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     #[clippy::version = "pre 1.29.0"]
  40     pub USELESS_VEC,
  41     perf,
  42     "useless `vec!`"
  43 }
  44
  45 impl_lint_pass!(UselessVec => [USELESS_VEC]);
  46
  47 impl<'tcx> LateLintPass<'tcx> for UselessVec {
  48     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
  49         // search for `&vec![_]` expressions where the adjusted type is `&[_]`
  50         if_chain! {
  51             if let ty::Ref(_, ty, _) = cx.typeck_results().expr_ty_adjusted(expr).kind();
  52             if let ty::Slice(..) = ty.kind();
  53             if let ExprKind::AddrOf(BorrowKind::Ref, mutability, addressee) = expr.kind;
  54             if let Some(vec_args) = higher::VecArgs::hir(cx, addressee);
  55             then {
  56                 self.check_vec_macro(cx, &vec_args, mutability, expr.span);
  57             }
  58         }
  59
  60         // search for `for _ in vec![…]`
  61         if_chain! {
  62             if let Some(higher::ForLoop { arg, .. }) = higher::ForLoop::hir(expr);
  63             if let Some(vec_args) = higher::VecArgs::hir(cx, arg);
  64             if is_copy(cx, vec_type(cx.typeck_results().expr_ty_adjusted(arg)));
  65             then {
  66                 // report the error around the `vec!` not inside `<std macros>:`
  67                 let span = arg.span.ctxt().outer_expn_data().call_site;
  68                 self.check_vec_macro(cx, &vec_args, Mutability::Not, span);
  69             }
  70         }
  71     }
  72 }
  73
  74 impl UselessVec {
  75     fn check_vec_macro<'tcx>(
  76         self,
  77         cx: &LateContext<'tcx>,
  78         vec_args: &higher::VecArgs<'tcx>,
  79         mutability: Mutability,
  80         span: Span,
  81     ) {
  82         let mut applicability = Applicability::MachineApplicable;
  83         let snippet = match *vec_args {
  84             higher::VecArgs::Repeat(elem, len) => {
  85                 if let Some((Constant::Int(len_constant), _)) = constant(cx, cx.typeck_results(), len) {
  86                     #[allow(clippy::cast_possible_truncation)]
  87                     if len_constant as u64 * size_of(cx, elem) > self.too_large_for_stack {
  88                         return;
  89                     }
  90
  91                     match mutability {
  92                         Mutability::Mut => {
  93                             format!(
  94                                 "&mut [{}; {}]",
  95                                 snippet_with_applicability(cx, elem.span, "elem", &mut applicability),
  96                                 snippet_with_applicability(cx, len.span, "len", &mut applicability)
  97                             )
  98                         },
  99                         Mutability::Not => {
 100                             format!(
 101                                 "&[{}; {}]",
 102                                 snippet_with_applicability(cx, elem.span, "elem", &mut applicability),
 103                                 snippet_with_applicability(cx, len.span, "len", &mut applicability)
 104                             )
 105                         },
 106                     }
 107                 } else {
 108                     return;
 109                 }
 110             },
 111             higher::VecArgs::Vec(args) => {
 112                 if let Some(last) = args.iter().last() {
 113                     #[allow(clippy::cast_possible_truncation)]
 114                     if args.len() as u64 * size_of(cx, last) > self.too_large_for_stack {
 115                         return;
 116                     }
 117                     let span = args[0].span.to(last.span);
 118
 119                     match mutability {
 120                         Mutability::Mut => {
 121                             format!(
 122                                 "&mut [{}]",
 123                                 snippet_with_applicability(cx, span, "..", &mut applicability)
 124                             )
 125                         },
 126                         Mutability::Not => {
 127                             format!("&[{}]", snippet_with_applicability(cx, span, "..", &mut applicability))
 128                         },
 129                     }
 130                 } else {
 131                     match mutability {
 132                         Mutability::Mut => "&mut []".into(),
 133                         Mutability::Not => "&[]".into(),
 134                     }
 135                 }
 136             },
 137         };
 138
 139         span_lint_and_sugg(
 140             cx,
 141             USELESS_VEC,
 142             span,
 143             "useless use of `vec!`",
 144             "you can use a slice directly",
 145             snippet,
 146             applicability,
 147         );
 148     }
 149 }
 150
 151 fn size_of(cx: &LateContext<'_>, expr: &Expr<'_>) -> u64 {
 152     let ty = cx.typeck_results().expr_ty_adjusted(expr);
 153     cx.layout_of(ty).map_or(0, |l| l.size.bytes())
 154 }
 155
 156 /// Returns the item type of the vector (i.e., the `T` in `Vec<T>`).
 157 fn vec_type(ty: Ty<'_>) -> Ty<'_> {
 158     if let ty::Adt(_, substs) = ty.kind() {
 159         substs.type_at(0)
 160     } else {
 161         panic!("The type of `vec!` is a not a struct?");
 162     }
 163 }