clippy_lints/src/needless_borrow.rs

   1 //! Checks for needless address of operations (`&`)
   2 //!
   3 //! This lint is **warn** by default
   4
   5 use rustc::lint::*;
   6 use rustc::hir::{ExprAddrOf, Expr, MutImmutable, Pat, PatKind, BindingMode};
   7 use rustc::ty;
   8 use rustc::ty::adjustment::{Adjustment, Adjust};
   9 use utils::{span_lint, in_macro};
  10
  11 /// **What it does:** Checks for address of operations (`&`) that are going to
  12 /// be dereferenced immediately by the compiler.
  13 ///
  14 /// **Why is this bad?** Suggests that the receiver of the expression borrows
  15 /// the expression.
  16 ///
  17 /// **Known problems:** None.
  18 ///
  19 /// **Example:**
  20 /// ```rust
  21 /// let x: &i32 = &&&&&&5;
  22 /// ```
  23 declare_lint! {
  24     pub NEEDLESS_BORROW,
  25     Warn,
  26     "taking a reference that is going to be automatically dereferenced"
  27 }
  28
  29 #[derive(Copy,Clone)]
  30 pub struct NeedlessBorrow;
  31
  32 impl LintPass for NeedlessBorrow {
  33     fn get_lints(&self) -> LintArray {
  34         lint_array!(NEEDLESS_BORROW)
  35     }
  36 }
  37
  38 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NeedlessBorrow {
  39     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, e: &'tcx Expr) {
  40         if in_macro(e.span) {
  41             return;
  42         }
  43         if let ExprAddrOf(MutImmutable, ref inner) = e.node {
  44             if let ty::TyRef(..) = cx.tables.expr_ty(inner).sty {
  45                 for adj3 in cx.tables.expr_adjustments(e).windows(3) {
  46                     if let [
  47                         Adjustment { kind: Adjust::Deref(_), .. },
  48                         Adjustment { kind: Adjust::Deref(_), .. },
  49                         Adjustment { kind: Adjust::Borrow(_), .. }
  50                     ] = *adj3 {
  51                         span_lint(cx,
  52                                   NEEDLESS_BORROW,
  53                                   e.span,
  54                                   "this expression borrows a reference that is immediately dereferenced by the \
  55                                    compiler");
  56                     }
  57                 }
  58             }
  59         }
  60     }
  61     fn check_pat(&mut self, cx: &LateContext<'a, 'tcx>, pat: &'tcx Pat) {
  62         if in_macro(pat.span) {
  63             return;
  64         }
  65         if_let_chain! {[
  66             let PatKind::Binding(BindingMode::BindByRef(MutImmutable), _, _, _) = pat.node,
  67             let ty::TyRef(_, ref tam) = cx.tables.pat_ty(pat).sty,
  68             tam.mutbl == MutImmutable,
  69             let ty::TyRef(_, ref tam) = tam.ty.sty,
  70             // only lint immutable refs, because borrowed `&mut T` cannot be moved out
  71             tam.mutbl == MutImmutable,
  72         ], {
  73             span_lint(cx, NEEDLESS_BORROW, pat.span, "this pattern creates a reference to a reference")
  74         }}
  75     }
  76 }