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

   1 use clippy_utils::diagnostics::span_lint_and_help;
   2 use clippy_utils::ty::{implements_trait, is_must_use_ty, match_type};
   3 use clippy_utils::{is_must_use_func_call, paths};
   4 use rustc_hir::{Local, PatKind};
   5 use rustc_lint::{LateContext, LateLintPass};
   6 use rustc_middle::lint::in_external_macro;
   7 use rustc_middle::ty::subst::GenericArgKind;
   8 use rustc_session::{declare_lint_pass, declare_tool_lint};
   9
  10 declare_clippy_lint! {
  11     /// ### What it does
  12     /// Checks for `let _ = <expr>` where expr is `#[must_use]`
  13     ///
  14     /// ### Why is this bad?
  15     /// It's better to explicitly handle the value of a `#[must_use]`
  16     /// expr
  17     ///
  18     /// ### Example
  19     /// ```rust
  20     /// fn f() -> Result<u32, u32> {
  21     ///     Ok(0)
  22     /// }
  23     ///
  24     /// let _ = f();
  25     /// // is_ok() is marked #[must_use]
  26     /// let _ = f().is_ok();
  27     /// ```
  28     #[clippy::version = "1.42.0"]
  29     pub LET_UNDERSCORE_MUST_USE,
  30     restriction,
  31     "non-binding `let` on a `#[must_use]` expression"
  32 }
  33
  34 declare_clippy_lint! {
  35     /// ### What it does
  36     /// Checks for `let _ = sync_lock`. This supports `mutex` and `rwlock` in
  37     /// `parking_lot`. For `std` locks see the `rustc` lint
  38     /// [`let_underscore_lock`](https://doc.rust-lang.org/nightly/rustc/lints/listing/deny-by-default.html#let-underscore-lock)
  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>` where the resulting type of expr implements `Future`
  65     ///
  66     /// ### Why is this bad?
  67     /// Futures must be polled for work to be done. The original intention was most likely to await the future
  68     /// and ignore the resulting value.
  69     ///
  70     /// ### Example
  71     /// ```rust
  72     /// async fn foo() -> Result<(), ()> {
  73     ///     Ok(())
  74     /// }
  75     /// let _ = foo();
  76     /// ```
  77     ///
  78     /// Use instead:
  79     /// ```rust
  80     /// # async fn context() {
  81     /// async fn foo() -> Result<(), ()> {
  82     ///     Ok(())
  83     /// }
  84     /// let _ = foo().await;
  85     /// # }
  86     /// ```
  87     #[clippy::version = "1.66"]
  88     pub LET_UNDERSCORE_FUTURE,
  89     suspicious,
  90     "non-binding `let` on a future"
  91 }
  92
  93 declare_lint_pass!(LetUnderscore => [LET_UNDERSCORE_MUST_USE, LET_UNDERSCORE_LOCK, LET_UNDERSCORE_FUTURE]);
  94
  95 const SYNC_GUARD_PATHS: [&[&str]; 3] = [
  96     &paths::PARKING_LOT_MUTEX_GUARD,
  97     &paths::PARKING_LOT_RWLOCK_READ_GUARD,
  98     &paths::PARKING_LOT_RWLOCK_WRITE_GUARD,
  99 ];
 100
 101 impl<'tcx> LateLintPass<'tcx> for LetUnderscore {
 102     fn check_local(&mut self, cx: &LateContext<'_>, local: &Local<'_>) {
 103         if !in_external_macro(cx.tcx.sess, local.span)
 104             && let PatKind::Wild = local.pat.kind
 105             && let Some(init) = local.init
 106         {
 107             let init_ty = cx.typeck_results().expr_ty(init);
 108             let contains_sync_guard = init_ty.walk().any(|inner| match inner.unpack() {
 109                 GenericArgKind::Type(inner_ty) => SYNC_GUARD_PATHS.iter().any(|path| match_type(cx, inner_ty, path)),
 110                 GenericArgKind::Lifetime(_) | GenericArgKind::Const(_) => false,
 111             });
 112             if contains_sync_guard {
 113                 span_lint_and_help(
 114                     cx,
 115                     LET_UNDERSCORE_LOCK,
 116                     local.span,
 117                     "non-binding `let` on a synchronization lock",
 118                     None,
 119                     "consider using an underscore-prefixed named \
 120                             binding or dropping explicitly with `std::mem::drop`",
 121                 );
 122             } else if let Some(future_trait_def_id) = cx.tcx.lang_items().future_trait()
 123                 && implements_trait(cx, cx.typeck_results().expr_ty(init), future_trait_def_id, &[]) {
 124                 span_lint_and_help(
 125                     cx,
 126                     LET_UNDERSCORE_FUTURE,
 127                     local.span,
 128                     "non-binding `let` on a future",
 129                     None,
 130                     "consider awaiting the future or dropping explicitly with `std::mem::drop`"
 131                 );
 132             } else if is_must_use_ty(cx, cx.typeck_results().expr_ty(init)) {
 133                 span_lint_and_help(
 134                     cx,
 135                     LET_UNDERSCORE_MUST_USE,
 136                     local.span,
 137                     "non-binding `let` on an expression with `#[must_use]` type",
 138                     None,
 139                     "consider explicitly using expression value",
 140                 );
 141             } else if is_must_use_func_call(cx, init) {
 142                 span_lint_and_help(
 143                     cx,
 144                     LET_UNDERSCORE_MUST_USE,
 145                     local.span,
 146                     "non-binding `let` on a result of a `#[must_use]` function",
 147                     None,
 148                     "consider explicitly using function result",
 149                 );
 150             }
 151         }
 152     }
 153 }