src/tools/clippy/clippy_lints/src/if_let_mutex.rs

   1 use clippy_utils::diagnostics::span_lint_and_help;
   2 use clippy_utils::higher;
   3 use clippy_utils::ty::is_type_diagnostic_item;
   4 use clippy_utils::SpanlessEq;
   5 use if_chain::if_chain;
   6 use rustc_hir::intravisit::{self as visit, NestedVisitorMap, Visitor};
   7 use rustc_hir::{Expr, ExprKind};
   8 use rustc_lint::{LateContext, LateLintPass};
   9 use rustc_middle::hir::map::Map;
  10 use rustc_session::{declare_lint_pass, declare_tool_lint};
  11
  12 declare_clippy_lint! {
  13     /// ### What it does
  14     /// Checks for `Mutex::lock` calls in `if let` expression
  15     /// with lock calls in any of the else blocks.
  16     ///
  17     /// ### Why is this bad?
  18     /// The Mutex lock remains held for the whole
  19     /// `if let ... else` block and deadlocks.
  20     ///
  21     /// ### Example
  22     /// ```rust,ignore
  23     /// if let Ok(thing) = mutex.lock() {
  24     ///     do_thing();
  25     /// } else {
  26     ///     mutex.lock();
  27     /// }
  28     /// ```
  29     /// Should be written
  30     /// ```rust,ignore
  31     /// let locked = mutex.lock();
  32     /// if let Ok(thing) = locked {
  33     ///     do_thing(thing);
  34     /// } else {
  35     ///     use_locked(locked);
  36     /// }
  37     /// ```
  38     pub IF_LET_MUTEX,
  39     correctness,
  40     "locking a `Mutex` in an `if let` block can cause deadlocks"
  41 }
  42
  43 declare_lint_pass!(IfLetMutex => [IF_LET_MUTEX]);
  44
  45 impl<'tcx> LateLintPass<'tcx> for IfLetMutex {
  46     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) {
  47         let mut arm_visit = ArmVisitor {
  48             mutex_lock_called: false,
  49             found_mutex: None,
  50             cx,
  51         };
  52         let mut op_visit = OppVisitor {
  53             mutex_lock_called: false,
  54             found_mutex: None,
  55             cx,
  56         };
  57         if let Some(higher::IfLet {
  58             let_expr,
  59             if_then,
  60             if_else: Some(if_else),
  61             ..
  62         }) = higher::IfLet::hir(cx, expr)
  63         {
  64             op_visit.visit_expr(let_expr);
  65             if op_visit.mutex_lock_called {
  66                 arm_visit.visit_expr(if_then);
  67                 arm_visit.visit_expr(if_else);
  68
  69                 if arm_visit.mutex_lock_called && arm_visit.same_mutex(cx, op_visit.found_mutex.unwrap()) {
  70                     span_lint_and_help(
  71                         cx,
  72                         IF_LET_MUTEX,
  73                         expr.span,
  74                         "calling `Mutex::lock` inside the scope of another `Mutex::lock` causes a deadlock",
  75                         None,
  76                         "move the lock call outside of the `if let ...` expression",
  77                     );
  78                 }
  79             }
  80         }
  81     }
  82 }
  83
  84 /// Checks if `Mutex::lock` is called in the `if let` expr.
  85 pub struct OppVisitor<'a, 'tcx> {
  86     mutex_lock_called: bool,
  87     found_mutex: Option<&'tcx Expr<'tcx>>,
  88     cx: &'a LateContext<'tcx>,
  89 }
  90
  91 impl<'tcx> Visitor<'tcx> for OppVisitor<'_, 'tcx> {
  92     type Map = Map<'tcx>;
  93
  94     fn visit_expr(&mut self, expr: &'tcx Expr<'_>) {
  95         if let Some(mutex) = is_mutex_lock_call(self.cx, expr) {
  96             self.found_mutex = Some(mutex);
  97             self.mutex_lock_called = true;
  98             return;
  99         }
 100         visit::walk_expr(self, expr);
 101     }
 102
 103     fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
 104         NestedVisitorMap::None
 105     }
 106 }
 107
 108 /// Checks if `Mutex::lock` is called in any of the branches.
 109 pub struct ArmVisitor<'a, 'tcx> {
 110     mutex_lock_called: bool,
 111     found_mutex: Option<&'tcx Expr<'tcx>>,
 112     cx: &'a LateContext<'tcx>,
 113 }
 114
 115 impl<'tcx> Visitor<'tcx> for ArmVisitor<'_, 'tcx> {
 116     type Map = Map<'tcx>;
 117
 118     fn visit_expr(&mut self, expr: &'tcx Expr<'tcx>) {
 119         if let Some(mutex) = is_mutex_lock_call(self.cx, expr) {
 120             self.found_mutex = Some(mutex);
 121             self.mutex_lock_called = true;
 122             return;
 123         }
 124         visit::walk_expr(self, expr);
 125     }
 126
 127     fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
 128         NestedVisitorMap::None
 129     }
 130 }
 131
 132 impl<'tcx, 'l> ArmVisitor<'tcx, 'l> {
 133     fn same_mutex(&self, cx: &LateContext<'_>, op_mutex: &Expr<'_>) -> bool {
 134         self.found_mutex
 135             .map_or(false, |arm_mutex| SpanlessEq::new(cx).eq_expr(op_mutex, arm_mutex))
 136     }
 137 }
 138
 139 fn is_mutex_lock_call<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) -> Option<&'tcx Expr<'tcx>> {
 140     if_chain! {
 141         if let ExprKind::MethodCall(path, _span, args, _) = &expr.kind;
 142         if path.ident.as_str() == "lock";
 143         let ty = cx.typeck_results().expr_ty(&args[0]);
 144         if is_type_diagnostic_item(cx, ty, sym!(mutex_type));
 145         then {
 146             Some(&args[0])
 147         } else {
 148             None
 149         }
 150     }
 151 }