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