diff --git a/compiler/rustc_session/src/options.rs b/compiler/rustc_session/src/options.rs
index f36fc29e97417..646aa10547f11 100644
--- a/compiler/rustc_session/src/options.rs
+++ b/compiler/rustc_session/src/options.rs
@@ -1286,7 +1286,7 @@ options! {
         "select processor to schedule for (`rustc --print target-cpus` for details)"),
     thinlto: Option<bool> = (None, parse_opt_bool, [TRACKED],
         "enable ThinLTO when possible"),
-    thir_unsafeck: bool = (false, parse_bool, [TRACKED],
+    thir_unsafeck: bool = (true, parse_bool, [TRACKED],
         "use the THIR unsafety checker (default: no)"),
     /// We default to 1 here since we want to behave like
     /// a sequential compiler for now. This'll likely be adjusted