clippy_lints/src/vec.rs

   1 use crate::consts::constant;
   2 use crate::utils::{higher, is_copy, snippet_with_applicability, span_lint_and_sugg};
   3 use if_chain::if_chain;
   4 use rustc::hir::*;
   5 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
   6 use rustc::ty::{self, Ty};
   7 use rustc::{declare_lint_pass, declare_tool_lint};
   8 use rustc_errors::Applicability;
   9 use syntax::source_map::Span;
  10
  11 declare_clippy_lint! {
  12     /// **What it does:** Checks for usage of `&vec![..]` when using `&[..]` would
  13     /// be possible.
  14     ///
  15     /// **Why is this bad?** This is less efficient.
  16     ///
  17     /// **Known problems:** None.
  18     ///
  19     /// **Example:**
  20     /// ```rust,ignore
  21     /// foo(&vec![1, 2])
  22     /// ```
  23     pub USELESS_VEC,
  24     perf,
  25     "useless `vec!`"
  26 }
  27
  28 declare_lint_pass!(UselessVec => [USELESS_VEC]);
  29
  30 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for UselessVec {
  31     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
  32         // search for `&vec![_]` expressions where the adjusted type is `&[_]`
  33         if_chain! {
  34             if let ty::Ref(_, ty, _) = cx.tables.expr_ty_adjusted(expr).sty;
  35             if let ty::Slice(..) = ty.sty;
  36             if let ExprKind::AddrOf(_, ref addressee) = expr.node;
  37             if let Some(vec_args) = higher::vec_macro(cx, addressee);
  38             then {
  39                 check_vec_macro(cx, &vec_args, expr.span);
  40             }
  41         }
  42
  43         // search for `for _ in vec![…]`
  44         if_chain! {
  45             if let Some((_, arg, _)) = higher::for_loop(expr);
  46             if let Some(vec_args) = higher::vec_macro(cx, arg);
  47             if is_copy(cx, vec_type(cx.tables.expr_ty_adjusted(arg)));
  48             then {
  49                 // report the error around the `vec!` not inside `<std macros>:`
  50                 let span = arg.span
  51                     .ctxt()
  52                     .outer_expn_info()
  53                     .map(|info| info.call_site)
  54                     .expect("unable to get call_site")
  55                     .ctxt()
  56                     .outer_expn_info()
  57                     .map(|info| info.call_site)
  58                     .expect("unable to get call_site");
  59                 check_vec_macro(cx, &vec_args, span);
  60             }
  61         }
  62     }
  63 }
  64
  65 fn check_vec_macro<'a, 'tcx>(cx: &LateContext<'a, 'tcx>, vec_args: &higher::VecArgs<'tcx>, span: Span) {
  66     let mut applicability = Applicability::MachineApplicable;
  67     let snippet = match *vec_args {
  68         higher::VecArgs::Repeat(elem, len) => {
  69             if constant(cx, cx.tables, len).is_some() {
  70                 format!(
  71                     "&[{}; {}]",
  72                     snippet_with_applicability(cx, elem.span, "elem", &mut applicability),
  73                     snippet_with_applicability(cx, len.span, "len", &mut applicability)
  74                 )
  75             } else {
  76                 return;
  77             }
  78         },
  79         higher::VecArgs::Vec(args) => {
  80             if let Some(last) = args.iter().last() {
  81                 let span = args[0].span.to(last.span);
  82
  83                 format!("&[{}]", snippet_with_applicability(cx, span, "..", &mut applicability))
  84             } else {
  85                 "&[]".into()
  86             }
  87         },
  88     };
  89
  90     span_lint_and_sugg(
  91         cx,
  92         USELESS_VEC,
  93         span,
  94         "useless use of `vec!`",
  95         "you can use a slice directly",
  96         snippet,
  97         applicability,
  98     );
  99 }
 100
 101 /// Returns the item type of the vector (i.e., the `T` in `Vec<T>`).
 102 fn vec_type(ty: Ty<'_>) -> Ty<'_> {
 103     if let ty::Adt(_, substs) = ty.sty {
 104         substs.type_at(0)
 105     } else {
 106         panic!("The type of `vec!` is a not a struct?");
 107     }
 108 }