clippy_lints/src/redundant_slicing.rs

   1 use clippy_utils::diagnostics::span_lint_and_sugg;
   2 use clippy_utils::source::snippet_with_context;
   3 use clippy_utils::ty::is_type_lang_item;
   4 use clippy_utils::{get_parent_expr, in_macro};
   5 use if_chain::if_chain;
   6 use rustc_errors::Applicability;
   7 use rustc_hir::{BorrowKind, Expr, ExprKind, LangItem, Mutability};
   8 use rustc_lint::{LateContext, LateLintPass};
   9 use rustc_middle::ty::TyS;
  10 use rustc_session::{declare_lint_pass, declare_tool_lint};
  11
  12 declare_clippy_lint! {
  13     /// **What it does:** Checks for redundant slicing expressions which use the full range, and
  14     /// do not change the type.
  15     ///
  16     /// **Why is this bad?** It unnecessarily adds complexity to the expression.
  17     ///
  18     /// **Known problems:** If the type being sliced has an implementation of `Index<RangeFull>`
  19     /// that actually changes anything then it can't be removed. However, this would be surprising
  20     /// to people reading the code and should have a note with it.
  21     ///
  22     /// **Example:**
  23     ///
  24     /// ```ignore
  25     /// fn get_slice(x: &[u32]) -> &[u32] {
  26     ///     &x[..]
  27     /// }
  28     /// ```
  29     /// Use instead:
  30     /// ```ignore
  31     /// fn get_slice(x: &[u32]) -> &[u32] {
  32     ///     x
  33     /// }
  34     /// ```
  35     pub REDUNDANT_SLICING,
  36     complexity,
  37     "redundant slicing of the whole range of a type"
  38 }
  39
  40 declare_lint_pass!(RedundantSlicing => [REDUNDANT_SLICING]);
  41
  42 impl LateLintPass<'_> for RedundantSlicing {
  43     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
  44         if in_macro(expr.span) {
  45             return;
  46         }
  47
  48         let ctxt = expr.span.ctxt();
  49         if_chain! {
  50             if let ExprKind::AddrOf(BorrowKind::Ref, mutability, addressee) = expr.kind;
  51             if addressee.span.ctxt() == ctxt;
  52             if let ExprKind::Index(indexed, range) = addressee.kind;
  53             if is_type_lang_item(cx, cx.typeck_results().expr_ty_adjusted(range), LangItem::RangeFull);
  54             if TyS::same_type(cx.typeck_results().expr_ty(expr), cx.typeck_results().expr_ty(indexed));
  55             then {
  56                 let mut app = Applicability::MachineApplicable;
  57                 let snip = snippet_with_context(cx, indexed.span, ctxt, "..", &mut app).0;
  58
  59                 let (reborrow_str, help_str) = if mutability == Mutability::Mut {
  60                     // The slice was used to reborrow the mutable reference.
  61                     ("&mut *", "reborrow the original value instead")
  62                 } else if matches!(
  63                     get_parent_expr(cx, expr),
  64                     Some(Expr {
  65                         kind: ExprKind::AddrOf(BorrowKind::Ref, Mutability::Mut, _),
  66                         ..
  67                     })
  68                 ) {
  69                     // The slice was used to make a temporary reference.
  70                     ("&*", "reborrow the original value instead")
  71                 } else {
  72                     ("", "use the original value instead")
  73                 };
  74
  75                 span_lint_and_sugg(
  76                     cx,
  77                     REDUNDANT_SLICING,
  78                     expr.span,
  79                     "redundant slicing of the whole range",
  80                     help_str,
  81                     format!("{}{}", reborrow_str, snip),
  82                     app,
  83                 );
  84             }
  85         }
  86     }
  87 }