Prepare for merging from rust-lang/rust

This updates the rust-version file to efd420c770bb179537c01063e98cb6990c439654.
diff --git a/src/doc/rustc-dev-guide/rust-version b/src/doc/rustc-dev-guide/rust-version
index f6b7efe..ce9f984 100644
--- a/src/doc/rustc-dev-guide/rust-version
+++ b/src/doc/rustc-dev-guide/rust-version
@@ -1 +1 @@
-460259d14de0274b97b8801e08cb2fe5f16fdac5
+efd420c770bb179537c01063e98cb6990c439654