Enable --show-type-layout for the rustc API docs

Author: camelid

src/bootstrap/doc.rs

@@ -549,6 +549,7 @@ impl Step for Rustc {
         cargo.rustdocflag("--enable-index-page");
         cargo.rustdocflag("-Zunstable-options");
         cargo.rustdocflag("-Znormalize-docs");
+        cargo.rustdocflag("--show-type-layout");
         compile::rustc_cargo(builder, &mut cargo, target);
 
         // Only include compiler crates, no dependencies of those, such as `libc`.