Add macro test for prelude collision suggestions.

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

@@ -0,0 +1,45 @@
+// run-rustfix
+// edition:2018
+// check-pass
+#![warn(rust_2021_prelude_collisions)]
+#![allow(unreachable_code)]
+
+macro_rules! foo {
+    () => {{
+        123;
+        S
+    }};
+}
+
+trait MyTry<T> {
+    fn try_into(self, _: u8);
+}
+
+struct S;
+
+impl MyTry<i32> for S {
+    fn try_into(self, _: u8) {}
+}
+
+trait TryFromU8: Sized {
+    fn try_from(_: u8);
+}
+
+impl TryFromU8 for u32 {
+    fn try_from(_: u8) {}
+}
+
+macro_rules! bar {
+    () => {
+        u32
+    };
+}
+
+fn main() {
+    MyTry::try_into(foo!(), todo!());
+    //~^ WARNING trait method `try_into` will become ambiguous in Rust 2021
+    //~| WARNING this is accepted in the current edition
+    <bar!() as TryFromU8>::try_from(0);
+    //~^ WARNING trait-associated function `try_from` will become ambiguous in Rust 2021
+    //~| WARNING this is accepted in the current edition
+}

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

@@ -0,0 +1,45 @@
+// run-rustfix
+// edition:2018
+// check-pass
+#![warn(rust_2021_prelude_collisions)]
+#![allow(unreachable_code)]
+
+macro_rules! foo {
+    () => {{
+        123;
+        S
+    }};
+}
+
+trait MyTry<T> {
+    fn try_into(self, _: u8);
+}
+
+struct S;
+
+impl MyTry<i32> for S {
+    fn try_into(self, _: u8) {}
+}
+
+trait TryFromU8: Sized {
+    fn try_from(_: u8);
+}
+
+impl TryFromU8 for u32 {
+    fn try_from(_: u8) {}
+}
+
+macro_rules! bar {
+    () => {
+        u32
+    };
+}
+
+fn main() {
+    foo!().try_into(todo!());
+    //~^ WARNING trait method `try_into` will become ambiguous in Rust 2021
+    //~| WARNING this is accepted in the current edition
+    <bar!()>::try_from(0);
+    //~^ WARNING trait-associated function `try_from` will become ambiguous in Rust 2021
+    //~| WARNING this is accepted in the current edition
+}

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

@@ -0,0 +1,25 @@
+warning: trait method `try_into` will become ambiguous in Rust 2021
+  --> $DIR/future-prelude-collision-macros.rs:39:5
+   |
+LL |     foo!().try_into(todo!());
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^ help: disambiguate the associated function: `MyTry::try_into(foo!(), todo!())`
+   |
+note: the lint level is defined here
+  --> $DIR/future-prelude-collision-macros.rs:4:9
+   |
+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 <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/prelude.html>
+
+warning: trait-associated function `try_from` will become ambiguous in Rust 2021
+  --> $DIR/future-prelude-collision-macros.rs:42:5
+   |
+LL |     <bar!()>::try_from(0);
+   |     ^^^^^^^^^^^^^^^^^^ help: disambiguate the associated function: `<bar!() as TryFromU8>::try_from`
+   |
+   = warning: this is accepted in the current edition (Rust 2018) but is a hard error in Rust 2021!
+   = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/prelude.html>
+
+warning: 2 warnings emitted
+