mutex_atomic.rs

use clippy_utils::diagnostics::span_lint;
use clippy_utils::ty::is_type_diagnostic_item;
use rustc_hir::Expr;
use rustc_lint::{LateContext, LateLintPass};
use rustc_middle::ty::{self, IntTy, Ty, UintTy};
use rustc_session::declare_lint_pass;
use rustc_span::sym;

declare_clippy_lint! {
    /// ### What it does
    /// Checks for usage of `Mutex<X>` where an atomic will do.
    ///
    /// ### Why restrict this?
    /// Using a mutex just to make access to a plain bool or
    /// reference sequential is shooting flies with cannons.
    /// `std::sync::atomic::AtomicBool` and `std::sync::atomic::AtomicPtr` are leaner and
    /// faster.
    ///
    /// On the other hand, `Mutex`es are, in general, easier to
    /// verify correctness. An atomic does not behave the same as
    /// an equivalent mutex. See [this issue](https://github.com/rust-lang/rust-clippy/issues/4295)'s
    /// commentary for more details.
    ///
    /// ### Known problems
    /// * This lint cannot detect if the mutex is actually used
    ///   for waiting before a critical section.
    /// * This lint has a false positive that warns without considering the case
    ///   where `Mutex` is used together with `Condvar`.
    ///
    /// ### Example
    /// ```no_run
    /// # let y = true;
    /// # use std::sync::Mutex;
    /// let x = Mutex::new(&y);
    /// ```
    ///
    /// Use instead:
    /// ```no_run
    /// # let y = true;
    /// # use std::sync::atomic::AtomicBool;
    /// let x = AtomicBool::new(y);
    /// ```
    #[clippy::version = "pre 1.29.0"]
    pub MUTEX_ATOMIC,
    restriction,
    "using a mutex where an atomic value could be used instead."
}

declare_clippy_lint! {
    /// ### What it does
    /// Checks for usage of `Mutex<X>` where `X` is an integral
    /// type.
    ///
    /// ### Why restrict this?
    /// Using a mutex just to make access to a plain integer
    /// sequential is
    /// shooting flies with cannons. `std::sync::atomic::AtomicUsize` is leaner and faster.
    ///
    /// On the other hand, `Mutex`es are, in general, easier to
    /// verify correctness. An atomic does not behave the same as
    /// an equivalent mutex. See [this issue](https://github.com/rust-lang/rust-clippy/issues/4295)'s
    /// commentary for more details.
    ///
    /// ### Known problems
    /// * This lint cannot detect if the mutex is actually used
    ///   for waiting before a critical section.
    /// * This lint has a false positive that warns without considering the case
    ///   where `Mutex` is used together with `Condvar`.
    /// * This lint suggest using `AtomicU64` instead of `Mutex<u64>`, but
    ///   `AtomicU64` is not available on some 32-bit platforms.
    ///
    /// ### Example
    /// ```no_run
    /// # use std::sync::Mutex;
    /// let x = Mutex::new(0usize);
    /// ```
    ///
    /// Use instead:
    /// ```no_run
    /// # use std::sync::atomic::AtomicUsize;
    /// let x = AtomicUsize::new(0usize);
    /// ```
    #[clippy::version = "pre 1.29.0"]
    pub MUTEX_INTEGER,
    restriction,
    "using a mutex for an integer type"
}

declare_lint_pass!(Mutex => [MUTEX_ATOMIC, MUTEX_INTEGER]);

impl<'tcx> LateLintPass<'tcx> for Mutex {
    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
        let ty = cx.typeck_results().expr_ty(expr);
        if let ty::Adt(_, subst) = ty.kind()
            && is_type_diagnostic_item(cx, ty, sym::Mutex)
        {
            let mutex_param = subst.type_at(0);
            if let Some(atomic_name) = get_atomic_name(mutex_param) {
                let msg = format!(
                    "consider using an `{atomic_name}` instead of a `Mutex` here; if you just want the locking \
                         behavior and not the internal type, consider using `Mutex<()>`"
                );
                match *mutex_param.kind() {
                    ty::Uint(t) if t != UintTy::Usize => span_lint(cx, MUTEX_INTEGER, expr.span, msg),
                    ty::Int(t) if t != IntTy::Isize => span_lint(cx, MUTEX_INTEGER, expr.span, msg),
                    _ => span_lint(cx, MUTEX_ATOMIC, expr.span, msg),
                }
            }
        }
    }
}

fn get_atomic_name(ty: Ty<'_>) -> Option<&'static str> {
    match ty.kind() {
        ty::Bool => Some("AtomicBool"),
        ty::Uint(uint_ty) => {
            match uint_ty {
                UintTy::U8 => Some("AtomicU8"),
                UintTy::U16 => Some("AtomicU16"),
                UintTy::U32 => Some("AtomicU32"),
                UintTy::U64 => Some("AtomicU64"),
                UintTy::Usize => Some("AtomicUsize"),
                // `AtomicU128` is unstable and only available on a few platforms: https://github.com/rust-lang/rust/issues/99069
                UintTy::U128 => None,
            }
        },
        ty::Int(int_ty) => {
            match int_ty {
                IntTy::I8 => Some("AtomicI8"),
                IntTy::I16 => Some("AtomicI16"),
                IntTy::I32 => Some("AtomicI32"),
                IntTy::I64 => Some("AtomicI64"),
                IntTy::Isize => Some("AtomicIsize"),
                // `AtomicU128` is unstable and only available on a few platforms: https://github.com/rust-lang/rust/issues/99069
                IntTy::I128 => None,
            }
        },
        ty::RawPtr(_, _) => Some("AtomicPtr"),
        _ => None,
    }
}