From 49852e129fc3074c984984f3e4ab1db5d34f6647 Mon Sep 17 00:00:00 2001 From: Daniel Henry-Mantilla Date: Thu, 20 Jul 2023 20:34:44 +0200 Subject: [PATCH] Initial release candidate --- .cargo/config | 17 ++ .github/workflows/CI.yml | 109 ++++++++++++ .github/workflows/future-proof.yml | 57 +++++++ .gitignore | 2 + Cargo.lock | 7 + Cargo.toml | 59 +++++++ LICENSE-APACHE | 201 ++++++++++++++++++++++ LICENSE-MIT | 21 +++ LICENSE-ZLIB | 19 +++ README.md | 39 +++++ cargo_publish.sh | 14 ++ change_version.sh | 14 ++ fix-docsrs-li-details-summary.html | 80 +++++++++ rust-toolchain.toml | 6 + src/lib.rs | 42 +++++ src/manually_drop.rs | 260 +++++++++++++++++++++++++++++ src/maybe_dangling.rs | 123 ++++++++++++++ 17 files changed, 1070 insertions(+) create mode 100644 .cargo/config create mode 100644 .github/workflows/CI.yml create mode 100644 .github/workflows/future-proof.yml create mode 100644 .gitignore create mode 100644 Cargo.lock create mode 100644 Cargo.toml create mode 100644 LICENSE-APACHE create mode 100644 LICENSE-MIT create mode 100644 LICENSE-ZLIB create mode 100644 README.md create mode 100755 cargo_publish.sh create mode 100755 change_version.sh create mode 100644 fix-docsrs-li-details-summary.html create mode 100644 rust-toolchain.toml create mode 100644 src/lib.rs create mode 100644 src/manually_drop.rs create mode 100644 src/maybe_dangling.rs diff --git a/.cargo/config b/.cargo/config new file mode 100644 index 0000000..3470e65 --- /dev/null +++ b/.cargo/config @@ -0,0 +1,17 @@ +# Templated by `cargo-generate` using https://github.com/danielhenrymantilla/proc-macro-template +[alias] +docs-rs = [ + "doc", "--no-deps", "--open", + "--features", "docs-rs", +] + +[build] +rustdocflags = [ + "--html-before-content", "fix-docsrs-li-details-summary.html", +] + +[doc.extern-map.registries] +crates-io = "https://docs.rs/" + +[profile.rust-analyzer] +inherits = "dev" diff --git a/.github/workflows/CI.yml b/.github/workflows/CI.yml new file mode 100644 index 0000000..a240e7e --- /dev/null +++ b/.github/workflows/CI.yml @@ -0,0 +1,109 @@ +name: CI + +concurrency: + group: ${{ github.workflow }}-${{ github.ref }} + cancel-in-progress: true + +on: + push: + branches: + - master + pull_request: + +jobs: + # == CHECK == # + check: + name: "Check beta stable and MSRV=1.65.0" + runs-on: ubuntu-latest + strategy: + matrix: + rust-toolchain: + - 1.65.0 + - stable + # Try to guard against a near-future regression. + - beta + cargo-locked: ['', '--locked'] + feature-nightly-dropck_eyepatch: ['', '--features nightly-dropck_eyepatch'] + exclude: + # MSRV guarantee only stands for `.lock`-tested dependencies. + - rust-toolchain: 1.65.0 + cargo-locked: '' + steps: + - name: Install Rust toolchain + uses: actions-rs/toolchain@v1 + with: + profile: minimal + toolchain: ${{ matrix.rust-toolchain }} + override: true + + - name: Clone repo + uses: actions/checkout@v2 + + - name: Update `Cargo.lock` + if: matrix.cargo-locked != '--locked' + run: cargo update -v + + - name: Cargo check + uses: actions-rs/cargo@v1 + env: + RUSTC_BOOTSTRAP: 1 + with: + command: check + args: ${{ matrix.cargo-locked }} ${{ matrix.feature-nightly-dropck_eyepatch }} + + # == BUILD & TEST == # + build-and-test: + name: Build and