Rollup merge of #87557 - rylev:fix-invalid-prelude-collision-error, r=nikomatsakis

Fix issue with autofix for ambiguous associated function from Rust 2021 prelude when struct is generic

Fixes #86940

The test cases and associated issue should make it clear what specifically this is meant to fix. The fix is slightly hacky in that we check against the literal source code of the call site for the presence of `<` in order to determine if the user has included the generics for the struct (meaning we don't need to include them for them).

r? ``@nikomatsakis``

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
+