src/tools/clippy/clippy_lints/src/mutex_atomic.rs

   1 //! Checks for uses of mutex where an atomic value could be used
   2 //!
   3 //! This lint is **warn** by default
   4
   5 use clippy_utils::diagnostics::span_lint;
   6 use clippy_utils::ty::is_type_diagnostic_item;
   7 use rustc_hir::Expr;
   8 use rustc_lint::{LateContext, LateLintPass};
   9 use rustc_middle::ty::{self, Ty};
  10 use rustc_session::{declare_lint_pass, declare_tool_lint};
  11 use rustc_span::sym;
  12
  13 declare_clippy_lint! {
  14     /// ### What it does
  15     /// Checks for usages of `Mutex<X>` where an atomic will do.
  16     ///
  17     /// ### Why is this bad?
  18     /// Using a mutex just to make access to a plain bool or
  19     /// reference sequential is shooting flies with cannons.
  20     /// `std::sync::atomic::AtomicBool` and `std::sync::atomic::AtomicPtr` are leaner and
  21     /// faster.
  22     ///
  23     /// ### Known problems
  24     /// This lint cannot detect if the mutex is actually used
  25     /// for waiting before a critical section.
  26     ///
  27     /// ### Example
  28     /// ```rust
  29     /// # let y = true;
  30     /// # use std::sync::Mutex;
  31     /// let x = Mutex::new(&y);
  32     /// ```
  33     ///
  34     /// Use instead:
  35     /// ```rust
  36     /// # let y = true;
  37     /// # use std::sync::atomic::AtomicBool;
  38     /// let x = AtomicBool::new(y);
  39     /// ```
  40     #[clippy::version = "pre 1.29.0"]
  41     pub MUTEX_ATOMIC,
  42     nursery,
  43     "using a mutex where an atomic value could be used instead"
  44 }
  45
  46 declare_clippy_lint! {
  47     /// ### What it does
  48     /// Checks for usages of `Mutex<X>` where `X` is an integral
  49     /// type.
  50     ///
  51     /// ### Why is this bad?
  52     /// Using a mutex just to make access to a plain integer
  53     /// sequential is
  54     /// shooting flies with cannons. `std::sync::atomic::AtomicUsize` is leaner and faster.
  55     ///
  56     /// ### Known problems
  57     /// This lint cannot detect if the mutex is actually used
  58     /// for waiting before a critical section.
  59     ///
  60     /// ### Example
  61     /// ```rust
  62     /// # use std::sync::Mutex;
  63     /// let x = Mutex::new(0usize);
  64     /// ```
  65     ///
  66     /// Use instead:
  67     /// ```rust
  68     /// # use std::sync::atomic::AtomicUsize;
  69     /// let x = AtomicUsize::new(0usize);
  70     /// ```
  71     #[clippy::version = "pre 1.29.0"]
  72     pub MUTEX_INTEGER,
  73     nursery,
  74     "using a mutex for an integer type"
  75 }
  76
  77 declare_lint_pass!(Mutex => [MUTEX_ATOMIC, MUTEX_INTEGER]);
  78
  79 impl<'tcx> LateLintPass<'tcx> for Mutex {
  80     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
  81         let ty = cx.typeck_results().expr_ty(expr);
  82         if let ty::Adt(_, subst) = ty.kind() {
  83             if is_type_diagnostic_item(cx, ty, sym::Mutex) {
  84                 let mutex_param = subst.type_at(0);
  85                 if let Some(atomic_name) = get_atomic_name(mutex_param) {
  86                     let msg = format!(
  87                         "consider using an `{}` instead of a `Mutex` here; if you just want the locking \
  88                          behavior and not the internal type, consider using `Mutex<()>`",
  89                         atomic_name
  90                     );
  91                     match *mutex_param.kind() {
  92                         ty::Uint(t) if t != ty::UintTy::Usize => span_lint(cx, MUTEX_INTEGER, expr.span, &msg),
  93                         ty::Int(t) if t != ty::IntTy::Isize => span_lint(cx, MUTEX_INTEGER, expr.span, &msg),
  94                         _ => span_lint(cx, MUTEX_ATOMIC, expr.span, &msg),
  95                     };
  96                 }
  97             }
  98         }
  99     }
 100 }
 101
 102 fn get_atomic_name(ty: Ty<'_>) -> Option<&'static str> {
 103     match ty.kind() {
 104         ty::Bool => Some("AtomicBool"),
 105         ty::Uint(_) => Some("AtomicUsize"),
 106         ty::Int(_) => Some("AtomicIsize"),
 107         ty::RawPtr(_) => Some("AtomicPtr"),
 108         _ => None,
 109     }
 110 }