test + runs-on: ${{ matrix.os }} + needs: [] + strategy: + fail-fast: false + matrix: + os: + - ubuntu-latest + - macos-latest + - windows-latest + rust-toolchain: + - 1.65.0 + - stable + steps: + - name: Install Rust toolchain + uses: actions-rs/toolchain@v1 + with: + profile: default + override: true + toolchain: ${{ matrix.rust-toolchain }} + + - name: Clone repo + uses: actions/checkout@v2 + + - name: cargo test --lib --tests + uses: actions-rs/cargo@v1 + with: + command: test + args: --lib --tests + + - name: cargo test --doc + if: matrix.rust-toolchain == 'stable' + uses: actions-rs/cargo@v1 + env: + RUSTC_BOOTSTRAP: 1 + with: + command: test + args: --features better-docs --doc + + required-jobs: + needs: + - check + - build-and-test + runs-on: ubuntu-latest + if: ${{ always() }} + steps: + - name: 'All required jobs' + run: | + RESULT=$(echo "${{ join(needs.*.result, '') }}" | sed -e "s/success//g") + if [ -n "$RESULT" ]; then + echo "❌" + false + fi + echo "✅" diff --git a/.github/workflows/future-proof.yml b/.github/workflows/future-proof.yml new file mode 100644 index 0000000..65ae41d --- /dev/null +++ b/.github/workflows/future-proof.yml @@ -0,0 +1,57 @@ +# Templated by `cargo-generate` using https://github.com/danielhenrymantilla/proc-macro-template +name: Cron CI + +on: + push: + branches: + - master + schedule: + - cron: '0 8 * * 1,5' + +jobs: + # == TEST == # + test-no-ui: + name: (Check & Build &) Test + runs-on: ${{ matrix.os }} + strategy: + fail-fast: false + matrix: + os: + - ubuntu-latest + - macos-latest + - windows-latest + rust-toolchain: + # Future-proof against compiler-regressions + - beta + - nightly + cargo-locked: ['--locked'] + feature-nightly-dropck_eyepatch: ['', '--features nightly-dropck_eyepatch'] + include: + # Also future-proof against semver breakage from dependencies. + - rust-toolchain: stable + cargo-locked: '' + steps: + - name: Install Rust toolchain + uses: actions-rs/toolchain@v1 + with: + profile: default + override: true + toolchain: ${{ matrix.rust-toolchain }} + + - name: Clone repo + uses: actions/checkout@v2 + + - name: Update `Cargo.lock` + if: matrix.cargo-locked != '--locked' + run: cargo update -v + + - name: Cargo test + uses: actions-rs/cargo@v1 + env: + RUSTC_BOOTSTRAP: 1 + with: + command: test + args: | + --features better-docs + ${{ matrix.cargo-locked }} + ${{ matrix.feature-nightly-dropck_eyepatch }} diff --git a/.gitignore b/.gitignore new file mode 100644 index 0000000..7654850 --- /dev/null +++ b/.gitignore @@ -0,0 +1,2 @@ +# Templated by `cargo-generate` using https://github.com/danielhenrymantilla/proc-macro-template +/target diff --git a/Cargo.lock b/Cargo.lock new file mode 100644 index 0000000..0f7bc73 --- /dev/null +++ b/Cargo.lock @@ -0,0 +1,7 @@ +# This file is automatically @generated by Cargo. +# It is not intended for manual editing. +version = 3 + +[[package]] +name = "maybe-dangling" +version = "0.1.0-rc1" diff --git a/Cargo.toml b/Cargo.toml new file mode 100644 index 0000000..448982a --- /dev/null +++ b/Cargo.toml @@ -0,0 +1,59 @@ +# Templated by `cargo-generate` using https://github.com/danielhenrymantilla/proc-macro-template +[package] +name = "maybe-dangling" +authors = [ + "Daniel Henry-Mantilla ", +] +version = "0.1.0-rc1" # Keep in sync +edition = "2021" +rust-version = "1.65.0" + +license = "Zlib OR MIT OR Apache-2.0" +repository = "https://github.com/danielhenrymantilla/maybe-dangling.rs" +documentation = "https://docs.rs/maybe-dangling" +readme = "README.md" + +description = "`ManuallyDrop` and `MaybeDangling` semantics in stable Rust as per https://github.com/rust-lang/rfcs/pull/3336" +keywords = ["unsafe", "dangling", "aliasing", "dereferenceable", "manuallydrop"] + +[features] +default = [ +] + +# Enable and use `#![feature(dropck_eyepatch)]` for `MaybeDangling`. +nightly-dropck_eyepatch = [] + +# unstable! Will probably use nightly to improve doc quality +better-docs = [] + +ui-tests = [ + "better-docs", +] + +# `docs-rs` is very similar to `better-docs`, but for it being allowed to enable +# other features as well, often to make sure optional/non-`default` features +# make it to the rendered docs (using `--all-features` instead is not that great). +docs-rs = [ + "better-docs", +] + +[dependencies] + +[dev-dependencies] + +[workspace] + +[package.metadata.docs.rs] +features = [ + "docs-rs", +] +rustdoc-args = [ + # Use (with or without `open`): + # ```md + #
Click to hide + # … + #
+ # ``` + # to get nice drop-down menus (and avoid the css bug with rustdoc). + "--html-before-content", "fix-docsrs-li-details-summary.html", +] diff --git a/LICENSE-APACHE b/LICENSE-APACHE new file mode 100644 index 0000000..8e4fc9d --- /dev/null +++ b/LICENSE-APACHE @@ -0,0 +1,201 @@ + Apache License + Version 2.0, January 2004 + http://www.apache.org/licenses/ + + TERMS AND CONDITIONS FOR USE, REPRODUCTION, AND DISTRIBUTION + + 1. Definitions. + + "License" shall mean the terms and conditions for use, reproduction, + and distribution as defined by Sections 1 through 9 of this document. + + "Licensor" shall mean the copyright owner or entity authorized by + the copyright owner that is granting the License. + + "Legal Entity" shall mean the union of the acting entity and all + other entities that control, are controlled by, or are under common + control with that entity. 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If you use this software + in a product, an acknowledgment in the product documentation would be + appreciated but is not required. +2. Altered source versions must be plainly marked as such, and must not be + misrepresented as being the original software. +3. This notice may not be removed or altered from any source distribution. diff --git a/README.md b/README.md new file mode 100644 index 0000000..37f40e9 --- /dev/null +++ b/README.md @@ -0,0 +1,39 @@ +# `::maybe-dangling` + +`ManuallyDrop` and `MaybeDangling` semantics in stable Rust as per + +[![Repository](https://img.shields.io/badge/repository-GitHub-brightgreen.svg)]( +https://github.com/danielhenrymantilla/maybe-dangling.rs) +[![Latest version](https://img.shields.io/crates/v/maybe-dangling.svg)]( +https://crates.io/crates/maybe-dangling) +[![Documentation](https://docs.rs/maybe-dangling/badge.svg)]( +https://docs.rs/maybe-dangling) +[![MSRV](https://img.shields.io/badge/MSRV-1.65.0-white)]( +https://gist.github.com/danielhenrymantilla/9b59de4db8e5f2467ed008b3c450527b) +[![License](https://img.shields.io/crates/l/maybe-dangling.svg)]( +https://github.com/danielhenrymantilla/maybe-dangling.rs/blob/master/LICENSE-ZLIB) +[![CI](https://github.com/danielhenrymantilla/maybe-dangling.rs/workflows/CI/badge.svg)]( +https://github.com/danielhenrymantilla/maybe-dangling.rs/actions) +[![no_std