clippy_lints/src/borrow_deref_ref.rs

   1 use crate::reference::DEREF_ADDROF;
   2 use clippy_utils::diagnostics::span_lint_and_then;
   3 use clippy_utils::source::snippet_opt;
   4 use clippy_utils::ty::implements_trait;
   5 use clippy_utils::{get_parent_expr, is_lint_allowed};
   6 use rustc_errors::Applicability;
   7 use rustc_hir::{ExprKind, UnOp};
   8 use rustc_lint::{LateContext, LateLintPass};
   9 use rustc_middle::mir::Mutability;
  10 use rustc_middle::ty;
  11 use rustc_session::{declare_lint_pass, declare_tool_lint};
  12
  13 declare_clippy_lint! {
  14     /// ### What it does
  15     /// Checks for `&*(&T)`.
  16     ///
  17     /// ### Why is this bad?
  18     /// Dereferencing and then borrowing a reference value has no effect in most cases.
  19     ///
  20     /// ### Known problems
  21     /// false negative on such code:
  22     /// ```
  23     /// let x = &12;
  24     /// let addr_x = &x as *const _ as usize;
  25     /// let addr_y = &&*x as *const _ as usize; // assert ok now, and lint triggerd.
  26     ///                                         // But if we fix it, assert will fail.
  27     /// assert_ne!(addr_x, addr_y);
  28     /// ```
  29     ///
  30     /// ### Example
  31     /// ```rust
  32     /// let s = &String::new();
  33     ///
  34     /// // Bad
  35     /// let a: &String = &* s;
  36     /// foo(&*s);
  37     ///
  38     /// // Good
  39     /// let a: &String = s;
  40     /// foo(&**s);
  41     ///
  42     /// fn foo(_: &str){ }
  43     /// ```
  44     #[clippy::version = "1.59.0"]
  45     pub BORROW_DEREF_REF,
  46     complexity,
  47     "deref on an immutable reference returns the same type as itself"
  48 }
  49
  50 declare_lint_pass!(BorrowDerefRef => [BORROW_DEREF_REF]);
  51
  52 impl LateLintPass<'_> for BorrowDerefRef {
  53     fn check_expr(&mut self, cx: &LateContext<'_>, e: &rustc_hir::Expr<'_>) {
  54         if_chain! {
  55             if !e.span.from_expansion();
  56             if let ExprKind::AddrOf(_, Mutability::Not, addrof_target) = e.kind;
  57             if !addrof_target.span.from_expansion();
  58             if let ExprKind::Unary(UnOp::Deref, deref_target) = addrof_target.kind;
  59             if !deref_target.span.from_expansion();
  60             if !matches!(deref_target.kind, ExprKind::Unary(UnOp::Deref, ..) );
  61             let ref_ty = cx.typeck_results().expr_ty(deref_target);
  62             if let ty::Ref(_, inner_ty, Mutability::Not) = ref_ty.kind();
  63             then{
  64
  65                 if let Some(parent_expr) = get_parent_expr(cx, e){
  66                     if matches!(parent_expr.kind, ExprKind::Unary(UnOp::Deref, ..)) &&
  67                        !is_lint_allowed(cx, DEREF_ADDROF, parent_expr.hir_id) {
  68                         return;
  69                     }
  70
  71                     // modification to `&mut &*x` is different from `&mut x`
  72                     if matches!(deref_target.kind, ExprKind::Path(..)
  73                                              | ExprKind::Field(..)
  74                                              | ExprKind::Index(..)
  75                                              | ExprKind::Unary(UnOp::Deref, ..))
  76                      && matches!(parent_expr.kind, ExprKind::AddrOf(_, Mutability::Mut, _)) {
  77                        return;
  78                     }
  79                 }
  80
  81                 span_lint_and_then(
  82                     cx,
  83                     BORROW_DEREF_REF,
  84                     e.span,
  85                     "deref on an immutable reference",
  86                     |diag| {
  87                         diag.span_suggestion(
  88                             e.span,
  89                             "if you would like to reborrow, try removing `&*`",
  90                             snippet_opt(cx, deref_target.span).unwrap(),
  91                             Applicability::MachineApplicable
  92                         );
  93
  94                         // has deref trait -> give 2 help
  95                         // doesn't have deref trait -> give 1 help
  96                         if let Some(deref_trait_id) = cx.tcx.lang_items().deref_trait(){
  97                             if !implements_trait(cx, *inner_ty, deref_trait_id, &[]) {
  98                                 return;
  99                             }
 100                         }
 101
 102                         diag.span_suggestion(
 103                             e.span,
 104                             "if you would like to deref, try using `&**`",
 105                             format!(
 106                                 "&**{}",
 107                                 &snippet_opt(cx, deref_target.span).unwrap(),
 108                              ),
 109                             Applicability::MaybeIncorrect
 110                         );
 111
 112                     }
 113                 );
 114
 115             }
 116         }
 117     }
 118 }