enable rust_2018_idioms for doctests

Signed-off-by: ozkanonur <work@onurozkan.dev>

compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs

@@ -1913,7 +1913,7 @@ fn is_late_bound_map(
     /// handles cycle detection as we go through the query system.
     ///
     /// This is necessary in the first place for the following case:
-    /// ```
+    /// ```rust,ignore (pseudo-Rust)
     /// type Alias<'a, T> = <T as Trait<'a>>::Assoc;
     /// fn foo<'a>(_: Alias<'a, ()>) -> Alias<'a, ()> { ... }
     /// ```