Add links w/r/t edition 2015 and dyn keyword

Author: benschulz

compiler/rustc_lint/src/unused.rs

@@ -1358,6 +1358,10 @@ impl EarlyLintPass for UnusedParens {
                             false
                         };
 
+                        // In edition 2015, dyn is a contextual keyword and `dyn::foo::Bar` is
+                        // parsed as a path, so parens are necessary to disambiguate. See
+                        //  - tests/ui/lint/unused/unused-parens-trait-obj-e2015.rs and
+                        //  - https://doc.rust-lang.org/reference/types/trait-object.html#r-type.trait-object.syntax-edition2018
                         let dyn2015_exception = cx.sess().psess.edition == Edition2015
                             && matches!(ty.kind, ast::TyKind::TraitObject(..))
                             && i == 0