Fix error with suggestion for how to disambiguate associated function when struct is generic

src/test/ui/rust-2021/future-prelude-collision-generic.fixed

@@ -0,0 +1,37 @@
+// test for https://github.com/rust-lang/rust/issues/86940
+// run-rustfix
+// edition:2018
+// check-pass
+#![warn(rust_2021_prelude_collisions)]
+#![allow(dead_code)]
+#![allow(unused_imports)]
+
+struct Generic<T, U>(T, U);
+
+trait MyFromIter {
+    fn from_iter(_: i32) -> Self;
+}
+
+impl MyFromIter for Generic<i32, i32> {
+    fn from_iter(x: i32) -> Self {
+        Self(x, x)
+    }
+}
+
+impl std::iter::FromIterator<i32> for Generic<i32, i32> {
+    fn from_iter<T: IntoIterator<Item = i32>>(_: T) -> Self {
+        todo!()
+    }
+}
+
+fn main() {
+    <Generic<_, _> as MyFromIter>::from_iter(1);
+    //~^ WARNING trait-associated function `from_iter` will become ambiguous in Rust 2021
+    //~| this is accepted in the current edition (Rust 2018)
+    <Generic::<i32, i32> as MyFromIter>::from_iter(1);
+    //~^ WARNING trait-associated function `from_iter` will become ambiguous in Rust 2021
+    //~| this is accepted in the current edition (Rust 2018)
+    <Generic::<_, _> as MyFromIter>::from_iter(1);
+    //~^ WARNING trait-associated function `from_iter` will become ambiguous in Rust 2021
+    //~| this is accepted in the current edition (Rust 2018)
+}

src/test/ui/rust-2021/future-prelude-collision-generic.rs

@@ -0,0 +1,37 @@
+// test for https://github.com/rust-lang/rust/issues/86940
+// run-rustfix
+// edition:2018
+// check-pass
+#![warn(rust_2021_prelude_collisions)]
+#![allow(dead_code)]
+#![allow(unused_imports)]
+
+struct Generic<T, U>(T, U);
+
+trait MyFromIter {
+    fn from_iter(_: i32) -> Self;
+}
+
+impl MyFromIter for Generic<i32, i32> {
+    fn from_iter(x: i32) -> Self {
+        Self(x, x)
+    }
+}
+
+impl std::iter::FromIterator<i32> for Generic<i32, i32> {
+    fn from_iter<T: IntoIterator<Item = i32>>(_: T) -> Self {
+        todo!()
+    }
+}
+
+fn main() {
+    Generic::from_iter(1);
+    //~^ WARNING trait-associated function `from_iter` will become ambiguous in Rust 2021
+    //~| this is accepted in the current edition (Rust 2018)
+    Generic::<i32, i32>::from_iter(1);
+    //~^ WARNING trait-associated function `from_iter` will become ambiguous in Rust 2021
+    //~| this is accepted in the current edition (Rust 2018)
+    Generic::<_, _>::from_iter(1);
+    //~^ WARNING trait-associated function `from_iter` will become ambiguous in Rust 2021
+    //~| this is accepted in the current edition (Rust 2018)
+}

src/test/ui/rust-2021/future-prelude-collision-generic.stderr

@@ -0,0 +1,34 @@
+warning: trait-associated function `from_iter` will become ambiguous in Rust 2021
+  --> $DIR/future-prelude-collision-generic.rs:28:5
+   |
+LL |     Generic::from_iter(1);
+   |     ^^^^^^^^^^^^^^^^^^ help: disambiguate the associated function: `<Generic<_, _> as MyFromIter>::from_iter`
+   |
+note: the lint level is defined here
+  --> $DIR/future-prelude-collision-generic.rs:5:9
+   |
+LL | #![warn(rust_2021_prelude_collisions)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = warning: this is accepted in the current edition (Rust 2018) but is a hard error in Rust 2021!
+   = note: for more information, see issue #85684 <https://github.com/rust-lang/rust/issues/85684>
+
+warning: trait-associated function `from_iter` will become ambiguous in Rust 2021
+  --> $DIR/future-prelude-collision-generic.rs:31:5
+   |
+LL |     Generic::<i32, i32>::from_iter(1);
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: disambiguate the associated function: `<Generic::<i32, i32> as MyFromIter>::from_iter`
+   |
+   = warning: this is accepted in the current edition (Rust 2018) but is a hard error in Rust 2021!
+   = note: for more information, see issue #85684 <https://github.com/rust-lang/rust/issues/85684>
+
+warning: trait-associated function `from_iter` will become ambiguous in Rust 2021
+  --> $DIR/future-prelude-collision-generic.rs:34:5
+   |
+LL |     Generic::<_, _>::from_iter(1);
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: disambiguate the associated function: `<Generic::<_, _> as MyFromIter>::from_iter`
+   |
+   = warning: this is accepted in the current edition (Rust 2018) but is a hard error in Rust 2021!
+   = note: for more information, see issue #85684 <https://github.com/rust-lang/rust/issues/85684>
+
+warning: 3 warnings emitted
+