auto merge of #13922 : Rufflewind/rust/patch-1, r=pnkfelix
auto merge of #13922 : Rufflewind/rust/patch-1, r=pnkfelix

Since rust-top-item-beg-re hasn't been defined yet, using defvar instead of
setq is more appropriate here (and also silences compilation warnings).

Wilfred/rust-mode,mrBliss/rust-mode,rust-lang/rust-mode,rust-lang/rust-mode