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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
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Original file line number | Diff line number | Diff line change |
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use std::{borrow::Borrow, ops::Deref}; | ||
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// 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}; | ||
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/// 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], | ||
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// +---------------------------------------+ | ||
// | 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>, | ||
} | ||
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/// 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() | ||
} | ||
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/// 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. | ||
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let owner = Box::new(owner); | ||
let bytes = slicer(&*owner)?; | ||
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Ok(OwnedSlice { bytes, owner }) | ||
} | ||
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impl Deref for OwnedSlice { | ||
type Target = [u8]; | ||
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#[inline] | ||
fn deref(&self) -> &[u8] { | ||
// Safety: | ||
// `self.bytes` is valid per the construction in `slice_owned` | ||
// (which is the only constructor) | ||
unsafe { &*self.bytes } | ||
} | ||
} | ||
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impl Borrow<[u8]> for OwnedSlice { | ||
#[inline] | ||
fn borrow(&self) -> &[u8] { | ||
self | ||
} | ||
} | ||
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// Safety: `OwnedSlice` is conceptually `(&'self.1 [u8], Box<dyn Send + Sync>)`, which is `Send` | ||
unsafe impl Send for OwnedSlice {} | ||
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// Safety: `OwnedSlice` is conceptually `(&'self.1 [u8], Box<dyn Send + Sync>)`, which is `Sync` | ||
unsafe impl Sync for OwnedSlice {} | ||
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#[cfg(test)] | ||
mod tests; |
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use std::{ | ||
ops::Deref, | ||
sync::{ | ||
atomic::{self, AtomicBool}, | ||
Arc, | ||
}, | ||
}; | ||
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use crate::{ | ||
owned_slice::{slice_owned, try_slice_owned, OwnedSlice}, | ||
OnDrop, | ||
}; | ||
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#[test] | ||
fn smoke() { | ||
let slice = slice_owned(vec![1, 2, 3, 4, 5, 6], Vec::as_slice); | ||
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assert_eq!(&*slice, [1, 2, 3, 4, 5, 6]); | ||
} | ||
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#[test] | ||
fn static_storage() { | ||
let slice = slice_owned(Box::new(String::from("what")), |_| b"bytes boo"); | ||
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assert_eq!(&*slice, b"bytes boo"); | ||
} | ||
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#[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..]); | ||
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assert_eq!(&*slice, &[1, 2, 3, 4, 5, 6][1..][..4][1..]); | ||
} | ||
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#[test] | ||
fn try_and_fail() { | ||
let res = try_slice_owned(vec![0], |v| v.get(12..).ok_or(())); | ||
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assert!(res.is_err()); | ||
} | ||
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#[test] | ||
fn boxed() { | ||
// It's important that we don't cause UB because of `Box`'es uniqueness | ||
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let boxed: Box<[u8]> = vec![1, 1, 2, 3, 5, 8, 13, 21].into_boxed_slice(); | ||
let slice = slice_owned(boxed, Deref::deref); | ||
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assert_eq!(&*slice, [1, 1, 2, 3, 5, 8, 13, 21]); | ||
} | ||
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#[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)); | ||
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let slice = slice_owned(d, |_| &[]); | ||
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assert_eq!(flag.load(atomic::Ordering::Relaxed), false); | ||
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drop(slice); | ||
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assert_eq!(flag.load(atomic::Ordering::Relaxed), true); | ||
} | ||
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#[test] | ||
fn send_sync() { | ||
crate::sync::assert_send::<OwnedSlice>(); | ||
crate::sync::assert_sync::<OwnedSlice>(); | ||
} |
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