Merge pull request #292 from ehuss/2024-prelude

Add 2024 prelude

src/SUMMARY.md

@@ -38,6 +38,7 @@
 ## Rust 2024
 
 - [Rust 2024 🚧](rust-2024/index.md)
+  - [Additions to the prelude](rust-2024/prelude.md)
   - [`unsafe_op_in_unsafe_fn` warning](rust-2024/unsafe-op-in-unsafe-fn.md)
   - [RPIT lifetime capture](rust-2024/rpit-lifetime-capture.md)
   - [Disallow references to `static mut`](rust-2024/static-mut-reference.md)

src/rust-2024/prelude.md

@@ -0,0 +1,69 @@
+# Additions to the prelude
+
+🚧 The 2024 Edition has not yet been released and hence this section is still "under construction".
+More information may be found in the tracking issue at <https://github.com/rust-lang/rust/issues/121042>.
+
+## Summary
+
+- The [`Future`] and [`IntoFuture`] traits are now part of the prelude.
+- This might make calls to trait methods ambiguous which could make some code fail to compile.
+
+[`Future`]: ../../std/future/trait.Future.html
+[`IntoFuture`]: ../../std/future/trait.IntoFuture.html
+
+## Details
+
+The [prelude of the standard library](../../std/prelude/index.html) is the module containing everything that is automatically imported in every module.
+It contains commonly used items such as `Option`, `Vec`, `drop`, and `Clone`.
+
+The Rust compiler prioritizes any manually imported items over those from the prelude,
+to make sure additions to the prelude will not break any existing code.
+For example, if you have a crate or module called `example` containing a `pub struct Option;`,
+then `use example::*;` will make `Option` unambiguously refer to the one from `example`;
+not the one from the standard library.
+
+However, adding a _trait_ to the prelude can break existing code in a subtle way.
+For example, a call to `x.poll()` which comes from a `MyPoller` trait might fail to compile if `std`'s `Future` is also imported, because the call to `poll` is now ambiguous and could come from either trait.
+
+As a solution, Rust 2024 will use a new prelude.
+It's identical to the current one, except for two new additions:
+
+- [`std::future::Future`][`Future`]
+- [`std::future::IntoFuture`][`IntoFuture`]
+
+## Migration
+
+🚧 The automatic migration for this has not yet been implemented.
+
+### Migration needed
+
+#### Conflicting trait methods
+
+When two traits that are in scope have the same method name, it is ambiguous which trait method should be used. For example:
+
+```rust,edition2021
+trait MyPoller {
+    // This name is the same as the `poll` method on the `Future` trait from `std`.
+    fn poll(&self) {
+        println!("polling");
+    }
+}
+
+impl<T> MyPoller for T {}
+
+fn main() {
+    // Pin<&mut async {}> implements both `std::future::Future` and `MyPoller`.
+    // If both traits are in scope (as would be the case in Rust 2024),
+    // then it becomes ambiguous which `poll` method to call
+    core::pin::pin!(async {}).poll();
+}
+```
+
+We can fix this by using fully qualified syntax:
+
+```rust,ignore
+fn main() {
+    // Now it is clear which trait method we're referring to
+    <_ as MyPoller>::poll(&core::pin::pin!(async {}));
+}
+```