compatible](https://img.shields.io/badge/no__std-compatible-success.svg)]( +https://github.com/rust-secure-code/safety-dance/) + + + +This crates offers two types, `ManuallyDrop`, and `MaybeDangling`, which do not carry +[aliasing/`dereferenceable`-ity properties](https://github.com/rust-lang/rfcs/pull/3336) w.r.t. the +`T` they each contain, which means they are allowed to: + 1. have some expired value inside of them, such as `T = &'expired …`, + 1. be fed to a function that does not inspect its value (such as `::core::mem::forget()`), + 1. exhibit well-defined behavior (no UB!). + +## References + + - ### The RFC that shall eventually and ultimately supersede this very crate: + + - The `miri` PR implementing the check against this: + + - The soundness problem of `::ouroboros` stemming from not using this: + + - The soundness problem of `::yoke` stemming from not using this: + + - [An URLO thread on the topic, and my post exposing the intention to write this very crate](https://users.rust-lang.org/t/unsafe-code-review-semi-owning-weak-rwlock-t-guard/95706/15?u=yandros) diff --git a/cargo_publish.sh b/cargo_publish.sh new file mode 100755 index 0000000..2666f81 --- /dev/null +++ b/cargo_publish.sh @@ -0,0 +1,14 @@ +#!/bin/sh + +set -euxo pipefail + +cargo update -vw +[[ -z "$(git status --porcelain)" ]] + + +for i in $(seq 10) +do + cargo publish && exit 0 + sleep 5 +done +cargo publish diff --git a/change_version.sh b/change_version.sh new file mode 100755 index 0000000..9760c9d --- /dev/null +++ b/change_version.sh @@ -0,0 +1,14 @@ +#!/bin/sh + +set -euxo pipefail + +find . \ + -type f \ + -name 'Cargo.toml' \ + -print \ + -a \ + -exec \ + sed -i -E "s/\"(=)?.*?\"( # Keep in sync)/\"\\1$1\"\\2/g" '{}' \ + \; + +cargo update -v -w diff --git a/fix-docsrs-li-details-summary.html b/fix-docsrs-li-details-summary.html new file mode 100644 index 0000000..ba7fde7 --- /dev/null +++ b/fix-docsrs-li-details-summary.html @@ -0,0 +1,80 @@ + diff --git a/rust-toolchain.toml b/rust-toolchain.toml new file mode 100644 index 0000000..6626c2a --- /dev/null +++ b/rust-toolchain.toml @@ -0,0 +1,6 @@ +[toolchain] +channel = '1.65.0' +profile = 'minimal' +components = [ + 'rustfmt', +] diff --git a/src/lib.rs b/src/lib.rs new file mode 100644 index 0000000..9a84d09 --- /dev/null +++ b/src/lib.rs @@ -0,0 +1,42 @@ +#![doc = include_str!("../README.md")] +#![no_std] +#![deny(unsafe_code)] + +#![cfg_attr(feature = "nightly-dropck_eyepatch", + feature(dropck_eyepatch), +)] + +pub use self::maybe_dangling::MaybeDangling; +mod maybe_dangling; + +pub use manually_drop::ManuallyDrop; +mod manually_drop; + +#[rustfmt::skip] +/// I really don't get the complexity of `cfg_if!`… +macro_rules! cfg_match { + ( + _ => { $($expansion:tt)* } $(,)? + ) => ( + $($expansion)* + ); + + ( + $cfg:meta => $expansion:tt $(, + $($($rest:tt)+)? )? + ) => ( + #[cfg($cfg)] + crate::cfg_match! { _ => $expansion } $($( + + #[cfg(not($cfg))] + crate::cfg_match! { $($rest)+ } )?)