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 utils::{span_lint, in_macro};
   9
  10 /// **What it does:** Checks for address of operations (`&`) that are going to
  11 /// be dereferenced immediately by the compiler.
  12 ///
  13 /// **Why is this bad?** Suggests that the receiver of the expression borrows
  14 /// the expression.
  15 ///
  16 /// **Known problems:** None.
  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<'a, 'tcx> LateLintPass<'a, 'tcx> for NeedlessBorrow {
  38     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, e: &'tcx Expr) {
  39         if in_macro(cx, e.span) {
  40             return;
  41         }
  42         if let ExprAddrOf(MutImmutable, ref inner) = e.node {
  43             if let ty::TyRef(..) = cx.tcx.tables().expr_ty(inner).sty {
  44                 if let Some(&ty::adjustment::Adjust::DerefRef { autoderefs, autoref, .. }) = cx.tcx.tables.borrow().adjustments.get(&e.id).map(|a| &a.kind) {
  45                     if autoderefs > 1 && 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     fn check_pat(&mut self, cx: &LateContext<'a, 'tcx>, pat: &'tcx Pat) {
  57         if in_macro(cx, pat.span) {
  58             return;
  59         }
  60         if let PatKind::Binding(BindingMode::BindByRef(MutImmutable), _, _, _) = pat.node {
  61             if let ty::TyRef(_, ref tam) = cx.tcx.tables().pat_ty(pat).sty {
  62                 if tam.mutbl == MutImmutable {
  63                     if let ty::TyRef(..) = tam.ty.sty {
  64                         span_lint(cx,
  65                                   NEEDLESS_BORROW,
  66                                   pat.span,
  67                                   "this pattern creates a reference to a reference")
  68                     }
  69                 }
  70             }
  71         }
  72     }
  73 }