src/tools/clippy/clippy_lints/src/vec.rs

   1 use rustc::hir::*;
   2 use rustc::lint::*;
   3 use rustc::ty::{self, Ty};
   4 use rustc::ty::subst::Substs;
   5 use rustc_const_eval::ConstContext;
   6 use syntax::codemap::Span;
   7 use utils::{higher, is_copy, snippet, span_lint_and_sugg};
   8
   9 /// **What it does:** Checks for usage of `&vec![..]` when using `&[..]` would
  10 /// be possible.
  11 ///
  12 /// **Why is this bad?** This is less efficient.
  13 ///
  14 /// **Known problems:** None.
  15 ///
  16 /// **Example:**
  17 /// ```rust,ignore
  18 /// foo(&vec![1, 2])
  19 /// ```
  20 declare_lint! {
  21     pub USELESS_VEC,
  22     Warn,
  23     "useless `vec!`"
  24 }
  25
  26 #[derive(Copy, Clone, Debug)]
  27 pub struct Pass;
  28
  29 impl LintPass for Pass {
  30     fn get_lints(&self) -> LintArray {
  31         lint_array!(USELESS_VEC)
  32     }
  33 }
  34
  35 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Pass {
  36     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
  37         // search for `&vec![_]` expressions where the adjusted type is `&[_]`
  38         if_let_chain!{[
  39             let ty::TyRef(_, ref ty) = cx.tables.expr_ty_adjusted(expr).sty,
  40             let ty::TySlice(..) = ty.ty.sty,
  41             let ExprAddrOf(_, ref addressee) = expr.node,
  42             let Some(vec_args) = higher::vec_macro(cx, addressee),
  43         ], {
  44             check_vec_macro(cx, &vec_args, expr.span);
  45         }}
  46
  47         // search for `for _ in vec![…]`
  48         if_let_chain!{[
  49             let Some((_, arg, _)) = higher::for_loop(expr),
  50             let Some(vec_args) = higher::vec_macro(cx, arg),
  51             is_copy(cx, vec_type(cx.tables.expr_ty_adjusted(arg))),
  52         ], {
  53             // report the error around the `vec!` not inside `<std macros>:`
  54             let span = arg.span.ctxt().outer().expn_info().map(|info| info.call_site).expect("unable to get call_site");
  55             check_vec_macro(cx, &vec_args, span);
  56         }}
  57     }
  58 }
  59
  60 fn check_vec_macro(cx: &LateContext, vec_args: &higher::VecArgs, span: Span) {
  61     let snippet = match *vec_args {
  62         higher::VecArgs::Repeat(elem, len) => {
  63             let parent_item = cx.tcx.hir.get_parent(len.id);
  64             let parent_def_id = cx.tcx.hir.local_def_id(parent_item);
  65             let substs = Substs::identity_for_item(cx.tcx, parent_def_id);
  66             if ConstContext::new(cx.tcx, cx.param_env.and(substs), cx.tables)
  67                 .eval(len)
  68                 .is_ok()
  69             {
  70                 format!("&[{}; {}]", snippet(cx, elem.span, "elem"), snippet(cx, len.span, "len")).into()
  71             } else {
  72                 return;
  73             }
  74         },
  75         higher::VecArgs::Vec(args) => {
  76             if let Some(last) = args.iter().last() {
  77                 let span = args[0].span.to(last.span);
  78
  79                 format!("&[{}]", snippet(cx, span, "..")).into()
  80             } else {
  81                 "&[]".into()
  82             }
  83         },
  84     };
  85
  86     span_lint_and_sugg(
  87         cx,
  88         USELESS_VEC,
  89         span,
  90         "useless use of `vec!`",
  91         "you can use a slice directly",
  92         snippet,
  93     );
  94 }
  95
  96 /// Return the item type of the vector (ie. the `T` in `Vec<T>`).
  97 fn vec_type(ty: Ty) -> Ty {
  98     if let ty::TyAdt(_, substs) = ty.sty {
  99         substs.type_at(0)
 100     } else {
 101         panic!("The type of `vec!` is a not a struct?");
 102     }
 103 }