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

   1 use clippy_utils::consts::{constant, Constant};
   2 use clippy_utils::diagnostics::span_lint_and_sugg;
   3 use clippy_utils::higher;
   4 use clippy_utils::source::snippet_with_applicability;
   5 use clippy_utils::ty::is_copy;
   6 use if_chain::if_chain;
   7 use rustc_errors::Applicability;
   8 use rustc_hir::{BorrowKind, Expr, ExprKind, Mutability};
   9 use rustc_lint::{LateContext, LateLintPass};
  10 use rustc_middle::ty::layout::LayoutOf;
  11 use rustc_middle::ty::{self, Ty};
  12 use rustc_session::{declare_tool_lint, impl_lint_pass};
  13 use rustc_span::source_map::Span;
  14
  15 #[allow(clippy::module_name_repetitions)]
  16 #[derive(Copy, Clone)]
  17 pub struct UselessVec {
  18     pub too_large_for_stack: u64,
  19 }
  20
  21 declare_clippy_lint! {
  22     /// ### What it does
  23     /// Checks for usage of `&vec![..]` when using `&[..]` would
  24     /// be possible.
  25     ///
  26     /// ### Why is this bad?
  27     /// This is less efficient.
  28     ///
  29     /// ### Example
  30     /// ```rust
  31     /// # fn foo(my_vec: &[u8]) {}
  32     ///
  33     /// // Bad
  34     /// foo(&vec![1, 2]);
  35     ///
  36     /// // Good
  37     /// foo(&[1, 2]);
  38     /// ```
  39     pub USELESS_VEC,
  40     perf,
  41     "useless `vec!`"
  42 }
  43
  44 impl_lint_pass!(UselessVec => [USELESS_VEC]);
  45
  46 impl<'tcx> LateLintPass<'tcx> for UselessVec {
  47     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
  48         // search for `&vec![_]` expressions where the adjusted type is `&[_]`
  49         if_chain! {
  50             if let ty::Ref(_, ty, _) = cx.typeck_results().expr_ty_adjusted(expr).kind();
  51             if let ty::Slice(..) = ty.kind();
  52             if let ExprKind::AddrOf(BorrowKind::Ref, mutability, addressee) = expr.kind;
  53             if let Some(vec_args) = higher::VecArgs::hir(cx, addressee);
  54             then {
  55                 self.check_vec_macro(cx, &vec_args, mutability, expr.span);
  56             }
  57         }
  58
  59         // search for `for _ in vec![…]`
  60         if_chain! {
  61             if let Some(higher::ForLoop { arg, .. }) = higher::ForLoop::hir(expr);
  62             if let Some(vec_args) = higher::VecArgs::hir(cx, arg);
  63             if is_copy(cx, vec_type(cx.typeck_results().expr_ty_adjusted(arg)));
  64             then {
  65                 // report the error around the `vec!` not inside `<std macros>:`
  66                 let span = arg.span.ctxt().outer_expn_data().call_site;
  67                 self.check_vec_macro(cx, &vec_args, Mutability::Not, span);
  68             }
  69         }
  70     }
  71 }
  72
  73 impl UselessVec {
  74     fn check_vec_macro<'tcx>(
  75         self,
  76         cx: &LateContext<'tcx>,
  77         vec_args: &higher::VecArgs<'tcx>,
  78         mutability: Mutability,
  79         span: Span,
  80     ) {
  81         let mut applicability = Applicability::MachineApplicable;
  82         let snippet = match *vec_args {
  83             higher::VecArgs::Repeat(elem, len) => {
  84                 if let Some((Constant::Int(len_constant), _)) = constant(cx, cx.typeck_results(), len) {
  85                     #[allow(clippy::cast_possible_truncation)]
  86                     if len_constant as u64 * size_of(cx, elem) > self.too_large_for_stack {
  87                         return;
  88                     }
  89
  90                     match mutability {
  91                         Mutability::Mut => {
  92                             format!(
  93                                 "&mut [{}; {}]",
  94                                 snippet_with_applicability(cx, elem.span, "elem", &mut applicability),
  95                                 snippet_with_applicability(cx, len.span, "len", &mut applicability)
  96                             )
  97                         },
  98                         Mutability::Not => {
  99                             format!(
 100                                 "&[{}; {}]",
 101                                 snippet_with_applicability(cx, elem.span, "elem", &mut applicability),
 102                                 snippet_with_applicability(cx, len.span, "len", &mut applicability)
 103                             )
 104                         },
 105                     }
 106                 } else {
 107                     return;
 108                 }
 109             },
 110             higher::VecArgs::Vec(args) => {
 111                 if let Some(last) = args.iter().last() {
 112                     #[allow(clippy::cast_possible_truncation)]
 113                     if args.len() as u64 * size_of(cx, last) > self.too_large_for_stack {
 114                         return;
 115                     }
 116                     let span = args[0].span.to(last.span);
 117
 118                     match mutability {
 119                         Mutability::Mut => {
 120                             format!(
 121                                 "&mut [{}]",
 122                                 snippet_with_applicability(cx, span, "..", &mut applicability)
 123                             )
 124                         },
 125                         Mutability::Not => {
 126                             format!("&[{}]", snippet_with_applicability(cx, span, "..", &mut applicability))
 127                         },
 128                     }
 129                 } else {
 130                     match mutability {
 131                         Mutability::Mut => "&mut []".into(),
 132                         Mutability::Not => "&[]".into(),
 133                     }
 134                 }
 135             },
 136         };
 137
 138         span_lint_and_sugg(
 139             cx,
 140             USELESS_VEC,
 141             span,
 142             "useless use of `vec!`",
 143             "you can use a slice directly",
 144             snippet,
 145             applicability,
 146         );
 147     }
 148 }
 149
 150 fn size_of(cx: &LateContext<'_>, expr: &Expr<'_>) -> u64 {
 151     let ty = cx.typeck_results().expr_ty_adjusted(expr);
 152     cx.layout_of(ty).map_or(0, |l| l.size.bytes())
 153 }
 154
 155 /// Returns the item type of the vector (i.e., the `T` in `Vec<T>`).
 156 fn vec_type(ty: Ty<'_>) -> Ty<'_> {
 157     if let ty::Adt(_, substs) = ty.kind() {
 158         substs.type_at(0)
 159     } else {
 160         panic!("The type of `vec!` is a not a struct?");
 161     }
 162 }