add inherent-method-collision test

Author: nikomatsakis

src/test/ui/rust-2021/inherent-method-collision.rs

@@ -0,0 +1,15 @@
+// Test that we do NOT warn for inherent methods invoked via `T::` form.
+//
+// check-pass
+
+#![deny(future_prelude_collision)]
+
+pub struct MySeq {}
+
+impl MySeq {
+    pub fn from_iter(_: impl IntoIterator<Item = u32>) {}
+}
+
+fn main() {
+    MySeq::from_iter(Some(22));
+}