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::TyRef;
   8 use utils::{span_lint, in_macro};
   9 use rustc::ty::adjustment::AutoAdjustment::AdjustDerefRef;
  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 LateLintPass for NeedlessBorrow {
  39     fn check_expr(&mut self, cx: &LateContext, e: &Expr) {
  40         if in_macro(cx, e.span) {
  41             return;
  42         }
  43         if let ExprAddrOf(MutImmutable, ref inner) = e.node {
  44             if let TyRef(..) = cx.tcx.expr_ty(inner).sty {
  45                 if let Some(&AdjustDerefRef(ref deref)) = cx.tcx.tables.borrow().adjustments.get(&e.id) {
  46                     if deref.autoderefs > 1 && deref.autoref.is_some() {
  47                         span_lint(cx,
  48                                   NEEDLESS_BORROW,
  49                                   e.span,
  50                                   "this expression borrows a reference that is immediately dereferenced by the \
  51                                    compiler");
  52                     }
  53                 }
  54             }
  55         }
  56     }
  57     fn check_pat(&mut self, cx: &LateContext, pat: &Pat) {
  58         if in_macro(cx, pat.span) {
  59             return;
  60         }
  61         if let PatKind::Binding(BindingMode::BindByRef(MutImmutable), _, _) = pat.node {
  62             if let TyRef(_, ref tam) = cx.tcx.pat_ty(pat).sty {
  63                 if tam.mutbl == MutImmutable {
  64                     if let TyRef(..) = tam.ty.sty {
  65                         span_lint(cx,
  66                                   NEEDLESS_BORROW,
  67                                   pat.span,
  68                                   "this pattern creates a reference to a reference")
  69                     }
  70                 }
  71             }
  72         }
  73     }
  74 }