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 declare_clippy_lint! {
  15     /// ### What it does
  16     /// Checks for redundant slicing expressions which use the full range, and
  17     /// do not change the type.
  18     ///
  19     /// ### Why is this bad?
  20     /// It unnecessarily adds complexity to the expression.
  21     ///
  22     /// ### Known problems
  23     /// If the type being sliced has an implementation of `Index<RangeFull>`
  24     /// that actually changes anything then it can't be removed. However, this would be surprising
  25     /// to people reading the code and should have a note with it.
  26     ///
  27     /// ### Example
  28     /// ```ignore
  29     /// fn get_slice(x: &[u32]) -> &[u32] {
  30     ///     &x[..]
  31     /// }
  32     /// ```
  33     /// Use instead:
  34     /// ```ignore
  35     /// fn get_slice(x: &[u32]) -> &[u32] {
  36     ///     x
  37     /// }
  38     /// ```
  39     #[clippy::version = "1.51.0"]
  40     pub REDUNDANT_SLICING,
  41     complexity,
  42     "redundant slicing of the whole range of a type"
  43 }
  44
  45 declare_clippy_lint! {
  46     /// ### What it does
  47     /// Checks for slicing expressions which are equivalent to dereferencing the
  48     /// value.
  49     ///
  50     /// ### Why is this bad?
  51     /// Some people may prefer to dereference rather than slice.
  52     ///
  53     /// ### Example
  54     /// ```rust
  55     /// let vec = vec![1, 2, 3];
  56     /// let slice = &vec[..];
  57     /// ```
  58     /// Use instead:
  59     /// ```rust
  60     /// let vec = vec![1, 2, 3];
  61     /// let slice = &*vec;
  62     /// ```
  63     #[clippy::version = "1.60.0"]
  64     pub DEREF_BY_SLICING,
  65     restriction,
  66     "slicing instead of dereferencing"
  67 }
  68
  69 declare_lint_pass!(RedundantSlicing => [REDUNDANT_SLICING, DEREF_BY_SLICING]);
  70
  71 static REDUNDANT_SLICING_LINT: (&Lint, &str) = (REDUNDANT_SLICING, "redundant slicing of the whole range");
  72 static DEREF_BY_SLICING_LINT: (&Lint, &str) = (DEREF_BY_SLICING, "slicing when dereferencing would work");
  73
  74 impl<'tcx> LateLintPass<'tcx> for RedundantSlicing {
  75     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
  76         if expr.span.from_expansion() {
  77             return;
  78         }
  79
  80         let ctxt = expr.span.ctxt();
  81         if_chain! {
  82             if let ExprKind::AddrOf(BorrowKind::Ref, mutability, addressee) = expr.kind;
  83             if addressee.span.ctxt() == ctxt;
  84             if let ExprKind::Index(indexed, range) = addressee.kind;
  85             if is_type_lang_item(cx, cx.typeck_results().expr_ty_adjusted(range), LangItem::RangeFull);
  86             then {
  87                 let (expr_ty, expr_ref_count) = peel_mid_ty_refs(cx.typeck_results().expr_ty(expr));
  88                 let (indexed_ty, indexed_ref_count) = peel_mid_ty_refs(cx.typeck_results().expr_ty(indexed));
  89                 let parent_expr = get_parent_expr(cx, expr);
  90                 let needs_parens_for_prefix = parent_expr.map_or(false, |parent| {
  91                     parent.precedence().order() > PREC_PREFIX
  92                 });
  93                 let mut app = Applicability::MachineApplicable;
  94
  95                 let ((lint, msg), help, sugg) = if expr_ty == indexed_ty {
  96                     if expr_ref_count > indexed_ref_count {
  97                         // Indexing takes self by reference and can't return a reference to that
  98                         // reference as it's a local variable. The only way this could happen is if
  99                         // `self` contains a reference to the `Self` type. If this occurs then the
 100                         // lint no longer applies as it's essentially a field access, which is not
 101                         // redundant.
 102                         return;
 103                     }
 104                     let deref_count = indexed_ref_count - expr_ref_count;
 105
 106                     let (lint, reborrow_str, help_str) = if mutability == Mutability::Mut {
 107                         // The slice was used to reborrow the mutable reference.
 108                         (DEREF_BY_SLICING_LINT, "&mut *", "reborrow the original value instead")
 109                     } else if matches!(
 110                         parent_expr,
 111                         Some(Expr {
 112                             kind: ExprKind::AddrOf(BorrowKind::Ref, Mutability::Mut, _),
 113                             ..
 114                         })
 115                     ) || cx.typeck_results().expr_adjustments(expr).first().map_or(false, |a| {
 116                         matches!(a.kind, Adjust::Borrow(AutoBorrow::Ref(_, AutoBorrowMutability::Mut { .. })))
 117                     }) {
 118                         // The slice was used to make a temporary reference.
 119                         (DEREF_BY_SLICING_LINT, "&*", "reborrow the original value instead")
 120                     } else if deref_count != 0 {
 121                         (DEREF_BY_SLICING_LINT, "", "dereference the original value instead")
 122                     } else {
 123                         (REDUNDANT_SLICING_LINT, "", "use the original value instead")
 124                     };
 125
 126                     let snip = snippet_with_context(cx, indexed.span, ctxt, "..", &mut app).0;
 127                     let sugg = if (deref_count != 0 || !reborrow_str.is_empty()) && needs_parens_for_prefix {
 128                         format!("({}{}{})", reborrow_str, "*".repeat(deref_count), snip)
 129                     } else {
 130                         format!("{}{}{}", reborrow_str, "*".repeat(deref_count), snip)
 131                     };
 132
 133                     (lint, help_str, sugg)
 134                 } else if let Some(target_id) = cx.tcx.lang_items().deref_target() {
 135                     if let Ok(deref_ty) = cx.tcx.try_normalize_erasing_regions(
 136                         cx.param_env,
 137                         cx.tcx.mk_projection(target_id, cx.tcx.mk_substs([GenericArg::from(indexed_ty)].into_iter())),
 138                     ) {
 139                         if deref_ty == expr_ty {
 140                             let snip = snippet_with_context(cx, indexed.span, ctxt, "..", &mut app).0;
 141                             let sugg = if needs_parens_for_prefix {
 142                                 format!("(&{}{}*{})", mutability.prefix_str(), "*".repeat(indexed_ref_count), snip)
 143                             } else {
 144                                 format!("&{}{}*{}", mutability.prefix_str(), "*".repeat(indexed_ref_count), snip)
 145                             };
 146                             (DEREF_BY_SLICING_LINT, "dereference the original value instead", sugg)
 147                         } else {
 148                             return;
 149                         }
 150                     } else {
 151                         return;
 152                     }
 153                 } else {
 154                     return;
 155                 };
 156
 157                 span_lint_and_sugg(
 158                     cx,
 159                     lint,
 160                     expr.span,
 161                     msg,
 162                     help,
 163                     sugg,
 164                     app,
 165                 );
 166             }
 167         }
 168     }
 169 }