Subpart7 for async drop (major1) - library changes

compiler/rustc_feature/src/unstable.rs

@@ -365,6 +365,8 @@ declare_features! (
     (unstable, associated_type_defaults, "1.2.0", Some(29661)),
     /// Allows `async || body` closures.
     (unstable, async_closure, "1.37.0", Some(62290)),
+    /// Allows implementing `AsyncDrop`.
+    (incomplete, async_drop, "CURRENT_RUSTC_VERSION", Some(126482)),
     /// Allows `#[track_caller]` on async functions.
     (unstable, async_fn_track_caller, "1.73.0", Some(110011)),
     /// Allows `for await` loops.

library/core/src/future/async_drop.rs

@@ -0,0 +1,51 @@
+#![unstable(feature = "async_drop", issue = "126482")]
+
+#[allow(unused_imports)]
+use core::future::Future;
+
+#[allow(unused_imports)]
+use crate::pin::Pin;
+#[allow(unused_imports)]
+use crate::task::{Context, Poll};
+
+/// Async version of Drop trait.
+///
+/// When a value is no longer needed, Rust will run a "destructor" on that value.
+/// The most common way that a value is no longer needed is when it goes out of
+/// scope. Destructors may still run in other circumstances, but we're going to
+/// focus on scope for the examples here. To learn about some of those other cases,
+/// please see [the reference] section on destructors.
+///
+/// [the reference]: https://doc.rust-lang.org/reference/destructors.html
+///
+/// ## `Copy` and ([`Drop`]|`AsyncDrop`) are exclusive
+///
+/// You cannot implement both [`Copy`] and ([`Drop`]|`AsyncDrop`) on the same type. Types that
+/// are `Copy` get implicitly duplicated by the compiler, making it very
+/// hard to predict when, and how often destructors will be executed. As such,
+/// these types cannot have destructors.
+#[cfg(not(bootstrap))]
+#[unstable(feature = "async_drop", issue = "126482")]
+#[lang = "async_drop"]
+pub trait AsyncDrop {
+    /// Executes the async destructor for this type.
+    ///
+    /// This method is called implicitly when the value goes out of scope,
+    /// and cannot be called explicitly.
+    ///
+    /// When this method has been called, `self` has not yet been deallocated.
+    /// That only happens after the method is over.
+    ///
+    /// # Panics
+    #[allow(async_fn_in_trait)]
+    async fn drop(self: Pin<&mut Self>);
+}
+
+/// Async drop.
+#[cfg(not(bootstrap))]
+#[unstable(feature = "async_drop", issue = "126482")]
+#[lang = "async_drop_in_place"]
+pub async unsafe fn async_drop_in_place<T: ?Sized>(_to_drop: *mut T) {
+    // Code here does not matter - this is replaced by the
+    // real implementation by the compiler.
+}

library/core/src/future/mod.rs

@@ -12,13 +12,17 @@
 use crate::ptr::NonNull;
 use crate::task::Context;
 
+mod async_drop;
 mod future;
 mod into_future;
 mod join;
 mod pending;
 mod poll_fn;
 mod ready;
 
+#[cfg(not(bootstrap))]
+#[unstable(feature = "async_drop", issue = "126482")]
+pub use async_drop::{async_drop_in_place, AsyncDrop};
 #[stable(feature = "into_future", since = "1.64.0")]
 pub use into_future::IntoFuture;
 #[stable(feature = "future_readiness_fns", since = "1.48.0")]

tests/ui/feature-gates/feature-gate-async-drop.rs

@@ -0,0 +1,16 @@
+//@ edition: 2021
+
+use std::future::AsyncDrop; //~ ERROR  use of unstable library feature 'async_drop'
+use std::pin::Pin;
+
+struct Foo {}
+
+impl Drop for Foo {
+    fn drop(&mut self) {}
+}
+
+impl AsyncDrop for Foo { //~ ERROR  use of unstable library feature 'async_drop'
+    async fn drop(self: Pin<&mut Self>) {} //~ ERROR  use of unstable library feature 'async_drop'
+}
+
+fn main() {}

tests/ui/feature-gates/feature-gate-async-drop.stderr

@@ -0,0 +1,33 @@
+error[E0658]: use of unstable library feature 'async_drop'
+  --> $DIR/feature-gate-async-drop.rs:3:5
+   |
+LL | use std::future::AsyncDrop;
+   |     ^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #126482 <https://github.com/rust-lang/rust/issues/126482> for more information
+   = help: add `#![feature(async_drop)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error[E0658]: use of unstable library feature 'async_drop'
+  --> $DIR/feature-gate-async-drop.rs:13:5
+   |
+LL |     async fn drop(self: Pin<&mut Self>) {}
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #126482 <https://github.com/rust-lang/rust/issues/126482> for more information
+   = help: add `#![feature(async_drop)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error[E0658]: use of unstable library feature 'async_drop'
+  --> $DIR/feature-gate-async-drop.rs:12:6
+   |
+LL | impl AsyncDrop for Foo {
+   |      ^^^^^^^^^
+   |
+   = note: see issue #126482 <https://github.com/rust-lang/rust/issues/126482> for more information
+   = help: add `#![feature(async_drop)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0658`.