src/tools/clippy/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 triggered.
  26     ///                                         // But if we fix it, assert will fail.
  27     /// assert_ne!(addr_x, addr_y);
  28     /// ```
  29     ///
  30     /// ### Example
  31     /// ```rust
  32     /// fn foo(_x: &str) {}
  33     ///
  34     /// let s = &String::new();
  35     ///
  36     /// let a: &String = &* s;
  37     /// foo(&*s);
  38     /// ```
  39     ///
  40     /// Use instead:
  41     /// ```rust
  42     /// # fn foo(_x: &str) {}
  43     /// # let s = &String::new();
  44     /// let a: &String = s;
  45     /// foo(&**s);
  46     /// ```
  47     #[clippy::version = "1.59.0"]
  48     pub BORROW_DEREF_REF,
  49     complexity,
  50     "deref on an immutable reference returns the same type as itself"
  51 }
  52
  53 declare_lint_pass!(BorrowDerefRef => [BORROW_DEREF_REF]);
  54
  55 impl LateLintPass<'_> for BorrowDerefRef {
  56     fn check_expr(&mut self, cx: &LateContext<'_>, e: &rustc_hir::Expr<'_>) {
  57         if_chain! {
  58             if !e.span.from_expansion();
  59             if let ExprKind::AddrOf(_, Mutability::Not, addrof_target) = e.kind;
  60             if !addrof_target.span.from_expansion();
  61             if let ExprKind::Unary(UnOp::Deref, deref_target) = addrof_target.kind;
  62             if !deref_target.span.from_expansion();
  63             if !matches!(deref_target.kind, ExprKind::Unary(UnOp::Deref, ..) );
  64             let ref_ty = cx.typeck_results().expr_ty(deref_target);
  65             if let ty::Ref(_, inner_ty, Mutability::Not) = ref_ty.kind();
  66             then{
  67
  68                 if let Some(parent_expr) = get_parent_expr(cx, e){
  69                     if matches!(parent_expr.kind, ExprKind::Unary(UnOp::Deref, ..)) &&
  70                        !is_lint_allowed(cx, DEREF_ADDROF, parent_expr.hir_id) {
  71                         return;
  72                     }
  73
  74                     // modification to `&mut &*x` is different from `&mut x`
  75                     if matches!(deref_target.kind, ExprKind::Path(..)
  76                                              | ExprKind::Field(..)
  77                                              | ExprKind::Index(..)
  78                                              | ExprKind::Unary(UnOp::Deref, ..))
  79                      && matches!(parent_expr.kind, ExprKind::AddrOf(_, Mutability::Mut, _)) {
  80                        return;
  81                     }
  82                 }
  83
  84                 span_lint_and_then(
  85                     cx,
  86                     BORROW_DEREF_REF,
  87                     e.span,
  88                     "deref on an immutable reference",
  89                     |diag| {
  90                         diag.span_suggestion(
  91                             e.span,
  92                             "if you would like to reborrow, try removing `&*`",
  93                             snippet_opt(cx, deref_target.span).unwrap(),
  94                             Applicability::MachineApplicable
  95                         );
  96
  97                         // has deref trait -> give 2 help
  98                         // doesn't have deref trait -> give 1 help
  99                         if let Some(deref_trait_id) = cx.tcx.lang_items().deref_trait(){
 100                             if !implements_trait(cx, *inner_ty, deref_trait_id, &[]) {
 101                                 return;
 102                             }
 103                         }
 104
 105                         diag.span_suggestion(
 106                             e.span,
 107                             "if you would like to deref, try using `&**`",
 108                             format!(
 109                                 "&**{}",
 110                                 &snippet_opt(cx, deref_target.span).unwrap(),
 111                              ),
 112                             Applicability::MaybeIncorrect
 113                         );
 114
 115                     }
 116                 );
 117
 118             }
 119         }
 120     }
 121 }