[rust] Enable index page for rustdoc

This is intended to be used as the entry point for fuchsia-docs.

Change-Id: I577bde6a7f87785d96c5fc03a5890ec6b140ff25
diff --git a/tools/devshell/contrib/lib/rust/rustdoc.py b/tools/devshell/contrib/lib/rust/rustdoc.py
index d6d6c51..6cca0c6 100755
--- a/tools/devshell/contrib/lib/rust/rustdoc.py
+++ b/tools/devshell/contrib/lib/rust/rustdoc.py
@@ -107,6 +107,7 @@
         )
     env["RUSTC"] = rustc
     env["RUSTDOC"] = rustdoc
+    env["RUSTDOCFLAGS"] = "-Z unstable-options --enable-index-page"
     env["RUST_BACKTRACE"] = "1"
     env["CC"] = clang_c_compiler
     if "fuchsia" in args.target: