message: Clean up slice access
diff: @@ -330,7 +330,7 @@ impl Term {
  | Term::If(start, _, _, ref body) => ByteSpan::new(start, body.span().end()),
  Term::Ann(ref term, ref ty) => term.span().to(ty.span()),
  Term::Arrow(ref ann, ref body) => ann.span().to(body.span()),
  - Term::App(ref fn_term, ref arg) => fn_term.span().to(arg[arg.len() - 1].span()),
  + Term::App(ref fn_term, ref arg) => fn_term.span().to(arg.last().unwrap().span()),
  Term::Proj(ref term, label_start, ref label) => term
  .span()
  .with_end(label_start + ByteOffset::from_str(label)),
  