? + ); + + // Bonus: expression-friendly syntax: `cfg_match!({ … })` + ({ + $($input:tt)* + }) => ({ + crate::cfg_match! { $($input)* } + }); +} +use cfg_match; diff --git a/src/manually_drop.rs b/src/manually_drop.rs new file mode 100644 index 0000000..bca6735 --- /dev/null +++ b/src/manually_drop.rs @@ -0,0 +1,260 @@ +use ::core::{ + cmp::*, + fmt::{self, Debug}, + hash::{self, Hash}, + mem::MaybeUninit as MU, + ops::{Deref, DerefMut}, +}; + +/// A wrapper to inhibit compiler from automatically calling `T`’s destructor. +/// This wrapper is 0-cost. +/// +/// See [`::core::mem::ManuallyDrop`] for more info. +/// +/// # Differences with [`::core::mem::ManuallyDrop`] +/// +/// - **No niches** +/// +/// The current implementation uses [`::core::mem::MaybeUninit`] to make +/// sure the aliasing and `dereferenceable`ity properties of the inner `T` +/// are properly disabled. +/// +/// The main side-effect of this implementation is that it disables +/// niches, thereby preventing **discriminant elision**. +/// +/// For instance, an [Option]<[ManuallyDrop]<[bool]>> will +/// occupy _two_ bytes rather than _one_. +/// +/// - It does not implement +/// [`Structural{Partial,}Eq`][::core::marker::StructuralEq]. +/// +/// - Note that once stdlib's own [`::core::mem::ManuallyDrop`] properly gets +/// its aliasing/`dereferenceable`ity properties removed, this crate shall +/// be updated to just reëxport it (using a `build.rs` to prevent MSRV +/// breakage). +/// +/// This means that the _lack of discriminant elision_ cannot be relied upon +/// either! +/// +/// - Other than that, this is a `#[repr(transparent)]` wrapper around `T`, +/// thereby having: +/// - equal [`Layout`][::core::alloc::Layout]; +/// - equal calling-convention ABI[^1] +/// +/// [^1]: this is assuming `MaybeUninit` has the same ABI as `T`, as it +/// currently advertises, despite that probably being a bad idea for +/// a "bag of bytes" `T`-ish wrapper, since it means that padding bytes +/// inside of `T` won't be preserved when working with a +/// `MaybeUninit`. So, if the stdlib were to break the current +/// ABI promise of `MaybeUninit` to cater to that problem, then this crate would +/// probably do so well, unless the `maybe_dangling` changes were to make it to +/// the stdlib first. +#[derive(Copy)] +#[repr(transparent)] +pub struct ManuallyDrop { + /// Until stdlib guarantees `MaybeDangling` semantics for its `ManuallyDrop`, + /// we have to polyfill it ourselves using `MaybeUninit`, the only type + /// known to date to feature such semantics. + /// + /// So doing, quite unfortunately, disables niche optimizations. + /// + /// # SAFETY INVARIANT: the value must always be init `MU`-wise. + value: MU, +} + +// SAFETY: as per the safety invariant above. +#[allow(unsafe_code)] +impl ManuallyDrop { + /// Wrap a value to be manually dropped. + /// + /// See [`::core::mem::ManuallyDrop::new()`] for more info. + #[inline] + pub const fn new(value: T) -> ManuallyDrop { + Self { + value: MU::new(value), + } + } + + /// Extracts the value from the `ManuallyDrop` container. + /// + /// See [`::core::mem::ManuallyDrop::into_inner()`] for more info. + #[inline] + pub const fn into_inner(slot: ManuallyDrop) -> T { + unsafe { MU::assume_init(slot.value) } + } + + /// Takes the value from the `ManuallyDrop` container out. + /// + /// See [`::core::mem::ManuallyDrop::take()`] for more info. + #[must_use = "if you don't need the value, you can use `ManuallyDrop::drop` instead"] + #[inline] + pub unsafe fn take(slot: &mut ManuallyDrop) -> T { + unsafe { + slot.value.as_ptr().read() + } + } + + /// Manually drops the contained value. + /// + /// See [`::core::mem::ManuallyDrop::drop()`] for more info. + #[inline] + pub unsafe fn drop(slot: &mut ManuallyDrop) { + unsafe { + slot.value.as_mut_ptr().drop_in_place() + } + } +} + +// Safety: as per the invariant mentioned above. +#[allow(unsafe_code)] +impl DerefMut for ManuallyDrop { + /// See [`::core::mem::ManuallyDrop::deref_mut()`] for more info. + #[inline] + fn deref_mut(&mut self) -> &mut T { + impl Deref for ManuallyDrop { + type Target = T; + + #[inline] + /// See [`::core::mem::ManuallyDrop::deref()`] for more info. + fn deref(self: &Self) -> &T { + unsafe { + self.value.assume_init_ref() + } + } + } + + unsafe { + self.value.assume_init_mut() + } + } +} + +impl Default for ManuallyDrop { + #[inline] + fn default() -> Self { + Self::new(T::default()) + } +} + +impl Clone for ManuallyDrop { + /// See [`::core::mem::ManuallyDrop::clone()`] for more info. + fn clone(self: &Self) -> Self { + Self::new(T::clone(self)) + } + + JustDerefTM! { + /// See [`::core::mem::ManuallyDrop::clone_from()`] for more info. + fn clone_from(self: &mut Self, source: &Self); + } +} + +JustDerefTM! { + impl Debug for ManuallyDrop { + /// See [`::core::mem::ManuallyDrop::fmt()`] for more info. + fn fmt(self: &Self, f: &mut fmt::Formatter<'_>) -> fmt::Result; + } + + impl Hash for ManuallyDrop { + /// See [`::core::mem::ManuallyDrop::hash()`] for more info. + fn hash<__H: hash::Hasher>(self: &Self, state: &mut __H); + } + + impl Ord for ManuallyDrop { + /// See [`::core::mem::ManuallyDrop::cmp()`] for more info. + fn cmp(self: &Self, other: &Self) -> Ordering; + } + + impl PartialOrd for ManuallyDrop { + /// See [`::core::mem::ManuallyDrop::partial_cmp()`] for more info. + fn partial_cmp(self: &Self, other: &Self) -> Option; + } + + impl PartialEq for ManuallyDrop { + /// See [`::core::mem::ManuallyDrop::eq()`] for more info. + fn eq(self: &Self, other: &Self) -> bool; + } + + impl Eq for ManuallyDrop {} +} + + + +macro_rules! JustDerefTM { + ( + $( + $(#$attr:tt)* + $($(@$if_unsafe:tt)? + unsafe + )? + impl + $($($Trait:ident)::+ for)? + ManuallyDrop + { + $($inner:tt)* + } + )* + ) => ( + $( + $(#$attr)* + $($($if_unsafe)? + unsafe + )? + impl + $($($Trait)::+ for)? + ManuallyDrop + { + JustDerefTM! { + $($inner)* + } + } + )* + $( + $(#$attr)* + $($($if_unsafe)? + unsafe + )? + impl + $($($Trait)::+ for)? + crate::MaybeDangling + { + JustDerefTM! { + $($inner)* + } + } + )* + ); + + ( + $( + $(#$attr:tt)* + $pub:vis + fn $fname:ident + $(<$H:ident $(: $Bound:path)?>)? + ( + $( + $arg_name:ident : $ArgTy:ty + ),* $(,)? + ) $(-> $Ret:ty)? ; + )* + ) => ( + $( + #[inline] + $(#$attr)* + #[doc = concat!( + "\nSee [`::core::mem::ManuallyDrop::", stringify!($fname), "()`] for more info." + )] + $pub + fn $fname + $(<$H $(: $Bound)?