Auto merge of #111671 - Dylan-DPC:rollup-1jy5r16, r=Dylan-DPC

Rollup of 6 pull requests

Successful merges:

 - #110145 (Share slice of bytes)
 - #111043 (Stabilize feature `cstr_is_empty`)
 - #111648 (Remove `LangItems::require`)
 - #111649 (Add derive for `core::marker::ConstParamTy`)
 - #111654 (Add a conversion from `&mut T` to `&mut UnsafeCell<T>`)
 - #111661 (Erase regions of type in `offset_of!`)

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup

3 files changed, 48 insertions, 13 deletions

diff --git a/compiler/rustc_data_structures/src/owned_slice.rs b/compiler/rustc_data_structures/src/owned_slice.rs
index 311a42aa42a..cbb3047d884 100644
--- a/compiler/rustc_data_structures/src/owned_slice.rs
+++ b/compiler/rustc_data_structures/src/owned_slice.rs
@@ -1,5 +1,6 @@
 use std::{borrow::Borrow, ops::Deref};
 
+use crate::sync::Lrc;
 // Use our fake Send/Sync traits when on not parallel compiler,
 // so that `OwnedSlice` only implements/requires Send/Sync
 // for parallel compiler builds.
@@ -7,7 +8,7 @@ use crate::sync::{Send, Sync};
 
 /// An owned slice.
 ///
-/// This is similar to `Box<[u8]>` but allows slicing and using anything as the
+/// This is similar to `Lrc<[u8]>` but allows slicing and using anything as the
 /// backing buffer.
 ///
 /// See [`slice_owned`] for `OwnedSlice` construction and examples.
@@ -16,6 +17,7 @@ use crate::sync::{Send, Sync};
 ///
 /// This is essentially a replacement for `owning_ref` which is a lot simpler
 /// and even sound! 🌸
+#[derive(Clone)]
 pub struct OwnedSlice {
     /// This is conceptually a `&'self.owner [u8]`.
     bytes: *const [u8],
@@ -31,7 +33,7 @@ pub struct OwnedSlice {
     //       \/
     //      ⊂(´･◡･⊂ )∘˚˳° (I am the phantom remnant of #97770)
     #[expect(dead_code)]
-    owner: Box<dyn Send + Sync>,
+    owner: Lrc<dyn Send + Sync>,
 }
 
 /// Makes an [`OwnedSlice`] out of an `owner` and a `slicer` function.
@@ -72,10 +74,10 @@ where
     O: Send + Sync + 'static,
     F: FnOnce(&O) -> Result<&[u8], E>,
 {
-    // We box the owner of the bytes, so it doesn't move.
+    // We wrap the owner of the bytes in, 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.
+    // before dropping, 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.
@@ -83,12 +85,39 @@ where
     // 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 owner = Lrc::new(owner);
     let bytes = slicer(&*owner)?;
 
     Ok(OwnedSlice { bytes, owner })
 }
 
+impl OwnedSlice {
+    /// Slice this slice by `slicer`.
+    ///
+    /// # 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[..]);
+    /// assert_eq!(&*slice, [1, 2, 3, 4]);
+    ///
+    /// let slice = slice.slice(|slice| &slice[1..][..2]);
+    /// assert_eq!(&*slice, [2, 3]);
+    /// ```
+    ///
+    pub fn slice(self, slicer: impl FnOnce(&[u8]) -> &[u8]) -> OwnedSlice {
+        // This is basically identical to `try_slice_owned`,
+        // `slicer` can only return slices of its argument or some static data,
+        // both of which are valid while `owner` is alive.
+
+        let bytes = slicer(&self);
+        OwnedSlice { bytes, ..self }
+    }
+}
+
 impl Deref for OwnedSlice {
     type Target = [u8];
 
@@ -108,11 +137,11 @@ impl Borrow<[u8]> for OwnedSlice {
     }
 }
 
-// Safety: `OwnedSlice` is conceptually `(&'self.1 [u8], Box<dyn Send + Sync>)`, which is `Send`
+// Safety: `OwnedSlice` is conceptually `(&'self.1 [u8], Arc<dyn Send + Sync>)`, which is `Send`
 #[cfg(parallel_compiler)]
 unsafe impl Send for OwnedSlice {}
 
-// Safety: `OwnedSlice` is conceptually `(&'self.1 [u8], Box<dyn Send + Sync>)`, which is `Sync`
+// Safety: `OwnedSlice` is conceptually `(&'self.1 [u8], Arc<dyn Send + Sync>)`, which is `Sync`
 #[cfg(parallel_compiler)]
 unsafe impl Sync for OwnedSlice {}
 
diff --git a/compiler/rustc_data_structures/src/owned_slice/tests.rs b/compiler/rustc_data_structures/src/owned_slice/tests.rs
index e151b8c2de0..1eb5378cd1a 100644
--- a/compiler/rustc_data_structures/src/owned_slice/tests.rs
+++ b/compiler/rustc_data_structures/src/owned_slice/tests.rs
@@ -26,7 +26,7 @@ fn static_storage() {
 }
 
 #[test]
-fn slice_the_slice() {
+fn slice_owned_the_slice() {
     let slice = slice_owned(vec![1, 2, 3, 4, 5, 6], Vec::as_slice);
     let slice = slice_owned(slice, |s| &s[1..][..4]);
     let slice = slice_owned(slice, |s| s);
@@ -36,6 +36,16 @@ fn slice_the_slice() {
 }
 
 #[test]
+fn slice_the_slice() {
+    let slice = slice_owned(vec![1, 2, 3, 4, 5, 6], Vec::as_slice)
+        .slice(|s| &s[1..][..4])
+        .slice(|s| s)
+        .slice(|s| &s[1..]);
+
+    assert_eq!(&*slice, &[1, 2, 3, 4, 5, 6][1..][..4][1..]);
+}
+
+#[test]
 fn try_and_fail() {
     let res = try_slice_owned(vec![0], |v| v.get(12..).ok_or(()));
 
diff --git a/compiler/rustc_data_structures/src/sync.rs b/compiler/rustc_data_structures/src/sync.rs
index 8a778866a77..6c3197d8ec2 100644
--- a/compiler/rustc_data_structures/src/sync.rs
+++ b/compiler/rustc_data_structures/src/sync.rs
@@ -40,7 +40,6 @@
 //! [^2] `MTLockRef` is a typedef.
 
 pub use crate::marker::*;
-use crate::owned_slice::OwnedSlice;
 use std::collections::HashMap;
 use std::hash::{BuildHasher, Hash};
 use std::ops::{Deref, DerefMut};
@@ -92,6 +91,7 @@ mod mode {
 }
 
 pub use mode::{is_dyn_thread_safe, set_dyn_thread_safe_mode};
+
 cfg_if! {
     if #[cfg(not(parallel_compiler))] {
         pub unsafe auto trait Send {}
@@ -244,8 +244,6 @@ cfg_if! {
             r
         }
 
-        pub type MetadataRef = OwnedSlice;
-
         pub use std::rc::Rc as Lrc;
         pub use std::rc::Weak as Weak;
         pub use std::cell::Ref as ReadGuard;
@@ -517,8 +515,6 @@ cfg_if! {
             }
         }
 
-        pub type MetadataRef = OwnedSlice;
-
         /// This makes locks panic if they are already held.
         /// It is only useful when you are running in a single thread
         const ERROR_CHECKING: bool = false;