Add test for #50571

Author: JohnTitor

src/test/ui/issues/issue-50571.rs

@@ -0,0 +1,6 @@
+trait Foo {
+    fn foo([a, b]: [i32; 2]) {}
+    //~^ ERROR: patterns aren't allowed in methods without bodies
+}
+
+fn main() {}

src/test/ui/issues/issue-50571.stderr

@@ -0,0 +1,13 @@
+error[E0642]: patterns aren't allowed in methods without bodies
+  --> $DIR/issue-50571.rs:2:12
+   |
+LL |     fn foo([a, b]: [i32; 2]) {}
+   |            ^^^^^^
+help: give this argument a name or use an underscore to ignore it
+   |
+LL |     fn foo(_: [i32; 2]) {}
+   |            ^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0642`.