Warn when [T; N].into_iter() is ambiguous in the new edition.

Cargo.lock

@@ -4405,6 +4405,7 @@ dependencies = [
  "rustc_hir_pretty",
  "rustc_index",
  "rustc_infer",
+ "rustc_lint",
  "rustc_macros",
  "rustc_middle",
  "rustc_session",

compiler/rustc_lint/src/lib.rs

@@ -62,6 +62,8 @@ mod traits;
 mod types;
 mod unused;
 
+pub use array_into_iter::ARRAY_INTO_ITER;
+
 use rustc_ast as ast;
 use rustc_hir as hir;
 use rustc_hir::def_id::LocalDefId;

compiler/rustc_typeck/Cargo.toml

@@ -26,3 +26,4 @@ rustc_index = { path = "../rustc_index" }
 rustc_infer = { path = "../rustc_infer" }
 rustc_trait_selection = { path = "../rustc_trait_selection" }
 rustc_ty_utils = { path = "../rustc_ty_utils" }
+rustc_lint = { path = "../rustc_lint" }

compiler/rustc_typeck/src/check/method/prelude2021.rs

@@ -5,7 +5,7 @@ use rustc_ast::Mutability;
 use rustc_errors::Applicability;
 use rustc_hir as hir;
 use rustc_middle::ty::subst::InternalSubsts;
-use rustc_middle::ty::{Adt, Ref, Ty};
+use rustc_middle::ty::{Adt, Array, Ref, Ty};
 use rustc_session::lint::builtin::RUST_2021_PRELUDE_COLLISIONS;
 use rustc_span::symbol::kw::Underscore;
 use rustc_span::symbol::{sym, Ident};
@@ -38,11 +38,28 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             return;
         }
 
-        // These are the method names that were added to prelude in Rust 2021
-        if !matches!(segment.ident.name, sym::try_into) {
+        // `try_into` was added to the prelude in Rust 2021.
+        // `into_iter` wasn't, but `[T; N].into_iter()` doesn't resolve to
+        // IntoIterator::into_iter before Rust 2021, which results in the same
+        // problem.
+        if !matches!(segment.ident.name, sym::try_into | sym::into_iter) {
             return;
         }
 
+        let prelude_or_array_lint = if segment.ident.name == sym::into_iter {
+            // The `into_iter` problem is only a thing for arrays.
+            if let Array(..) = self_ty.kind() {
+                // In this case, it wasn't really a prelude addition that was the problem.
+                // Instead, the problem is that the array-into_iter hack will no longer apply in Rust 2021.
+                rustc_lint::ARRAY_INTO_ITER
+            } else {
+                // No problem in this case.
+                return;
+            }
+        } else {
+            RUST_2021_PRELUDE_COLLISIONS
+        };
+
         // No need to lint if method came from std/core, as that will now be in the prelude
         if matches!(self.tcx.crate_name(pick.item.def_id.krate), sym::std | sym::core) {
             return;
@@ -69,7 +86,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             // Inherent impls only require not relying on autoref and autoderef in order to
             // ensure that the trait implementation won't be used
             self.tcx.struct_span_lint_hir(
-                RUST_2021_PRELUDE_COLLISIONS,
+                prelude_or_array_lint,
                 self_expr.hir_id,
                 self_expr.span,
                 |lint| {
@@ -130,7 +147,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             // trait implementations require full disambiguation to not clash with the new prelude
             // additions (i.e. convert from dot-call to fully-qualified call)
             self.tcx.struct_span_lint_hir(
-                RUST_2021_PRELUDE_COLLISIONS,
+                prelude_or_array_lint,
                 call_expr.hir_id,
                 call_expr.span,
                 |lint| {

src/test/ui/rust-2021/array-into-iter-ambiguous.fixed

@@ -0,0 +1,27 @@
+// See https://github.com/rust-lang/rust/issues/88475
+// run-rustfix
+// edition:2018
+// check-pass
+#![warn(array_into_iter)]
+#![allow(unused)]
+
+struct FooIter;
+
+trait MyIntoIter {
+    fn into_iter(self) -> FooIter;
+}
+
+impl<T, const N: usize> MyIntoIter for [T; N] {
+    fn into_iter(self) -> FooIter {
+        FooIter
+    }
+}
+
+struct Point;
+
+pub fn main() {
+    let points: [Point; 1] = [Point];
+    let y = MyIntoIter::into_iter(points);
+    //~^ WARNING trait method `into_iter` will become ambiguous in Rust 2021
+    //~| WARNING this changes meaning in Rust 2021
+}

src/test/ui/rust-2021/array-into-iter-ambiguous.rs

@@ -0,0 +1,27 @@
+// See https://github.com/rust-lang/rust/issues/88475
+// run-rustfix
+// edition:2018
+// check-pass
+#![warn(array_into_iter)]
+#![allow(unused)]
+
+struct FooIter;
+
+trait MyIntoIter {
+    fn into_iter(self) -> FooIter;
+}
+
+impl<T, const N: usize> MyIntoIter for [T; N] {
+    fn into_iter(self) -> FooIter {
+        FooIter
+    }
+}
+
+struct Point;
+
+pub fn main() {
+    let points: [Point; 1] = [Point];
+    let y = points.into_iter();
+    //~^ WARNING trait method `into_iter` will become ambiguous in Rust 2021
+    //~| WARNING this changes meaning in Rust 2021
+}

src/test/ui/rust-2021/array-into-iter-ambiguous.stderr

@@ -0,0 +1,16 @@
+warning: trait method `into_iter` will become ambiguous in Rust 2021
+  --> $DIR/array-into-iter-ambiguous.rs:24:13
+   |
+LL |     let y = points.into_iter();
+   |             ^^^^^^^^^^^^^^^^^^ help: disambiguate the associated function: `MyIntoIter::into_iter(points)`
+   |
+note: the lint level is defined here
+  --> $DIR/array-into-iter-ambiguous.rs:5:9
+   |
+LL | #![warn(array_into_iter)]
+   |         ^^^^^^^^^^^^^^^
+   = warning: this changes meaning in Rust 2021
+   = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/IntoIterator-for-arrays.html>
+
+warning: 1 warning emitted
+