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::{in_macro, snippet_opt, span_lint_and_then};
   6 use if_chain::if_chain;
   7 use rustc::hir::{BindingAnnotation, Expr, ExprKind, Item, MutImmutable, Pat, PatKind};
   8 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
   9 use rustc::ty;
  10 use rustc::ty::adjustment::{Adjust, Adjustment};
  11 use rustc::{declare_tool_lint, lint_array};
  12 use rustc_errors::Applicability;
  13 use syntax::ast::NodeId;
  14
  15 /// **What it does:** Checks for address of operations (`&`) that are going to
  16 /// be dereferenced immediately by the compiler.
  17 ///
  18 /// **Why is this bad?** Suggests that the receiver of the expression borrows
  19 /// the expression.
  20 ///
  21 /// **Example:**
  22 /// ```rust
  23 /// let x: &i32 = &&&&&&5;
  24 /// ```
  25 ///
  26 /// **Known problems:** None.
  27 declare_clippy_lint! {
  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<NodeId>,
  36 }
  37
  38 impl LintPass for NeedlessBorrow {
  39     fn get_lints(&self) -> LintArray {
  40         lint_array!(NEEDLESS_BORROW)
  41     }
  42 }
  43
  44 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NeedlessBorrow {
  45     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, e: &'tcx Expr) {
  46         if in_macro(e.span) || self.derived_item.is_some() {
  47             return;
  48         }
  49         if let ExprKind::AddrOf(MutImmutable, ref inner) = e.node {
  50             if let ty::Ref(..) = cx.tables.expr_ty(inner).sty {
  51                 for adj3 in cx.tables.expr_adjustments(e).windows(3) {
  52                     if let [Adjustment {
  53                         kind: Adjust::Deref(_), ..
  54                     }, Adjustment {
  55                         kind: Adjust::Deref(_), ..
  56                     }, Adjustment {
  57                         kind: Adjust::Borrow(_),
  58                         ..
  59                     }] = *adj3
  60                     {
  61                         span_lint_and_then(
  62                             cx,
  63                             NEEDLESS_BORROW,
  64                             e.span,
  65                             "this expression borrows a reference that is immediately dereferenced \
  66                              by the compiler",
  67                             |db| {
  68                                 if let Some(snippet) = snippet_opt(cx, inner.span) {
  69                                     db.span_suggestion_with_applicability(
  70                                         e.span,
  71                                         "change this to",
  72                                         snippet,
  73                                         Applicability::MachineApplicable,
  74                                     );
  75                                 }
  76                             },
  77                         );
  78                     }
  79                 }
  80             }
  81         }
  82     }
  83     fn check_pat(&mut self, cx: &LateContext<'a, 'tcx>, pat: &'tcx Pat) {
  84         if in_macro(pat.span) || self.derived_item.is_some() {
  85             return;
  86         }
  87         if_chain! {
  88             if let PatKind::Binding(BindingAnnotation::Ref, _, name, _) = pat.node;
  89             if let ty::Ref(_, tam, mutbl) = cx.tables.pat_ty(pat).sty;
  90             if mutbl == MutImmutable;
  91             if let ty::Ref(_, _, mutbl) = tam.sty;
  92             // only lint immutable refs, because borrowed `&mut T` cannot be moved out
  93             if mutbl == MutImmutable;
  94             then {
  95                 span_lint_and_then(
  96                     cx,
  97                     NEEDLESS_BORROW,
  98                     pat.span,
  99                     "this pattern creates a reference to a reference",
 100                     |db| {
 101                         if let Some(snippet) = snippet_opt(cx, name.span) {
 102                             db.span_suggestion_with_applicability(
 103                                 pat.span,
 104                                 "change this to",
 105                                 snippet,
 106                                 Applicability::MachineApplicable,
 107                             );
 108                         }
 109                     }
 110                 )
 111             }
 112         }
 113     }
 114
 115     fn check_item(&mut self, _: &LateContext<'a, 'tcx>, item: &'tcx Item) {
 116         if item.attrs.iter().any(|a| a.check_name("automatically_derived")) {
 117             debug_assert!(self.derived_item.is_none());
 118             self.derived_item = Some(item.id);
 119         }
 120     }
 121
 122     fn check_item_post(&mut self, _: &LateContext<'a, 'tcx>, item: &'tcx Item) {
 123         if let Some(id) = self.derived_item {
 124             if item.id == id {
 125                 self.derived_item = None;
 126             }
 127         }
 128     }
 129 }