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};
   7 use rustc::ty::TyRef;
   8 use utils::{span_lint, in_macro};
   9 use rustc::ty::adjustment::AutoAdjustment::AdjustDerefRef;
  10
  11 /// **What it does:** This lint checks for address of operations (`&`) that are going to be
  12 /// dereferenced immediately by the compiler
  13 ///
  14 /// **Why is this bad?** Suggests that the receiver of the expression borrows the expression.
  15 ///
  16 /// **Known problems:**
  17 ///
  18 /// **Example:**
  19 /// ```rust
  20 /// let x: &i32 = &&&&&&5;
  21 /// ```
  22 declare_lint! {
  23     pub NEEDLESS_BORROW,
  24     Warn,
  25     "taking a reference that is going to be automatically dereferenced"
  26 }
  27
  28 #[derive(Copy,Clone)]
  29 pub struct NeedlessBorrow;
  30
  31 impl LintPass for NeedlessBorrow {
  32     fn get_lints(&self) -> LintArray {
  33         lint_array!(NEEDLESS_BORROW)
  34     }
  35 }
  36
  37 impl LateLintPass for NeedlessBorrow {
  38     fn check_expr(&mut self, cx: &LateContext, e: &Expr) {
  39         if in_macro(cx, e.span) {
  40             return;
  41         }
  42         if let ExprAddrOf(MutImmutable, ref inner) = e.node {
  43             if let TyRef(..) = cx.tcx.expr_ty(inner).sty {
  44                 if let Some(&AdjustDerefRef(ref deref)) = cx.tcx.tables.borrow().adjustments.get(&e.id) {
  45                     if deref.autoderefs > 1 && deref.autoref.is_some() {
  46                         span_lint(cx,
  47                                   NEEDLESS_BORROW,
  48                                   e.span,
  49                                   "this expression borrows a reference that is immediately dereferenced by the \
  50                                    compiler");
  51                     }
  52                 }
  53             }
  54         }
  55     }
  56 }