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::source::snippet_opt;
   7 use if_chain::if_chain;
   8 use rustc_errors::Applicability;
   9 use rustc_hir::{BindingAnnotation, BorrowKind, Expr, ExprKind, Mutability, Pat, PatKind};
  10 use rustc_lint::{LateContext, LateLintPass};
  11 use rustc_middle::ty;
  12 use rustc_middle::ty::adjustment::{Adjust, Adjustment};
  13 use rustc_session::{declare_tool_lint, impl_lint_pass};
  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
  40 impl_lint_pass!(NeedlessBorrow => [NEEDLESS_BORROW]);
  41
  42 impl<'tcx> LateLintPass<'tcx> for NeedlessBorrow {
  43     fn check_expr(&mut self, cx: &LateContext<'tcx>, e: &'tcx Expr<'_>) {
  44         if e.span.from_expansion() {
  45             return;
  46         }
  47         if let ExprKind::AddrOf(BorrowKind::Ref, Mutability::Not, inner) = e.kind {
  48             if let ty::Ref(_, ty, _) = cx.typeck_results().expr_ty(inner).kind() {
  49                 for adj3 in cx.typeck_results().expr_adjustments(e).windows(3) {
  50                     if let [Adjustment {
  51                         kind: Adjust::Deref(_), ..
  52                     }, Adjustment {
  53                         kind: Adjust::Deref(_), ..
  54                     }, Adjustment {
  55                         kind: Adjust::Borrow(_),
  56                         ..
  57                     }] = *adj3
  58                     {
  59                         span_lint_and_then(
  60                             cx,
  61                             NEEDLESS_BORROW,
  62                             e.span,
  63                             &format!(
  64                                 "this expression borrows a reference (`&{}`) that is immediately dereferenced \
  65                              by the compiler",
  66                                 ty
  67                             ),
  68                             |diag| {
  69                                 if let Some(snippet) = snippet_opt(cx, inner.span) {
  70                                     diag.span_suggestion(
  71                                         e.span,
  72                                         "change this to",
  73                                         snippet,
  74                                         Applicability::MachineApplicable,
  75                                     );
  76                                 }
  77                             },
  78                         );
  79                     }
  80                 }
  81             }
  82         }
  83     }
  84     fn check_pat(&mut self, cx: &LateContext<'tcx>, pat: &'tcx Pat<'_>) {
  85         if pat.span.from_expansion() {
  86             return;
  87         }
  88         if_chain! {
  89             if let PatKind::Binding(BindingAnnotation::Ref, .., name, _) = pat.kind;
  90             if let ty::Ref(_, tam, mutbl) = *cx.typeck_results().pat_ty(pat).kind();
  91             if mutbl == Mutability::Not;
  92             if let ty::Ref(_, _, mutbl) = *tam.kind();
  93             // only lint immutable refs, because borrowed `&mut T` cannot be moved out
  94             if mutbl == Mutability::Not;
  95             then {
  96                 span_lint_and_then(
  97                     cx,
  98                     NEEDLESS_BORROW,
  99                     pat.span,
 100                     "this pattern creates a reference to a reference",
 101                     |diag| {
 102                         if let Some(snippet) = snippet_opt(cx, name.span) {
 103                             diag.span_suggestion(
 104                                 pat.span,
 105                                 "change this to",
 106                                 snippet,
 107                                 Applicability::MachineApplicable,
 108                             );
 109                         }
 110                     }
 111                 )
 112             }
 113         }
 114     }
 115 }