Use defvar for rust-top-item-beg-re instead
Use defvar for rust-top-item-beg-re instead

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