Add UI tests for future_prelude_collision lint

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

@@ -535,7 +535,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                             let trait_name = tcx.def_path_str(pick.item.container.assert_trait());
 
                             let mut lint = lint.build(&format!(
-                                "trait method `{}` will become ambiguous in Rust 2021",
+                                "trait-associated function `{}` will become ambiguous in Rust 2021",
                                 method_name.name
                             ));
 

src/test/ui/lint/future-prelude-collision.fixed

@@ -0,0 +1,57 @@
+// run-rustfix
+// edition:2018
+// check-pass
+
+trait TryIntoU32 {
+    fn try_into(self) -> Result<u32, ()>;
+}
+
+impl TryIntoU32 for u8 {
+    fn try_into(self) -> Result<u32, ()> {
+        Ok(self as u32)
+    }
+}
+
+trait TryFromU8: Sized {
+    fn try_from(x: u8) -> Result<Self, ()>;
+}
+
+impl TryFromU8 for u32 {
+    fn try_from(x: u8) -> Result<Self, ()> {
+        Ok(x as u32)
+    }
+}
+
+trait FromByteIterator {
+    fn from_iter<T>(iter: T) -> Self
+        where T: Iterator<Item = u8>;
+}
+
+impl FromByteIterator for Vec<u8> {
+    fn from_iter<T>(iter: T) -> Self
+        where T: Iterator<Item = u8>
+    {
+        iter.collect()
+    }
+}
+
+fn main() {
+    // test dot-call that will break in 2021 edition
+    let _: u32 = TryIntoU32::try_into(3u8).unwrap();
+    //~^ WARNING trait method `try_into` will become ambiguous in Rust 2021
+
+    // test associated function call that will break in 2021 edition
+    let _ = <u32 as TryFromU8>::try_from(3u8).unwrap();
+    //~^ WARNING trait-associated function `try_from` will become ambiguous in Rust 2021
+
+    // test reverse turbofish too
+    let _ = <Vec<u8> as FromByteIterator>::from_iter(vec![1u8, 2, 3, 4, 5, 6].into_iter());
+    //~^ WARNING trait-associated function `from_iter` will become ambiguous in Rust 2021
+
+    // negative testing lint (this line should *not* emit a warning)
+    let _: u32 = TryFromU8::try_from(3u8).unwrap();
+
+    // test type omission
+    let _: u32 = <_ as TryFromU8>::try_from(3u8).unwrap();
+    //~^ WARNING trait-associated function `try_from` will become ambiguous in Rust 2021
+}

src/test/ui/lint/future-prelude-collision.rs

@@ -0,0 +1,57 @@
+// run-rustfix
+// edition:2018
+// check-pass
+
+trait TryIntoU32 {
+    fn try_into(self) -> Result<u32, ()>;
+}
+
+impl TryIntoU32 for u8 {
+    fn try_into(self) -> Result<u32, ()> {
+        Ok(self as u32)
+    }
+}
+
+trait TryFromU8: Sized {
+    fn try_from(x: u8) -> Result<Self, ()>;
+}
+
+impl TryFromU8 for u32 {
+    fn try_from(x: u8) -> Result<Self, ()> {
+        Ok(x as u32)
+    }
+}
+
+trait FromByteIterator {
+    fn from_iter<T>(iter: T) -> Self
+        where T: Iterator<Item = u8>;
+}
+
+impl FromByteIterator for Vec<u8> {
+    fn from_iter<T>(iter: T) -> Self
+        where T: Iterator<Item = u8>
+    {
+        iter.collect()
+    }
+}
+
+fn main() {
+    // test dot-call that will break in 2021 edition
+    let _: u32 = 3u8.try_into().unwrap();
+    //~^ WARNING trait method `try_into` will become ambiguous in Rust 2021
+
+    // test associated function call that will break in 2021 edition
+    let _ = u32::try_from(3u8).unwrap();
+    //~^ WARNING trait-associated function `try_from` will become ambiguous in Rust 2021
+
+    // test reverse turbofish too
+    let _ = <Vec<u8>>::from_iter(vec![1u8, 2, 3, 4, 5, 6].into_iter());
+    //~^ WARNING trait-associated function `from_iter` will become ambiguous in Rust 2021
+
+    // negative testing lint (this line should *not* emit a warning)
+    let _: u32 = TryFromU8::try_from(3u8).unwrap();
+
+    // test type omission
+    let _: u32 = <_>::try_from(3u8).unwrap();
+    //~^ WARNING trait-associated function `try_from` will become ambiguous in Rust 2021
+}

src/test/ui/lint/future-prelude-collision.stderr

@@ -0,0 +1,28 @@
+warning: trait method `try_into` will become ambiguous in Rust 2021
+  --> $DIR/future-prelude-collision.rs:40:18
+   |
+LL |     let _: u32 = 3u8.try_into().unwrap();
+   |                  ^^^^^^^^^^^^^^ help: disambiguate the associated function: `TryIntoU32::try_into(3u8)`
+   |
+   = note: `#[warn(future_prelude_collision)]` on by default
+
+warning: trait-associated function `try_from` will become ambiguous in Rust 2021
+  --> $DIR/future-prelude-collision.rs:44:13
+   |
+LL |     let _ = u32::try_from(3u8).unwrap();
+   |             ^^^^^^^^^^^^^ help: disambiguate the associated function: `<u32 as TryFromU8>::try_from`
+
+warning: trait-associated function `from_iter` will become ambiguous in Rust 2021
+  --> $DIR/future-prelude-collision.rs:48:13
+   |
+LL |     let _ = <Vec<u8>>::from_iter(vec![1u8, 2, 3, 4, 5, 6].into_iter());
+   |             ^^^^^^^^^^^^^^^^^^^^ help: disambiguate the associated function: `<Vec<u8> as FromByteIterator>::from_iter`
+
+warning: trait-associated function `try_from` will become ambiguous in Rust 2021
+  --> $DIR/future-prelude-collision.rs:55:18
+   |
+LL |     let _: u32 = <_>::try_from(3u8).unwrap();
+   |                  ^^^^^^^^^^^^^ help: disambiguate the associated function: `<_ as TryFromU8>::try_from`
+
+warning: 4 warnings emitted
+