Implement &pin const self and &pin mut self sugars

tests/pretty/pin-ergonomics.rs

@@ -0,0 +1,24 @@
+//@ pp-exact
+
+#![feature(pin_ergonomics)]
+#![allow(dead_code, incomplete_features)]
+
+struct Foo;
+
+impl Foo {
+    fn baz(&pin mut self) {}
+
+    fn baz_const(&pin const self) {}
+
+    fn baz_lt<'a>(&'a pin mut self) {}
+
+    fn baz_const_lt(&'_ pin const self) {}
+}
+
+fn foo(_: &pin mut Foo) {}
+fn foo_lt<'a>(_: &'a pin mut Foo) {}
+
+fn foo_const(_: &pin const Foo) {}
+fn foo_const_lt(_: &'_ pin const Foo) {}
+
+fn main() {}