diff --git a/compiler/rustc_lint/src/lib.rs b/compiler/rustc_lint/src/lib.rs
index eaf9360cc3358..e694b9c52a944 100644
--- a/compiler/rustc_lint/src/lib.rs
+++ b/compiler/rustc_lint/src/lib.rs
@@ -337,9 +337,7 @@ fn register_builtins(store: &mut LintStore) {
         UNUSED_ASSIGNMENTS,
         DEAD_CODE,
         UNUSED_MUT,
-        // FIXME: add this lint when it becomes stable,
-        // see https://github.com/rust-lang/rust/issues/115585.
-        // UNREACHABLE_CFG_SELECT_PREDICATES,
+        UNREACHABLE_CFG_SELECT_PREDICATES,
         UNREACHABLE_CODE,
         UNREACHABLE_PATTERNS,
         UNUSED_MUST_USE,
diff --git a/tests/ui/lint/issue-70819-dont-override-forbid-in-same-scope.stderr b/tests/ui/lint/issue-70819-dont-override-forbid-in-same-scope.stderr
index 506791fd17269..f40fb73acbb8b 100644
--- a/tests/ui/lint/issue-70819-dont-override-forbid-in-same-scope.stderr
+++ b/tests/ui/lint/issue-70819-dont-override-forbid-in-same-scope.stderr
@@ -431,3 +431,21 @@ note: the lint level is defined here
 LL | #![forbid(forbidden_lint_groups)]
    |           ^^^^^^^^^^^^^^^^^^^^^
 
+Future breakage diagnostic:
+error: warn(unused) incompatible with previous forbid
+  --> $DIR/issue-70819-dont-override-forbid-in-same-scope.rs:22:13
+   |
+LL |     #![forbid(unused)]
+   |               ------ `forbid` level set here
+LL |     #![deny(unused)]
+LL |     #![warn(unused)]
+   |             ^^^^^^ overruled by previous forbid
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #81670 <https://github.com/rust-lang/rust/issues/81670>
+note: the lint level is defined here
+  --> $DIR/issue-70819-dont-override-forbid-in-same-scope.rs:17:11
+   |
+LL | #![forbid(forbidden_lint_groups)]
+   |           ^^^^^^^^^^^^^^^^^^^^^
+
diff --git a/tests/ui/lint/outer-forbid.stderr b/tests/ui/lint/outer-forbid.stderr
index 7810ca223f8ad..abe5959176d4a 100644
--- a/tests/ui/lint/outer-forbid.stderr
+++ b/tests/ui/lint/outer-forbid.stderr
@@ -471,3 +471,21 @@ note: the lint level is defined here
 LL | #![forbid(forbidden_lint_groups)]
    |           ^^^^^^^^^^^^^^^^^^^^^
 
+Future breakage diagnostic:
+error: allow(unused) incompatible with previous forbid
+  --> $DIR/outer-forbid.rs:25:9
+   |
+LL | #![forbid(unused, non_snake_case)]
+   |           ------ `forbid` level set here
+...
+LL | #[allow(unused)]
+   |         ^^^^^^ overruled by previous forbid
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #81670 <https://github.com/rust-lang/rust/issues/81670>
+note: the lint level is defined here
+  --> $DIR/outer-forbid.rs:18:11
+   |
+LL | #![forbid(forbidden_lint_groups)]
+   |           ^^^^^^^^^^^^^^^^^^^^^
+