Make some lint doctests compatible with --stage=0

Author: Zalathar

compiler/rustc_lint/src/if_let_rescope.rs

@@ -24,7 +24,7 @@ declare_lint! {
 /// ### Example
 ///
 /// ```rust,edition2021
-/// #![feature(if_let_rescope)]
+/// #![cfg_attr(not(bootstrap), feature(if_let_rescope))] // Simplify this in bootstrap bump.
 /// #![warn(if_let_rescope)]
 /// #![allow(unused_variables)]
 ///

compiler/rustc_lint_defs/src/builtin.rs

@@ -1871,6 +1871,7 @@ declare_lint! {
 /// ### Example
 ///
 /// ```rust,compile_fail
+/// # #[cfg_attr(bootstrap)] compile_error!(); // Remove this in bootstrap bump.
 /// #![deny(elided_named_lifetimes)]
 /// struct Foo;
 /// impl Foo {