Add migration lint for 2024 prelude additions

This adds the migration lint for the newly ambiguous methods `poll` and
`into_future`. When these methods are used on types implementing the
respective traits, it will be ambiguous in the future, which can lead to
hard errors or behavior changes depending on the exact circumstances.

Author: Noratrieb

compiler/rustc_hir_typeck/src/method/prelude_edition_lints.rs

@@ -9,7 +9,7 @@ use rustc_hir as hir;
 use rustc_lint::{ARRAY_INTO_ITER, BOXED_SLICE_INTO_ITER};
 use rustc_middle::span_bug;
 use rustc_middle::ty::{self, Ty};
-use rustc_session::lint::builtin::RUST_2021_PRELUDE_COLLISIONS;
+use rustc_session::lint::builtin::{RUST_2021_PRELUDE_COLLISIONS, RUST_2024_PRELUDE_COLLISIONS};
 use rustc_span::symbol::kw::{Empty, Underscore};
 use rustc_span::symbol::{sym, Ident};
 use rustc_span::Span;
@@ -35,6 +35,21 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         let (prelude_or_array_lint, edition) = match segment.ident.name {
             // `try_into` was added to the prelude in Rust 2021.
             sym::try_into if !span.at_least_rust_2021() => (RUST_2021_PRELUDE_COLLISIONS, "2021"),
+            // `Future::poll` was added to the prelude in Rust 2024.
+            sym::poll
+                // We check that the self type is `Pin<&mut _>` to avoid false positives for this common name.
+                if !span.at_least_rust_2024()
+                    && let ty::Adt(adt_def, args) = self_ty.kind()
+                    && self.tcx.is_lang_item(adt_def.did(), hir::LangItem::Pin)
+                    && let ty::Ref(_, _, ty::Mutability::Mut) =
+                        args[0].as_type().unwrap().kind() =>
+            {
+                (RUST_2024_PRELUDE_COLLISIONS, "2024")
+            }
+            // `IntoFuture::into_future` was added to the prelude in Rust 2024.
+            sym::into_future if !span.at_least_rust_2024() => {
+                (RUST_2024_PRELUDE_COLLISIONS, "2024")
+            }
             // `into_iter` wasn't added to the prelude,
             // but `[T; N].into_iter()` doesn't resolve to IntoIterator::into_iter
             // before Rust 2021, which results in the same problem.

compiler/rustc_lint_defs/src/builtin.rs

@@ -91,6 +91,7 @@ declare_lint_pass! {
         RUST_2021_PREFIXES_INCOMPATIBLE_SYNTAX,
         RUST_2021_PRELUDE_COLLISIONS,
         RUST_2024_INCOMPATIBLE_PAT,
+        RUST_2024_PRELUDE_COLLISIONS,
         SELF_CONSTRUCTOR_FROM_OUTER_ITEM,
         SEMICOLON_IN_EXPRESSIONS_FROM_MACROS,
         SINGLE_USE_LIFETIMES,
@@ -3750,6 +3751,46 @@ declare_lint! {
     };
 }
 
+declare_lint! {
+    /// The `rust_2024_prelude_collisions` lint detects the usage of trait methods which are ambiguous
+    /// with traits added to the prelude in future editions.
+    ///
+    /// ### Example
+    ///
+    /// ```rust,edition2021,compile_fail
+    /// #![deny(rust_2024_prelude_collisions)]
+    /// trait Meow {
+    ///     fn poll(&self) {}
+    /// }
+    /// impl<T> Meow for T {}
+    ///
+    /// fn main() {
+    ///     core::pin::pin!(async {}).poll();
+    ///     //                        ^^^^^^
+    ///     // This call to try_into matches both Future::poll and Meow::poll as
+    ///     // `Future` has been added to the Rust prelude in 2024 edition.
+    /// }
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// Rust 2024, introduces two new additions to the standard library's prelude:
+    /// `Future` and `IntoFuture`. This results in an ambiguity as to which method/function
+    /// to call when an existing `poll`/`into_future` method is called via dot-call syntax or
+    /// a `poll`/`into_future` associated function is called directly on a type.
+    ///
+    pub RUST_2024_PRELUDE_COLLISIONS,
+    Allow,
+    "detects the usage of trait methods which are ambiguous with traits added to the \
+        prelude in future editions",
+    @future_incompatible = FutureIncompatibleInfo {
+        reason: FutureIncompatibilityReason::EditionError(Edition::Edition2024),
+        reference: "<https://doc.rust-lang.org/nightly/edition-guide/rust-2024/prelude.html>",
+    };
+}
+
 declare_lint! {
     /// The `rust_2021_prefixes_incompatible_syntax` lint detects identifiers that will be parsed as a
     /// prefix instead in Rust 2021.

