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};
11 declare_clippy_lint! {
13 /// Checks for `let _ = <expr>` where expr is `#[must_use]`
15 /// ### Why is this bad?
16 /// It's better to explicitly handle the value of a `#[must_use]`
21 /// fn f() -> Result<u32, u32> {
26 /// // is_ok() is marked #[must_use]
27 /// let _ = f().is_ok();
29 #[clippy::version = "1.42.0"]
30 pub LET_UNDERSCORE_MUST_USE,
32 "non-binding let on a `#[must_use]` expression"
35 declare_clippy_lint! {
37 /// Checks for `let _ = sync_lock`.
38 /// This supports `mutex` and `rwlock` in `std::sync` and `parking_lot`.
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.
51 /// let _ = mutex.lock();
56 /// let _lock = mutex.lock();
58 #[clippy::version = "1.43.0"]
59 pub LET_UNDERSCORE_LOCK,
61 "non-binding let on a synchronization lock"
64 declare_clippy_lint! {
66 /// Checks for `let _ = <expr>`
67 /// where expr has a type that implements `Drop`
69 /// ### Why is this bad?
70 /// This statement immediately drops the initializer
71 /// expression instead of extending its lifetime to the end of the scope, which
72 /// is often not intended. To extend the expression's lifetime to the end of the
73 /// scope, use an underscore-prefixed name instead (i.e. _var). If you want to
74 /// explicitly drop the expression, `std::mem::drop` conveys your intention
75 /// better and is less error-prone.
82 /// impl Drop for Droppable {
83 /// fn drop(&mut self) {}
86 /// let _ = Droppable;
95 /// let _droppable = Droppable;
97 /// // dropped at end of scope
100 #[clippy::version = "1.50.0"]
101 pub LET_UNDERSCORE_DROP,
103 "non-binding let on a type that implements `Drop`"
106 declare_lint_pass!(LetUnderscore => [LET_UNDERSCORE_MUST_USE, LET_UNDERSCORE_LOCK, LET_UNDERSCORE_DROP]);
108 const SYNC_GUARD_PATHS: [&[&str]; 5] = [
110 &paths::RWLOCK_READ_GUARD,
111 &paths::RWLOCK_WRITE_GUARD,
112 &paths::PARKING_LOT_RAWMUTEX,
113 &paths::PARKING_LOT_RAWRWLOCK,
116 impl<'tcx> LateLintPass<'tcx> for LetUnderscore {
117 fn check_local(&mut self, cx: &LateContext<'_>, local: &Local<'_>) {
118 if in_external_macro(cx.tcx.sess, local.span) {
123 if let PatKind::Wild = local.pat.kind;
124 if let Some(init) = local.init;
126 let init_ty = cx.typeck_results().expr_ty(init);
127 let contains_sync_guard = init_ty.walk().any(|inner| match inner.unpack() {
128 GenericArgKind::Type(inner_ty) => {
129 SYNC_GUARD_PATHS.iter().any(|path| match_type(cx, inner_ty, path))
132 GenericArgKind::Lifetime(_) | GenericArgKind::Const(_) => false,
134 if contains_sync_guard {
139 "non-binding let on a synchronization lock",
141 "consider using an underscore-prefixed named \
142 binding or dropping explicitly with `std::mem::drop`"
144 } else if init_ty.needs_drop(cx.tcx, cx.param_env) {
149 "non-binding `let` on a type that implements `Drop`",
151 "consider using an underscore-prefixed named \
152 binding or dropping explicitly with `std::mem::drop`"
154 } else if is_must_use_ty(cx, cx.typeck_results().expr_ty(init)) {
157 LET_UNDERSCORE_MUST_USE,
159 "non-binding let on an expression with `#[must_use]` type",
161 "consider explicitly using expression value"
163 } else if is_must_use_func_call(cx, init) {
166 LET_UNDERSCORE_MUST_USE,
168 "non-binding let on a result of a `#[must_use]` function",
170 "consider explicitly using function result"