>)? + ( + $( + $arg_name : $ArgTy + ),* + ) $(-> $Ret)? + { + T::$fname($($arg_name),*) + } + )* + ); +} +use JustDerefTM; diff --git a/src/maybe_dangling.rs b/src/maybe_dangling.rs new file mode 100644 index 0000000..7296342 --- /dev/null +++ b/src/maybe_dangling.rs @@ -0,0 +1,123 @@ +use { + ::core::{ + mem::{ManuallyDrop as StdMD}, + }, + crate::{ + ManuallyDrop, + }, +}; + +/// Like [`crate::ManuallyDrop`] but for having `drop` glue. +/// This wrapper is 0-cost. +/// +/// In other words, a [MaybeDangling]\ is just like `T`, but +/// for having been stripped of aliasing/`dereferenceable`-ity properties. +/// +/// Its usage should be quite rare and advanced: if you are intending to keep +/// hold of a potentially dangling / exhausted value, chances are you won't +/// want implicit/automatic drop glue of it without having previously checked +/// for lack of exhaustion ⚠️. +/// +/// That is, it is strongly advisable to be using +/// [crate::ManuallyDrop]\ instead! +/// +/// ### Opting into `#[may_dangle]` and the `dropck_eyepatch` +/// +/// Ironically, for this drop glue to be as smooth as it should be, the unstable +/// `#[may_dangle]` feature is needed. +/// +/// But by virtue of being unstable, it cannot be offered by this crate on +/// stable Rust. +/// +/// For the adventurous `nightly` users, you can enable the +/// `nightly-dropck_eyepatch` Cargo feature to opt into the usage of the +/// eponymous `rustc` feature so as to get the `Drop` impl amended accordingly. +pub struct MaybeDangling { + value: ManuallyDrop, + #[cfg(feature = "nightly-dropck_eyepatch")] + #[allow(nonstandard_style)] + _owns_T: ::core::marker::PhantomData, +} + +impl MaybeDangling { + pub const fn new(value: T) -> MaybeDangling { + Self { + value: ManuallyDrop::new(value), + #[cfg(feature = "nightly-dropck_eyepatch")] + _owns_T: ::core::marker::PhantomData, + } + } + + /// Extracts the value from the `MaybeDangling` container. + /// + /// See [`::core::mem::ManuallyDrop::into_inner()`] for more info. + #[inline] + pub fn into_inner(slot: MaybeDangling) -> T { + #![allow(unsafe_code)] + // Safety: this is the defuse inherent drop glue pattern. + unsafe { + ManuallyDrop::take(&mut StdMD::new(slot).value) + } + } +} + +// The main difference with `ManuallyDrop`: automatic drop glue! +crate::cfg_match! { + feature = "nightly-dropck_eyepatch" => { + unsafe impl<#[may_dangle] T> Drop for MaybeDangling { + fn drop(&mut self) { + #![allow(unsafe_code)] + unsafe { + ManuallyDrop::drop(&mut self.value) + } + } + } + }, + + _ => { + impl Drop for MaybeDangling { + fn drop(&mut self) { + #![allow(unsafe_code)] + unsafe { + ManuallyDrop::drop(&mut self.value) + } + } + } + }, +} + + +impl ::core::ops::DerefMut for MaybeDangling { + #[inline] + fn deref_mut(&mut self) -> &mut T { + impl ::core::ops::Deref for MaybeDangling { + type Target = T; + + #[inline] + fn deref(self: &Self) -> &T { + let Self { value, .. } = self; + value + } + } + + let Self { value, .. } = self; + value + } +} + +impl Default for MaybeDangling { + #[inline] + fn default() -> Self { + Self::new(T::default()) + } +} + +impl Clone for MaybeDangling { + fn clone(self: &Self) -> Self { + Self::new(T::clone(self)) + } + + fn clone_from(self: &mut Self, source: &Self) { + T::clone_from(self, source) + } +}