diff --git a/src/bootstrap/builder.rs b/src/bootstrap/builder.rs
index 917abde9de1ce..6ccf8b1d5221c 100644
--- a/src/bootstrap/builder.rs
+++ b/src/bootstrap/builder.rs
@@ -988,10 +988,20 @@ impl<'a> Builder<'a> {
             }
         };
 
-        if use_new_symbol_mangling {
-            rustflags.arg("-Zsymbol-mangling-version=v0");
+        // cfg(bootstrap) -- drop the compiler.stage == 0 branch.
+        if compiler.stage == 0 {
+            if use_new_symbol_mangling {
+                rustflags.arg("-Zsymbol-mangling-version=v0");
+            } else {
+                rustflags.arg("-Zsymbol-mangling-version=legacy");
+            }
         } else {
-            rustflags.arg("-Zsymbol-mangling-version=legacy");
+            if use_new_symbol_mangling {
+                rustflags.arg("-Csymbol-mangling-version=v0");
+            } else {
+                rustflags.arg("-Csymbol-mangling-version=legacy");
+                rustflags.arg("-Zunstable-options");
+            }
         }
 
         // FIXME: It might be better to use the same value for both `RUSTFLAGS` and `RUSTDOCFLAGS`,