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