clippy_lints/src/needless_borrow.rs

   1 //! Checks for needless address of operations (`&`)
   2 //!
   3 //! This lint is **warn** by default
   4
   5 use clippy_utils::diagnostics::span_lint_and_then;
   6 use clippy_utils::is_automatically_derived;
   7 use clippy_utils::source::snippet_opt;
   8 use if_chain::if_chain;
   9 use rustc_errors::Applicability;
  10 use rustc_hir::{BindingAnnotation, BorrowKind, Expr, ExprKind, Item, Mutability, Pat, PatKind};
  11 use rustc_lint::{LateContext, LateLintPass};
  12 use rustc_middle::ty;
  13 use rustc_middle::ty::adjustment::{Adjust, Adjustment};
  14 use rustc_session::{declare_tool_lint, impl_lint_pass};
  15 use rustc_span::def_id::LocalDefId;
  16
  17 declare_clippy_lint! {
  18     /// **What it does:** Checks for address of operations (`&`) that are going to
  19     /// be dereferenced immediately by the compiler.
  20     ///
  21     /// **Why is this bad?** Suggests that the receiver of the expression borrows
  22     /// the expression.
  23     ///
  24     /// **Known problems:** None.
  25     ///
  26     /// **Example:**
  27     /// ```rust
  28     /// // Bad
  29     /// let x: &i32 = &&&&&&5;
  30     ///
  31     /// // Good
  32     /// let x: &i32 = &5;
  33     /// ```
  34     pub NEEDLESS_BORROW,
  35     nursery,
  36     "taking a reference that is going to be automatically dereferenced"
  37 }
  38
  39 #[derive(Default)]
  40 pub struct NeedlessBorrow {
  41     derived_item: Option<LocalDefId>,
  42 }
  43
  44 impl_lint_pass!(NeedlessBorrow => [NEEDLESS_BORROW]);
  45
  46 impl<'tcx> LateLintPass<'tcx> for NeedlessBorrow {
  47     fn check_expr(&mut self, cx: &LateContext<'tcx>, e: &'tcx Expr<'_>) {
  48         if e.span.from_expansion() || self.derived_item.is_some() {
  49             return;
  50         }
  51         if let ExprKind::AddrOf(BorrowKind::Ref, Mutability::Not, inner) = e.kind {
  52             if let ty::Ref(_, ty, _) = cx.typeck_results().expr_ty(inner).kind() {
  53                 for adj3 in cx.typeck_results().expr_adjustments(e).windows(3) {
  54                     if let [Adjustment {
  55                         kind: Adjust::Deref(_), ..
  56                     }, Adjustment {
  57                         kind: Adjust::Deref(_), ..
  58                     }, Adjustment {
  59                         kind: Adjust::Borrow(_),
  60                         ..
  61                     }] = *adj3
  62                     {
  63                         span_lint_and_then(
  64                             cx,
  65                             NEEDLESS_BORROW,
  66                             e.span,
  67                             &format!(
  68                                 "this expression borrows a reference (`&{}`) that is immediately dereferenced \
  69                              by the compiler",
  70                                 ty
  71                             ),
  72                             |diag| {
  73                                 if let Some(snippet) = snippet_opt(cx, inner.span) {
  74                                     diag.span_suggestion(
  75                                         e.span,
  76                                         "change this to",
  77                                         snippet,
  78                                         Applicability::MachineApplicable,
  79                                     );
  80                                 }
  81                             },
  82                         );
  83                     }
  84                 }
  85             }
  86         }
  87     }
  88     fn check_pat(&mut self, cx: &LateContext<'tcx>, pat: &'tcx Pat<'_>) {
  89         if pat.span.from_expansion() || self.derived_item.is_some() {
  90             return;
  91         }
  92         if_chain! {
  93             if let PatKind::Binding(BindingAnnotation::Ref, .., name, _) = pat.kind;
  94             if let ty::Ref(_, tam, mutbl) = *cx.typeck_results().pat_ty(pat).kind();
  95             if mutbl == Mutability::Not;
  96             if let ty::Ref(_, _, mutbl) = *tam.kind();
  97             // only lint immutable refs, because borrowed `&mut T` cannot be moved out
  98             if mutbl == Mutability::Not;
  99             then {
 100                 span_lint_and_then(
 101                     cx,
 102                     NEEDLESS_BORROW,
 103                     pat.span,
 104                     "this pattern creates a reference to a reference",
 105                     |diag| {
 106                         if let Some(snippet) = snippet_opt(cx, name.span) {
 107                             diag.span_suggestion(
 108                                 pat.span,
 109                                 "change this to",
 110                                 snippet,
 111                                 Applicability::MachineApplicable,
 112                             );
 113                         }
 114                     }
 115                 )
 116             }
 117         }
 118     }
 119
 120     fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx Item<'_>) {
 121         let attrs = cx.tcx.hir().attrs(item.hir_id());
 122         if is_automatically_derived(attrs) {
 123             debug_assert!(self.derived_item.is_none());
 124             self.derived_item = Some(item.def_id);
 125         }
 126     }
 127
 128     fn check_item_post(&mut self, _: &LateContext<'tcx>, item: &'tcx Item<'_>) {
 129         if let Some(id) = self.derived_item {
 130             if item.def_id == id {
 131                 self.derived_item = None;
 132             }
 133         }
 134     }
 135 }