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