src/tools/clippy/clippy_lints/src/redundant_slicing.rs

   1 use clippy_utils::diagnostics::span_lint_and_sugg;
   2 use clippy_utils::get_parent_expr;
   3 use clippy_utils::source::snippet_with_context;
   4 use clippy_utils::ty::{is_type_lang_item, peel_mid_ty_refs};
   5 use if_chain::if_chain;
   6 use rustc_ast::util::parser::PREC_PREFIX;
   7 use rustc_errors::Applicability;
   8 use rustc_hir::{BorrowKind, Expr, ExprKind, LangItem, Mutability};
   9 use rustc_lint::{LateContext, LateLintPass, Lint};
  10 use rustc_middle::ty::adjustment::{Adjust, AutoBorrow, AutoBorrowMutability};
  11 use rustc_middle::ty::subst::GenericArg;
  12 use rustc_session::{declare_lint_pass, declare_tool_lint};
  13
  14 use std::iter;
  15
  16 declare_clippy_lint! {
  17     /// ### What it does
  18     /// Checks for redundant slicing expressions which use the full range, and
  19     /// do not change the type.
  20     ///
  21     /// ### Why is this bad?
  22     /// It unnecessarily adds complexity to the expression.
  23     ///
  24     /// ### Known problems
  25     /// If the type being sliced has an implementation of `Index<RangeFull>`
  26     /// that actually changes anything then it can't be removed. However, this would be surprising
  27     /// to people reading the code and should have a note with it.
  28     ///
  29     /// ### Example
  30     /// ```ignore
  31     /// fn get_slice(x: &[u32]) -> &[u32] {
  32     ///     &x[..]
  33     /// }
  34     /// ```
  35     /// Use instead:
  36     /// ```ignore
  37     /// fn get_slice(x: &[u32]) -> &[u32] {
  38     ///     x
  39     /// }
  40     /// ```
  41     #[clippy::version = "1.51.0"]
  42     pub REDUNDANT_SLICING,
  43     complexity,
  44     "redundant slicing of the whole range of a type"
  45 }
  46
  47 declare_clippy_lint! {
  48     /// ### What it does
  49     /// Checks for slicing expressions which are equivalent to dereferencing the
  50     /// value.
  51     ///
  52     /// ### Why is this bad?
  53     /// Some people may prefer to dereference rather than slice.
  54     ///
  55     /// ### Example
  56     /// ```rust
  57     /// let vec = vec![1, 2, 3];
  58     /// let slice = &vec[..];
  59     /// ```
  60     /// Use instead:
  61     /// ```rust
  62     /// let vec = vec![1, 2, 3];
  63     /// let slice = &*vec;
  64     /// ```
  65     #[clippy::version = "1.61.0"]
  66     pub DEREF_BY_SLICING,
  67     restriction,
  68     "slicing instead of dereferencing"
  69 }
  70
  71 declare_lint_pass!(RedundantSlicing => [REDUNDANT_SLICING, DEREF_BY_SLICING]);
  72
  73 static REDUNDANT_SLICING_LINT: (&Lint, &str) = (REDUNDANT_SLICING, "redundant slicing of the whole range");
  74 static DEREF_BY_SLICING_LINT: (&Lint, &str) = (DEREF_BY_SLICING, "slicing when dereferencing would work");
  75
  76 impl<'tcx> LateLintPass<'tcx> for RedundantSlicing {
  77     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
  78         if expr.span.from_expansion() {
  79             return;
  80         }
  81
  82         let ctxt = expr.span.ctxt();
  83         if_chain! {
  84             if let ExprKind::AddrOf(BorrowKind::Ref, mutability, addressee) = expr.kind;
  85             if addressee.span.ctxt() == ctxt;
  86             if let ExprKind::Index(indexed, range) = addressee.kind;
  87             if is_type_lang_item(cx, cx.typeck_results().expr_ty_adjusted(range), LangItem::RangeFull);
  88             then {
  89                 let (expr_ty, expr_ref_count) = peel_mid_ty_refs(cx.typeck_results().expr_ty(expr));
  90                 let (indexed_ty, indexed_ref_count) = peel_mid_ty_refs(cx.typeck_results().expr_ty(indexed));
  91                 let parent_expr = get_parent_expr(cx, expr);
  92                 let needs_parens_for_prefix = parent_expr.map_or(false, |parent| {
  93                     parent.precedence().order() > PREC_PREFIX
  94                 });
  95                 let mut app = Applicability::MachineApplicable;
  96
  97                 let ((lint, msg), help, sugg) = if expr_ty == indexed_ty {
  98                     if expr_ref_count > indexed_ref_count {
  99                         // Indexing takes self by reference and can't return a reference to that
 100                         // reference as it's a local variable. The only way this could happen is if
 101                         // `self` contains a reference to the `Self` type. If this occurs then the
 102                         // lint no longer applies as it's essentially a field access, which is not
 103                         // redundant.
