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     fn name(&self) -> &'static str {
  44         "NeedlessBorrow"
  45     }
  46 }
  47
  48 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NeedlessBorrow {
  49     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, e: &'tcx Expr) {
  50         if in_macro(e.span) || self.derived_item.is_some() {
  51             return;
  52         }
  53         if let ExprKind::AddrOf(MutImmutable, ref inner) = e.node {
  54             if let ty::Ref(..) = cx.tables.expr_ty(inner).sty {
  55                 for adj3 in cx.tables.expr_adjustments(e).windows(3) {
  56                     if let [Adjustment {
  57                         kind: Adjust::Deref(_), ..
  58                     }, Adjustment {
  59                         kind: Adjust::Deref(_), ..
  60                     }, Adjustment {
  61                         kind: Adjust::Borrow(_),
  62                         ..
  63                     }] = *adj3
  64                     {
  65                         span_lint_and_then(
  66                             cx,
  67                             NEEDLESS_BORROW,
  68                             e.span,
  69                             "this expression borrows a reference that is immediately dereferenced \
  70                              by the compiler",
  71                             |db| {
  72                                 if let Some(snippet) = snippet_opt(cx, inner.span) {
  73                                     db.span_suggestion(
  74                                         e.span,
  75                                         "change this to",
  76                                         snippet,
  77                                         Applicability::MachineApplicable,
  78                                     );
  79                                 }
  80                             },
  81                         );
  82                     }
  83                 }
  84             }
  85         }
  86     }
  87     fn check_pat(&mut self, cx: &LateContext<'a, 'tcx>, pat: &'tcx Pat) {
  88         if in_macro(pat.span) || self.derived_item.is_some() {
  89             return;
  90         }
  91         if_chain! {
  92             if let PatKind::Binding(BindingAnnotation::Ref, _, name, _) = pat.node;
  93             if let ty::Ref(_, tam, mutbl) = cx.tables.pat_ty(pat).sty;
  94             if mutbl == MutImmutable;
  95             if let ty::Ref(_, _, mutbl) = tam.sty;
  96             // only lint immutable refs, because borrowed `&mut T` cannot be moved out
  97             if mutbl == MutImmutable;
  98             then {
  99                 span_lint_and_then(
 100                     cx,
 101                     NEEDLESS_BORROW,
 102                     pat.span,
 103                     "this pattern creates a reference to a reference",
 104                     |db| {
 105                         if let Some(snippet) = snippet_opt(cx, name.span) {
 106                             db.span_suggestion(
 107                                 pat.span,
 108                                 "change this to",
 109                                 snippet,
 110                                 Applicability::MachineApplicable,
 111                             );
 112                         }
 113                     }
 114                 )
 115             }
 116         }
 117     }
 118
 119     fn check_item(&mut self, _: &LateContext<'a, 'tcx>, item: &'tcx Item) {
 120         if item.attrs.iter().any(|a| a.check_name("automatically_derived")) {
 121             debug_assert!(self.derived_item.is_none());
 122             self.derived_item = Some(item.id);
 123         }
 124     }
 125
 126     fn check_item_post(&mut self, _: &LateContext<'a, 'tcx>, item: &'tcx Item) {
 127         if let Some(id) = self.derived_item {
 128             if item.id == id {
 129                 self.derived_item = None;
 130             }
 131         }
 132     }
 133 }