tests/ui/rust-2024/prelude-migration/future-poll-already-future.rs

@@ -0,0 +1,17 @@
+//@ revisions: e2021 e2024
+//@[e2021] edition: 2021
+//@[e2024] edition: 2024
+//@[e2024] compile-flags: -Zunstable-options
+//@ check-pass
+
+#![deny(rust_2024_prelude_collisions)]
+
+use std::future::Future;
+
+fn main() {
+    core::pin::pin!(async {}).poll(&mut context());
+}
+
+fn context() -> core::task::Context<'static> {
+    loop {}
+}

tests/ui/rust-2024/prelude-migration/future-poll-async-block.e2021.fixed

@@ -0,0 +1,21 @@
+//@ revisions: e2021 e2024
+//@[e2021] edition: 2021
+//@[e2021] run-rustfix
+//@[e2024] edition: 2024
+//@[e2024] compile-flags: -Zunstable-options
+//@[e2024] check-pass
+
+#![deny(rust_2024_prelude_collisions)]
+trait Meow {
+    fn poll(&self, _ctx: &mut core::task::Context<'_>) {}
+}
+impl<T> Meow for T {}
+fn main() {
+    Meow::poll(&core::pin::pin!(async {}), &mut context());
+    //[e2021]~^ ERROR trait method `poll` will become ambiguous in Rust 2024
+    //[e2021]~| WARN this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+}
+
+fn context() -> core::task::Context<'static> {
+    loop {}
+}

tests/ui/rust-2024/prelude-migration/future-poll-async-block.e2021.stderr

@@ -0,0 +1,16 @@
+error: trait method `poll` will become ambiguous in Rust 2024
+  --> $DIR/future-poll-async-block.rs:14:5
+   |
+LL |     core::pin::pin!(async {}).poll(&mut context());
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: disambiguate the associated function: `Meow::poll(&core::pin::pin!(async {}), &mut context())`
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2024/prelude.html>
+note: the lint level is defined here
+  --> $DIR/future-poll-async-block.rs:8:9
+   |
+LL | #![deny(rust_2024_prelude_collisions)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+

tests/ui/rust-2024/prelude-migration/future-poll-async-block.rs

@@ -0,0 +1,21 @@
+//@ revisions: e2021 e2024
+//@[e2021] edition: 2021
+//@[e2021] run-rustfix
+//@[e2024] edition: 2024
+//@[e2024] compile-flags: -Zunstable-options
+//@[e2024] check-pass
+
+#![deny(rust_2024_prelude_collisions)]
+trait Meow {
+    fn poll(&self, _ctx: &mut core::task::Context<'_>) {}
+}
+impl<T> Meow for T {}
+fn main() {
+    core::pin::pin!(async {}).poll(&mut context());
+    //[e2021]~^ ERROR trait method `poll` will become ambiguous in Rust 2024
+    //[e2021]~| WARN this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+}
+
+fn context() -> core::task::Context<'static> {
+    loop {}
+}

tests/ui/rust-2024/prelude-migration/future-poll-not-future-pinned.e2021.fixed

@@ -0,0 +1,21 @@
+//@ revisions: e2021 e2024
+//@[e2021] edition: 2021
+//@[e2021] run-rustfix
+//@[e2024] edition: 2024
+//@[e2024] compile-flags: -Zunstable-options
+//@[e2024] check-pass
+
+#![deny(rust_2024_prelude_collisions)]
+trait Meow {
+    fn poll(&self) {}
+}
+impl<T> Meow for T {}
+fn main() {
+    // This is a deliberate false positive.
+    // While `()` does not implement `Future` and can therefore not be ambiguous, we
+    // do not check that in the lint, as that introduces additional complexities.
+    // Just checking whether the self type is `Pin<&mut _>` is enough.
+    Meow::poll(&core::pin::pin!(()));
+    //[e2021]~^ ERROR trait method `poll` will become ambiguous in Rust 2024
+    //[e2021]~| WARN this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+}

tests/ui/rust-2024/prelude-migration/future-poll-not-future-pinned.e2021.stderr

@@ -0,0 +1,16 @@
+error: trait method `poll` will become ambiguous in Rust 2024
+  --> $DIR/future-poll-not-future-pinned.rs:18:5
+   |
+LL |     core::pin::pin!(()).poll();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: disambiguate the associated function: `Meow::poll(&core::pin::pin!(()))`
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2024/prelude.html>
+note: the lint level is defined here
+  --> $DIR/future-poll-not-future-pinned.rs:8:9
+   |
+LL | #![deny(rust_2024_prelude_collisions)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+

tests/ui/rust-2024/prelude-migration/future-poll-not-future-pinned.rs

@@ -0,0 +1,21 @@
+//@ revisions: e2021 e2024
+//@[e2021] edition: 2021
+//@[e2021] run-rustfix
+//@[e2024] edition: 2024
+//@[e2024] compile-flags: -Zunstable-options
+//@[e2024] check-pass
+
+#![deny(rust_2024_prelude_collisions)]
+trait Meow {
+    fn poll(&self) {}
+}
+impl<T> Meow for T {}
+fn main() {
+    // This is a deliberate false positive.
+    // While `()` does not implement `Future` and can therefore not be ambiguous, we
+    // do not check that in the lint, as that introduces additional complexities.
+    // Just checking whether the self type is `Pin<&mut _>` is enough.
+    core::pin::pin!(()).poll();
+    //[e2021]~^ ERROR trait method `poll` will become ambiguous in Rust 2024
+    //[e2021]~| WARN this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+}

tests/ui/rust-2024/prelude-migration/future-poll-not-future.rs

@@ -0,0 +1,15 @@
+//@ revisions: e2021 e2024
+//@[e2021] edition: 2021
+//@[e2024] edition: 2024
+//@[e2024] compile-flags: -Zunstable-options
+//@ check-pass
+
+#![deny(rust_2024_prelude_collisions)]
+trait Meow {
+    fn poll(&self) {}
+}
+impl<T> Meow for T {}
+fn main() {
+    // As the self type here is not `Pin<&mut _>`, the lint does not fire.
+    ().poll();
+}