clippy_lints/src/needless_borrow.rs

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