src/tools/clippy/clippy_lints/src/let_underscore.rs

   1 use clippy_utils::diagnostics::span_lint_and_help;
   2 use clippy_utils::ty::{is_must_use_ty, match_type};
   3 use clippy_utils::{is_must_use_func_call, paths};
   4 use if_chain::if_chain;
   5 use rustc_hir::{Local, PatKind};
   6 use rustc_lint::{LateContext, LateLintPass};
   7 use rustc_middle::lint::in_external_macro;
   8 use rustc_middle::ty::subst::GenericArgKind;
   9 use rustc_session::{declare_lint_pass, declare_tool_lint};
  10
  11 declare_clippy_lint! {
  12     /// ### What it does
  13     /// Checks for `let _ = <expr>` where expr is `#[must_use]`
  14     ///
  15     /// ### Why is this bad?
  16     /// It's better to explicitly handle the value of a `#[must_use]`
  17     /// expr
  18     ///
  19     /// ### Example
  20     /// ```rust
  21     /// fn f() -> Result<u32, u32> {
  22     ///     Ok(0)
  23     /// }
  24     ///
  25     /// let _ = f();
  26     /// // is_ok() is marked #[must_use]
  27     /// let _ = f().is_ok();
  28     /// ```
  29     #[clippy::version = "1.42.0"]
  30     pub LET_UNDERSCORE_MUST_USE,
  31     restriction,
  32     "non-binding let on a `#[must_use]` expression"
  33 }
  34
  35 declare_clippy_lint! {
  36     /// ### What it does
  37     /// Checks for `let _ = sync_lock`.
  38     /// This supports `mutex` and `rwlock` in `std::sync` and `parking_lot`.
  39     ///
  40     /// ### Why is this bad?
  41     /// This statement immediately drops the lock instead of
  42     /// extending its lifetime to the end of the scope, which is often not intended.
  43     /// To extend lock lifetime to the end of the scope, use an underscore-prefixed
  44     /// name instead (i.e. _lock). If you want to explicitly drop the lock,
  45     /// `std::mem::drop` conveys your intention better and is less error-prone.
  46     ///
  47     /// ### Example
  48     /// ```rust,ignore
  49     /// let _ = mutex.lock();
  50     /// ```
  51     ///
  52     /// Use instead:
  53     /// ```rust,ignore
  54     /// let _lock = mutex.lock();
  55     /// ```
  56     #[clippy::version = "1.43.0"]
  57     pub LET_UNDERSCORE_LOCK,
  58     correctness,
  59     "non-binding let on a synchronization lock"
  60 }
  61
  62 declare_clippy_lint! {
  63     /// ### What it does
  64     /// Checks for `let _ = <expr>`
  65     /// where expr has a type that implements `Drop`
  66     ///
  67     /// ### Why is this bad?
  68     /// This statement immediately drops the initializer
  69     /// expression instead of extending its lifetime to the end of the scope, which
  70     /// is often not intended. To extend the expression's lifetime to the end of the
  71     /// scope, use an underscore-prefixed name instead (i.e. _var). If you want to
  72     /// explicitly drop the expression, `std::mem::drop` conveys your intention
  73     /// better and is less error-prone.
  74     ///
  75     /// ### Example
  76     /// ```rust
  77     /// # struct DroppableItem;
  78     /// {
  79     ///     let _ = DroppableItem;
  80     ///     //                   ^ dropped here
  81     ///     /* more code */
  82     /// }
  83     /// ```
  84     ///
  85     /// Use instead:
  86     /// ```rust
  87     /// # struct DroppableItem;
  88     /// {
  89     ///     let _droppable = DroppableItem;
  90     ///     /* more code */
  91     ///     // dropped at end of scope
  92     /// }
  93     /// ```
  94     #[clippy::version = "1.50.0"]
  95     pub LET_UNDERSCORE_DROP,
  96     pedantic,
  97     "non-binding let on a type that implements `Drop`"
  98 }
  99
 100 declare_lint_pass!(LetUnderscore => [LET_UNDERSCORE_MUST_USE, LET_UNDERSCORE_LOCK, LET_UNDERSCORE_DROP]);
 101
 102 const SYNC_GUARD_PATHS: [&[&str]; 6] = [
 103     &paths::MUTEX_GUARD,
 104     &paths::RWLOCK_READ_GUARD,
 105     &paths::RWLOCK_WRITE_GUARD,
 106     &paths::PARKING_LOT_MUTEX_GUARD,
 107     &paths::PARKING_LOT_RWLOCK_READ_GUARD,
 108     &paths::PARKING_LOT_RWLOCK_WRITE_GUARD,
 109 ];
 110
 111 impl<'tcx> LateLintPass<'tcx> for LetUnderscore {
 112     fn check_local(&mut self, cx: &LateContext<'_>, local: &Local<'_>) {
 113         if in_external_macro(cx.tcx.sess, local.span) {
 114             return;
 115         }
 116
 117         if_chain! {
 118             if let PatKind::Wild = local.pat.kind;
 119             if let Some(init) = local.init;
 120             then {
 121                 let init_ty = cx.typeck_results().expr_ty(init);
 122                 let contains_sync_guard = init_ty.walk().any(|inner| match inner.unpack() {
 123                     GenericArgKind::Type(inner_ty) => {
 124                         SYNC_GUARD_PATHS.iter().any(|path| match_type(cx, inner_ty, path))
 125                     },
 126
 127                     GenericArgKind::Lifetime(_) | GenericArgKind::Const(_) => false,
 128                 });
 129                 if contains_sync_guard {
 130                     span_lint_and_help(
 131                         cx,
 132                         LET_UNDERSCORE_LOCK,
 133                         local.span,
 134                         "non-binding let on a synchronization lock",
 135                         None,
 136                         "consider using an underscore-prefixed named \
 137                             binding or dropping explicitly with `std::mem::drop`"
 138                     );
 139                 } else if init_ty.needs_drop(cx.tcx, cx.param_env) {
 140                     span_lint_and_help(
 141                         cx,
 142                         LET_UNDERSCORE_DROP,
 143                         local.span,
 144                         "non-binding `let` on a type that implements `Drop`",
 145                         None,
 146                         "consider using an underscore-prefixed named \
 147                             binding or dropping explicitly with `std::mem::drop`"
 148                     );
 149                 } else if is_must_use_ty(cx, cx.typeck_results().expr_ty(init)) {
 150                     span_lint_and_help(
 151                         cx,
 152                         LET_UNDERSCORE_MUST_USE,
 153                         local.span,
 154                         "non-binding let on an expression with `#[must_use]` type",
 155                         None,
 156                         "consider explicitly using expression value"
 157                     );
 158                 } else if is_must_use_func_call(cx, init) {
 159                     span_lint_and_help(
 160                         cx,
 161                         LET_UNDERSCORE_MUST_USE,
 162                         local.span,
 163                         "non-binding let on a result of a `#[must_use]` function",
 164                         None,
 165                         "consider explicitly using function result"
 166                     );
 167                 }
 168             }
 169         }
 170     }
 171 }