clippy_lints/src/vec.rs

   1 use crate::consts::{constant, Constant};
   2 use crate::rustc_target::abi::LayoutOf;
   3 use crate::utils::{higher, is_copy, snippet_with_applicability, span_lint_and_sugg};
   4 use if_chain::if_chain;
   5 use rustc_errors::Applicability;
   6 use rustc_hir::{BorrowKind, Expr, ExprKind};
   7 use rustc_lint::{LateContext, LateLintPass};
   8 use rustc_middle::ty::{self, Ty};
   9 use rustc_session::{declare_tool_lint, impl_lint_pass};
  10 use rustc_span::source_map::Span;
  11
  12 #[allow(clippy::module_name_repetitions)]
  13 #[derive(Copy, Clone)]
  14 pub struct UselessVec {
  15     pub too_large_for_stack: u64,
  16 }
  17
  18 declare_clippy_lint! {
  19     /// **What it does:** Checks for usage of `&vec![..]` when using `&[..]` would
  20     /// be possible.
  21     ///
  22     /// **Why is this bad?** This is less efficient.
  23     ///
  24     /// **Known problems:** None.
  25     ///
  26     /// **Example:**
  27     /// ```rust
  28     /// # fn foo(my_vec: &[u8]) {}
  29     ///
  30     /// // Bad
  31     /// foo(&vec![1, 2]);
  32     ///
  33     /// // Good
  34     /// foo(&[1, 2]);
  35     /// ```
  36     pub USELESS_VEC,
  37     perf,
  38     "useless `vec!`"
  39 }
  40
  41 impl_lint_pass!(UselessVec => [USELESS_VEC]);
  42
  43 impl<'tcx> LateLintPass<'tcx> for UselessVec {
  44     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
  45         // search for `&vec![_]` expressions where the adjusted type is `&[_]`
  46         if_chain! {
  47             if let ty::Ref(_, ty, _) = cx.typeck_results().expr_ty_adjusted(expr).kind();
  48             if let ty::Slice(..) = ty.kind();
  49             if let ExprKind::AddrOf(BorrowKind::Ref, _, ref addressee) = expr.kind;
  50             if let Some(vec_args) = higher::vec_macro(cx, addressee);
  51             then {
  52                 self.check_vec_macro(cx, &vec_args, expr.span);
  53             }
  54         }
  55
  56         // search for `for _ in vec![…]`
  57         if_chain! {
  58             if let Some((_, arg, _, _)) = higher::for_loop(expr);
  59             if let Some(vec_args) = higher::vec_macro(cx, arg);
  60             if is_copy(cx, vec_type(cx.typeck_results().expr_ty_adjusted(arg)));
  61             then {
  62                 // report the error around the `vec!` not inside `<std macros>:`
  63                 let span = arg.span
  64                     .ctxt()
  65                     .outer_expn_data()
  66                     .call_site
  67                     .ctxt()
  68                     .outer_expn_data()
  69                     .call_site;
  70                 self.check_vec_macro(cx, &vec_args, span);
  71             }
  72         }
  73     }
  74 }
  75
  76 impl UselessVec {
  77     fn check_vec_macro<'tcx>(self, cx: &LateContext<'tcx>, vec_args: &higher::VecArgs<'tcx>, span: Span) {
  78         let mut applicability = Applicability::MachineApplicable;
  79         let snippet = match *vec_args {
  80             higher::VecArgs::Repeat(elem, len) => {
  81                 if let Some((Constant::Int(len_constant), _)) = constant(cx, cx.typeck_results(), len) {
  82                     #[allow(clippy::cast_possible_truncation)]
  83                     if len_constant as u64 * size_of(cx, elem) > self.too_large_for_stack {
  84                         return;
  85                     }
  86
  87                     format!(
  88                         "&[{}; {}]",
  89                         snippet_with_applicability(cx, elem.span, "elem", &mut applicability),
  90                         snippet_with_applicability(cx, len.span, "len", &mut applicability)
  91                     )
  92                 } else {
  93                     return;
  94                 }
  95             },
  96             higher::VecArgs::Vec(args) => {
  97                 if let Some(last) = args.iter().last() {
  98                     #[allow(clippy::cast_possible_truncation)]
  99                     if args.len() as u64 * size_of(cx, last) > self.too_large_for_stack {
 100                         return;
 101                     }
 102                     let span = args[0].span.to(last.span);
 103
 104                     format!("&[{}]", snippet_with_applicability(cx, span, "..", &mut applicability))
 105                 } else {
 106                     "&[]".into()
 107                 }
 108             },
 109         };
 110
 111         span_lint_and_sugg(
 112             cx,
 113             USELESS_VEC,
 114             span,
 115             "useless use of `vec!`",
 116             "you can use a slice directly",
 117             snippet,
 118             applicability,
 119         );
 120     }
 121 }
 122
 123 fn size_of(cx: &LateContext<'_>, expr: &Expr<'_>) -> u64 {
 124     let ty = cx.typeck_results().expr_ty_adjusted(expr);
 125     cx.layout_of(ty).map_or(0, |l| l.size.bytes())
 126 }
 127
 128 /// Returns the item type of the vector (i.e., the `T` in `Vec<T>`).
 129 fn vec_type(ty: Ty<'_>) -> Ty<'_> {
 130     if let ty::Adt(_, substs) = ty.kind() {
 131         substs.type_at(0)
 132     } else {
 133         panic!("The type of `vec!` is a not a struct?");
 134     }
 135 }