 104                         return;
 105                     }
 106                     let deref_count = indexed_ref_count - expr_ref_count;
 107
 108                     let (lint, reborrow_str, help_str) = if mutability == Mutability::Mut {
 109                         // The slice was used to reborrow the mutable reference.
 110                         (DEREF_BY_SLICING_LINT, "&mut *", "reborrow the original value instead")
 111                     } else if matches!(
 112                         parent_expr,
 113                         Some(Expr {
 114                             kind: ExprKind::AddrOf(BorrowKind::Ref, Mutability::Mut, _),
 115                             ..
 116                         })
 117                     ) || cx.typeck_results().expr_adjustments(expr).first().map_or(false, |a| {
 118                         matches!(a.kind, Adjust::Borrow(AutoBorrow::Ref(_, AutoBorrowMutability::Mut { .. })))
 119                     }) {
 120                         // The slice was used to make a temporary reference.
 121                         (DEREF_BY_SLICING_LINT, "&*", "reborrow the original value instead")
 122                     } else if deref_count != 0 {
 123                         (DEREF_BY_SLICING_LINT, "", "dereference the original value instead")
 124                     } else {
 125                         (REDUNDANT_SLICING_LINT, "", "use the original value instead")
 126                     };
 127
 128                     let snip = snippet_with_context(cx, indexed.span, ctxt, "..", &mut app).0;
 129                     let sugg = if (deref_count != 0 || !reborrow_str.is_empty()) && needs_parens_for_prefix {
 130                         format!("({reborrow_str}{}{snip})", "*".repeat(deref_count))
 131                     } else {
 132                         format!("{reborrow_str}{}{snip}", "*".repeat(deref_count))
 133                     };
 134
 135                     (lint, help_str, sugg)
 136                 } else if let Some(target_id) = cx.tcx.lang_items().deref_target() {
 137                     if let Ok(deref_ty) = cx.tcx.try_normalize_erasing_regions(
 138                         cx.param_env,
 139                         cx.tcx.mk_projection(target_id, cx.tcx.mk_substs(iter::once(GenericArg::from(indexed_ty)))),
 140                     ) {
 141                         if deref_ty == expr_ty {
 142                             let snip = snippet_with_context(cx, indexed.span, ctxt, "..", &mut app).0;
 143                             let sugg = if needs_parens_for_prefix {
 144                                 format!("(&{}{}*{snip})", mutability.prefix_str(), "*".repeat(indexed_ref_count))
 145                             } else {
 146                                 format!("&{}{}*{snip}", mutability.prefix_str(), "*".repeat(indexed_ref_count))
 147                             };
 148                             (DEREF_BY_SLICING_LINT, "dereference the original value instead", sugg)
 149                         } else {
 150                             return;
 151                         }
 152                     } else {
 153                         return;
 154                     }
 155                 } else {
 156                     return;
 157                 };
 158
 159                 span_lint_and_sugg(
 160                     cx,
 161                     lint,
 162                     expr.span,
 163                     msg,
 164                     help,
 165                     sugg,
 166                     app,
 167                 );
 168             }
 169         }
 170     }
 171 }