@
+use std::{borrow::Borrow, ops::Deref};
+
+// Use our fake Send/Sync traits when on not parallel compiler,
+// so that `OwnedSlice` only implements/requires Send/Sync
+// for parallel compiler builds.
+use crate::sync::{Send, Sync};
+
+/// An owned slice.
+///
+/// This is similar to `Box<[u8]>` but allows slicing and using anything as the
+/// backing buffer.
+///
+/// See [`slice_owned`] for `OwnedSlice` construction and examples.
+///
+/// ---------------------------------------------------------------------------
+///
+/// This is essentially a replacement for `owning_ref` which is a lot simpler
+/// and even sound! 🌸
+pub struct OwnedSlice {
+    /// This is conceptually a `&'self.owner [u8]`.
+    bytes: *const [u8],
+
+    // +---------------------------------------+
+    // | We expect `dead_code` lint here,      |
+    // | because we don't want to accidentally |
+    // | touch the owner — otherwise the owner |
+    // | could invalidate out `bytes` pointer  |
+    // |                                       |
+    // | so be quite                           |
+    // +----+  +-------------------------------+
+    //       \/
+    //      ⊂(´･◡･⊂ )∘˚˳°
+    #[expect(dead_code)]
+    owner: Box<dyn Send + Sync>,
+}
+
+/// Makes an [`OwnedSlice`] out of an `owner` and a `slicer` function.
+///
+/// ## Examples
+///
+/// ```rust
+/// # use rustc_data_structures::owned_slice::{OwnedSlice, slice_owned};
+/// let vec = vec![1, 2, 3, 4];
+///
+/// // Identical to slicing via `&v[1..3]` but produces an owned slice
+/// let slice: OwnedSlice = slice_owned(vec, |v| &v[1..3]);
+/// assert_eq!(&*slice, [2, 3]);
+/// ```
+///
+/// ```rust
+/// # use rustc_data_structures::owned_slice::{OwnedSlice, slice_owned};
+/// # use std::ops::Deref;
+/// let vec = vec![1, 2, 3, 4];
+///
+/// // Identical to slicing via `&v[..]` but produces an owned slice
+/// let slice: OwnedSlice = slice_owned(vec, Deref::deref);
+/// assert_eq!(&*slice, [1, 2, 3, 4]);
+/// ```
+pub fn slice_owned<O, F>(owner: O, slicer: F) -> OwnedSlice
+where
+    O: Send + Sync + 'static,
+    F: Fn(&O) -> &[u8],
+{
+    try_slice_owned(owner, |x| Ok::<_, !>(slicer(x))).into_ok()
+}
+
+/// Makes an [`OwnedSlice`] out of an `owner` and a `slicer` function that can fail.
+///
+/// See [`slice_owned`] for the infallible version.
+pub fn try_slice_owned<O, F, E>(owner: O, slicer: F) -> Result<OwnedSlice, E>
+where
+    O: Send + Sync + 'static,
+    F: Fn(&O) -> Result<&[u8], E>,
+{
+    // We box the owner of the bytes, so it doesn't move.
+    //
+    // Since the owner does not move and we don't access it in any way
+    // before drop, there is nothing that can invalidate the bytes pointer.
+    //
+    // Thus, "extending" the lifetime of the reference returned from `F` is fine.
+    // We pretend that we pass it a reference that lives as long as the returned slice.
+    //
+    // N.B. the HRTB on the `slicer` is important — without it the caller could provide
+    // a short lived slice, unrelated to the owner.
+
+    let owner = Box::new(owner);
+    let bytes = slicer(&*owner)?;
+
+    Ok(OwnedSlice { bytes, owner })
+}
+
+impl Deref for OwnedSlice {
+    type Target = [u8];
+
+    fn deref(&self) -> &[u8] {
+        // Safety:
+        // `self.bytes` is valid per the construction in `slice_owned`
+        // (which is the only constructor)
+        unsafe { &*self.bytes }
+    }
+}
+
+impl Borrow<[u8]> for OwnedSlice {
+    fn borrow(&self) -> &[u8] {
+        self
+    }
+}
+
+// Safety: `OwnedSlice` is conceptually `(&'self.1 [u8], Box<dyn Send + Sync>)`, which is `Send`
+unsafe impl Send for OwnedSlice {}
+
+// Safety: `OwnedSlice` is conceptually `(&'self.1 [u8], Box<dyn Send + Sync>)`, which is `Sync`
+unsafe impl Sync for OwnedSlice {}

compiler/rustc_data_structures/src/sync.rs

@@ -57,11 +57,8 @@ mod vec;
 
 cfg_if! {
     if #[cfg(not(parallel_compiler))] {
-        pub auto trait Send {}
-        pub auto trait Sync {}
-
-        impl<T> Send for T {}
-        impl<T> Sync for T {}
+        pub unsafe auto trait Send {}
+        pub unsafe auto trait Sync {}
 
         #[macro_export]
         macro_rules! rustc_erase_owner {
@@ -69,6 +66,8 @@ cfg_if! {
                 $v.erase_owner()
             }
         }
+        unsafe impl<T> Send for T {}
+        unsafe impl<T> Sync for T {}
 
         use std::ops::Add;