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