WIP: Find the imports that were used to reach a method

And add tests for some corner cases we have to consider.

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

@@ -0,0 +1,52 @@
+// run-rustfix
+// edition:2018
+// check-pass
+#![warn(future_prelude_collision)]
+#![allow(dead_code)]
+
+mod m {
+    pub trait TryIntoU32 {
+        fn try_into(self) -> Result<u32, ()>;
+    }
+
+    impl TryIntoU32 for u8 {
+        fn try_into(self) -> Result<u32, ()> {
+            Ok(self as u32)
+        }
+    }
+
+    pub trait AnotherTrick {}
+}
+
+mod a {
+    use crate::m::TryIntoU32;
+
+    fn main() {
+        // In this case, we can just use `TryIntoU32`
+        let _: u32 = 3u8.try_into().unwrap();
+    }
+}
+
+mod b {
+    use crate::m::AnotherTrick as TryIntoU32;
+    use crate::m::TryIntoU32 as _;
+
+    fn main() {
+        // In this case, a `TryIntoU32::try_into` rewrite will not work, and we need to use
+        // the path `crate::m::TryIntoU32` (with which it was imported).
+        let _: u32 = 3u8.try_into().unwrap();
+    }
+}
+
+mod c {
+    use super::m::TryIntoU32 as _;
+    use crate::m::AnotherTrick as TryIntoU32;
+
+    fn main() {
+        // In this case, a `TryIntoU32::try_into` rewrite will not work, and we need to use
+        // the path `super::m::TryIntoU32` (with which it was imported).
+        let _: u32 = 3u8.try_into().unwrap();
+    }
+}
+
+fn main() {}

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

@@ -0,0 +1,33 @@
+// run-rustfix
+// edition:2018
+// check-pass
+#![warn(future_prelude_collision)]
+#![allow(dead_code)]
+
+mod m {
+    pub trait TryIntoU32 {
+        fn try_into(self) -> Result<u32, ()>;
+    }
+
+    impl TryIntoU32 for u8 {
+        fn try_into(self) -> Result<u32, ()> {
+            Ok(self as u32)
+        }
+    }
+
+    pub trait AnotherTrick {}
+}
+
+mod d {
+    use crate::m::AnotherTrick as TryIntoU32;
+    use crate::m::*;
+
+    fn main() {
+        // Here, `TryIntoU32` is imported but shadowed, but in that case we don't permit its methods
+        // to be available.
+        let _: u32 = 3u8.try_into().unwrap();
+        //~^ ERROR no method name `try_into` found
+    }
+}
+
+fn main() {}