Auto merge of #109971 - WaffleLapkin:yeet_ownership, r=Nilstrieb

Yeet `owning_ref`

Based on the discussions from #109948

This replaces `owning_ref` with a far simpler & safer abstraction.

Fixes #109974

compiler/rustc_codegen_ssa/src/back/metadata.rs

@@ -13,8 +13,7 @@ use object::{
 use snap::write::FrameEncoder;
 
 use rustc_data_structures::memmap::Mmap;
-use rustc_data_structures::owning_ref::OwningRef;
-use rustc_data_structures::rustc_erase_owner;
+use rustc_data_structures::owned_slice::try_slice_owned;
 use rustc_data_structures::sync::MetadataRef;
 use rustc_metadata::fs::METADATA_FILENAME;
 use rustc_metadata::EncodedMetadata;
@@ -42,10 +41,10 @@ fn load_metadata_with(
 ) -> Result<MetadataRef, String> {
     let file =
         File::open(path).map_err(|e| format!("failed to open file '{}': {}", path.display(), e))?;
-    let data = unsafe { Mmap::map(file) }
-        .map_err(|e| format!("failed to mmap file '{}': {}", path.display(), e))?;
-    let metadata = OwningRef::new(data).try_map(f)?;
-    return Ok(rustc_erase_owner!(metadata.map_owner_box()));
+
+    unsafe { Mmap::map(file) }
+        .map_err(|e| format!("failed to mmap file '{}': {}", path.display(), e))
+        .and_then(|mmap| try_slice_owned(mmap, |mmap| f(mmap)))
 }
 
 impl MetadataLoader for DefaultMetadataLoader {

compiler/rustc_data_structures/src/lib.rs

@@ -1,5 +1,5 @@
 //! Various data structures used by the Rust compiler. The intention
-//! is that code in here should be not be *specific* to rustc, so that
+//! is that code in here should not be *specific* to rustc, so that
 //! it can be easily unit tested and so forth.
 //!
 //! # Note
@@ -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)]
@@ -59,7 +61,6 @@ pub mod intern;
 pub mod jobserver;
 pub mod macros;
 pub mod obligation_forest;
-pub mod owning_ref;
 pub mod sip128;
 pub mod small_c_str;
 pub mod small_str;
@@ -82,6 +83,7 @@ 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/memmap.rs

@@ -2,9 +2,7 @@ use std::fs::File;
 use std::io;
 use std::ops::{Deref, DerefMut};
 
-use crate::owning_ref::StableAddress;
-
-/// A trivial wrapper for [`memmap2::Mmap`] that implements [`StableAddress`].
+/// A trivial wrapper for [`memmap2::Mmap`] (or `Vec<u8>` on WASM).
 #[cfg(not(target_arch = "wasm32"))]
 pub struct Mmap(memmap2::Mmap);
 
@@ -46,12 +44,6 @@ impl AsRef<[u8]> for Mmap {
     }
 }
 
-// SAFETY: On architectures other than WASM, mmap is used as backing storage. The address of this
-// memory map is stable. On WASM, `Vec<u8>` is used as backing storage. The `Mmap` type doesn't
-// export any function that can cause the `Vec` to be re-allocated. As such the address of the
-// bytes inside this `Vec` is stable.
-unsafe impl StableAddress for Mmap {}
-
 #[cfg(not(target_arch = "wasm32"))]
 pub struct MmapMut(memmap2::MmapMut);
 

compiler/rustc_data_structures/src/owned_slice.rs

@@ -0,0 +1,118 @@
+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 quiet                           |
+    // +----+  +-------------------------------+
+    //       \/
+    //      ⊂(´･◡･⊂ )∘˚˳° (I am the phantom remnant of #97770)
+    #[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: FnOnce(&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: FnOnce(&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];
+
+    #[inline]
+    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 {
+    #[inline]
+    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 {}
+
+#[cfg(test)]
+mod tests;

compiler/rustc_data_structures/src/owned_slice/tests.rs

@@ -0,0 +1,74 @@
+use std::{
+    ops::Deref,
+    sync::{
+        atomic::{self, AtomicBool},
+        Arc,
+    },
+};
+
+use crate::{
+    owned_slice::{slice_owned, try_slice_owned, OwnedSlice},
+    OnDrop,
+};
+
+#[test]
+fn smoke() {
+    let slice = slice_owned(vec![1, 2, 3, 4, 5, 6], Vec::as_slice);
+
+    assert_eq!(&*slice, [1, 2, 3, 4, 5, 6]);
+}
+
+#[test]
+fn static_storage() {
+    let slice = slice_owned(Box::new(String::from("what")), |_| b"bytes boo");
+
+    assert_eq!(&*slice, b"bytes boo");
+}
+
+#[test]
+fn slice_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);
+    let slice = slice_owned(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(()));
+
+    assert!(res.is_err());
+}
+
+#[test]
+fn boxed() {
+    // It's important that we don't cause UB because of `Box`'es uniqueness
+
+    let boxed: Box<[u8]> = vec![1, 1, 2, 3, 5, 8, 13, 21].into_boxed_slice();
+    let slice = slice_owned(boxed, Deref::deref);
+
+    assert_eq!(&*slice, [1, 1, 2, 3, 5, 8, 13, 21]);
+}
+
+#[test]
+fn drop_drops() {
+    let flag = Arc::new(AtomicBool::new(false));
+    let flag_prime = Arc::clone(&flag);
+    let d = OnDrop(move || flag_prime.store(true, atomic::Ordering::Relaxed));
+
+    let slice = slice_owned(d, |_| &[]);
+
+    assert_eq!(flag.load(atomic::Ordering::Relaxed), false);
+
+    drop(slice);
+
+    assert_eq!(flag.load(atomic::Ordering::Relaxed), true);
+}
+
+#[test]
+fn send_sync() {
+    crate::sync::assert_send::<OwnedSlice>();
+    crate::sync::assert_sync::<OwnedSlice>();
+}