1 use if_chain::if_chain;
2 use rustc_hir::{Local, PatKind};
3 use rustc_lint::{LateContext, LateLintPass};
4 use rustc_middle::lint::in_external_macro;
5 use rustc_session::{declare_lint_pass, declare_tool_lint};
7 use crate::utils::{is_must_use_func_call, is_must_use_ty, match_type, paths, span_lint_and_help};
10 /// **What it does:** Checks for `let _ = <expr>`
11 /// where expr is #[must_use]
13 /// **Why is this bad?** It's better to explicitly
14 /// handle the value of a #[must_use] expr
16 /// **Known problems:** None.
20 /// fn f() -> Result<u32, u32> {
25 /// // is_ok() is marked #[must_use]
26 /// let _ = f().is_ok();
28 pub LET_UNDERSCORE_MUST_USE,
30 "non-binding let on a `#[must_use]` expression"
33 declare_clippy_lint! {
34 /// **What it does:** Checks for `let _ = sync_lock`
36 /// **Why is this bad?** This statement immediately drops the lock instead of
37 /// extending it's lifetime to the end of the scope, which is often not intended.
38 /// To extend lock lifetime to the end of the scope, use an underscore-prefixed
39 /// name instead (i.e. _lock). If you want to explicitly drop the lock,
40 /// `std::mem::drop` conveys your intention better and is less error-prone.
42 /// **Known problems:** None.
48 /// let _ = mutex.lock();
53 /// let _lock = mutex.lock();
55 pub LET_UNDERSCORE_LOCK,
57 "non-binding let on a synchronization lock"
60 declare_lint_pass!(LetUnderscore => [LET_UNDERSCORE_MUST_USE, LET_UNDERSCORE_LOCK]);
62 const SYNC_GUARD_PATHS: [&[&str]; 3] = [
64 &paths::RWLOCK_READ_GUARD,
65 &paths::RWLOCK_WRITE_GUARD,
68 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for LetUnderscore {
69 fn check_local(&mut self, cx: &LateContext<'_, '_>, local: &Local<'_>) {
70 if in_external_macro(cx.tcx.sess, local.span) {
75 if let PatKind::Wild = local.pat.kind;
76 if let Some(ref init) = local.init;
78 let check_ty = |ty| SYNC_GUARD_PATHS.iter().any(|path| match_type(cx, ty, path));
79 if cx.tables.expr_ty(init).walk().any(check_ty) {
84 "non-binding let on a synchronization lock",
85 "consider using an underscore-prefixed named \
86 binding or dropping explicitly with `std::mem::drop`"
88 } else if is_must_use_ty(cx, cx.tables.expr_ty(init)) {
91 LET_UNDERSCORE_MUST_USE,
93 "non-binding let on an expression with `#[must_use]` type",
94 "consider explicitly using expression value"
96 } else if is_must_use_func_call(cx, init) {
99 LET_UNDERSCORE_MUST_USE,
101 "non-binding let on a result of a `#[must_use]` function",
102 "consider explicitly using function result"