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 pub LET_UNDERSCORE_MUST_USE,
31 "non-binding let on a `#[must_use]` expression"
34 declare_clippy_lint! {
36 /// Checks for `let _ = sync_lock`
38 /// ### Why is this bad?
39 /// This statement immediately drops the lock instead of
40 /// extending its lifetime to the end of the scope, which is often not intended.
41 /// To extend lock lifetime to the end of the scope, use an underscore-prefixed
42 /// name instead (i.e. _lock). If you want to explicitly drop the lock,
43 /// `std::mem::drop` conveys your intention better and is less error-prone.
49 /// let _ = mutex.lock();
54 /// let _lock = mutex.lock();
56 pub LET_UNDERSCORE_LOCK,
58 "non-binding let on a synchronization lock"
61 declare_clippy_lint! {
63 /// Checks for `let _ = <expr>`
64 /// where expr has a type that implements `Drop`
66 /// ### Why is this bad?
67 /// This statement immediately drops the initializer
68 /// expression instead of extending its lifetime to the end of the scope, which
69 /// is often not intended. To extend the expression's lifetime to the end of the
70 /// scope, use an underscore-prefixed name instead (i.e. _var). If you want to
71 /// explicitly drop the expression, `std::mem::drop` conveys your intention
72 /// better and is less error-prone.
79 /// impl Drop for Droppable {
80 /// fn drop(&mut self) {}
83 /// let _ = Droppable;
92 /// let _droppable = Droppable;
94 /// // dropped at end of scope
97 pub LET_UNDERSCORE_DROP,
99 "non-binding let on a type that implements `Drop`"
102 declare_lint_pass!(LetUnderscore => [LET_UNDERSCORE_MUST_USE, LET_UNDERSCORE_LOCK, LET_UNDERSCORE_DROP]);
104 const SYNC_GUARD_PATHS: [&[&str]; 3] = [
106 &paths::RWLOCK_READ_GUARD,
107 &paths::RWLOCK_WRITE_GUARD,
110 impl<'tcx> LateLintPass<'tcx> for LetUnderscore {
111 fn check_local(&mut self, cx: &LateContext<'_>, local: &Local<'_>) {
112 if in_external_macro(cx.tcx.sess, local.span) {
117 if let PatKind::Wild = local.pat.kind;
118 if let Some(init) = local.init;
120 let init_ty = cx.typeck_results().expr_ty(init);
121 let contains_sync_guard = init_ty.walk(cx.tcx).any(|inner| match inner.unpack() {
122 GenericArgKind::Type(inner_ty) => {
123 SYNC_GUARD_PATHS.iter().any(|path| match_type(cx, inner_ty, path))
126 GenericArgKind::Lifetime(_) | GenericArgKind::Const(_) => false,
128 if contains_sync_guard {
133 "non-binding let on a synchronization lock",
135 "consider using an underscore-prefixed named \
136 binding or dropping explicitly with `std::mem::drop`"
138 } else if init_ty.needs_drop(cx.tcx, cx.param_env) {
143 "non-binding `let` on a type that implements `Drop`",
145 "consider using an underscore-prefixed named \
146 binding or dropping explicitly with `std::mem::drop`"
148 } else if is_must_use_ty(cx, cx.typeck_results().expr_ty(init)) {
151 LET_UNDERSCORE_MUST_USE,
153 "non-binding let on an expression with `#[must_use]` type",
155 "consider explicitly using expression value"
157 } else if is_must_use_func_call(cx, init) {
160 LET_UNDERSCORE_MUST_USE,
162 "non-binding let on a result of a `#[must_use]` function",
164 "consider explicitly using function result"