rust-clippy/clippy_lints/src/let_underscore.rs

171 lines
5.9 KiB
Rust
Raw Normal View History

use clippy_utils::diagnostics::span_lint_and_help;
use clippy_utils::ty::{is_must_use_ty, match_type};
use clippy_utils::{is_must_use_func_call, paths};
use if_chain::if_chain;
2020-04-01 19:18:16 +00:00
use rustc_hir::{Local, PatKind};
2020-01-12 06:08:41 +00:00
use rustc_lint::{LateContext, LateLintPass};
use rustc_middle::lint::in_external_macro;
use rustc_middle::ty::subst::GenericArgKind;
2020-01-11 11:37:08 +00:00
use rustc_session::{declare_lint_pass, declare_tool_lint};
declare_clippy_lint! {
/// ### What it does
/// Checks for `let _ = <expr>` where expr is `#[must_use]`
///
/// ### Why is this bad?
/// It's better to explicitly handle the value of a `#[must_use]`
/// expr
///
/// ### Example
/// ```rust
/// fn f() -> Result<u32, u32> {
/// Ok(0)
/// }
///
/// let _ = f();
/// // is_ok() is marked #[must_use]
/// let _ = f().is_ok();
/// ```
#[clippy::version = "1.42.0"]
pub LET_UNDERSCORE_MUST_USE,
restriction,
2020-01-06 06:30:43 +00:00
"non-binding let on a `#[must_use]` expression"
}
2020-01-27 14:34:30 +00:00
declare_clippy_lint! {
/// ### What it does
/// Checks for `let _ = sync_lock`.
/// This supports `mutex` and `rwlock` in `std::sync` and `parking_lot`.
2020-01-27 14:34:30 +00:00
///
/// ### Why is this bad?
/// This statement immediately drops the lock instead of
/// extending its lifetime to the end of the scope, which is often not intended.
/// To extend lock lifetime to the end of the scope, use an underscore-prefixed
/// name instead (i.e. _lock). If you want to explicitly drop the lock,
/// `std::mem::drop` conveys your intention better and is less error-prone.
2020-01-27 14:34:30 +00:00
///
/// ### Example
2020-01-27 14:34:30 +00:00
/// ```rust,ignore
/// let _ = mutex.lock();
/// ```
///
2022-06-05 19:24:41 +00:00
/// Use instead:
2020-01-27 14:34:30 +00:00
/// ```rust,ignore
/// let _lock = mutex.lock();
/// ```
#[clippy::version = "1.43.0"]
2020-01-27 14:34:30 +00:00
pub LET_UNDERSCORE_LOCK,
correctness,
"non-binding let on a synchronization lock"
}
declare_clippy_lint! {
/// ### What it does
/// Checks for `let _ = <expr>`
/// where expr has a type that implements `Drop`
///
/// ### Why is this bad?
/// This statement immediately drops the initializer
/// expression instead of extending its lifetime to the end of the scope, which
/// is often not intended. To extend the expression's lifetime to the end of the
/// scope, use an underscore-prefixed name instead (i.e. _var). If you want to
/// explicitly drop the expression, `std::mem::drop` conveys your intention
/// better and is less error-prone.
///
/// ### Example
2022-06-05 19:24:41 +00:00
/// ```rust
/// # struct DroppableItem;
/// {
2022-06-05 19:24:41 +00:00
/// let _ = DroppableItem;
/// // ^ dropped here
/// /* more code */
/// }
/// ```
///
2022-06-05 19:24:41 +00:00
/// Use instead:
/// ```rust
/// # struct DroppableItem;
/// {
2022-06-05 19:24:41 +00:00
/// let _droppable = DroppableItem;
/// /* more code */
/// // dropped at end of scope
/// }
/// ```
#[clippy::version = "1.50.0"]
pub LET_UNDERSCORE_DROP,
pedantic,
"non-binding let on a type that implements `Drop`"
}
declare_lint_pass!(LetUnderscore => [LET_UNDERSCORE_MUST_USE, LET_UNDERSCORE_LOCK, LET_UNDERSCORE_DROP]);
2020-01-27 14:34:30 +00:00
const SYNC_GUARD_PATHS: [&[&str]; 5] = [
&paths::MUTEX_GUARD,
&paths::RWLOCK_READ_GUARD,
&paths::RWLOCK_WRITE_GUARD,
&paths::PARKING_LOT_RAWMUTEX,
&paths::PARKING_LOT_RAWRWLOCK,
];
impl<'tcx> LateLintPass<'tcx> for LetUnderscore {
fn check_local(&mut self, cx: &LateContext<'_>, local: &Local<'_>) {
2020-04-01 19:18:16 +00:00
if in_external_macro(cx.tcx.sess, local.span) {
return;
}
if_chain! {
if let PatKind::Wild = local.pat.kind;
if let Some(init) = local.init;
then {
2020-07-17 08:47:04 +00:00
let init_ty = cx.typeck_results().expr_ty(init);
2022-01-12 03:19:52 +00:00
let contains_sync_guard = init_ty.walk().any(|inner| match inner.unpack() {
GenericArgKind::Type(inner_ty) => {
SYNC_GUARD_PATHS.iter().any(|path| match_type(cx, inner_ty, path))
},
GenericArgKind::Lifetime(_) | GenericArgKind::Const(_) => false,
});
if contains_sync_guard {
span_lint_and_help(
cx,
LET_UNDERSCORE_LOCK,
2020-04-01 19:18:16 +00:00
local.span,
"non-binding let on a synchronization lock",
None,
"consider using an underscore-prefixed named \
binding or dropping explicitly with `std::mem::drop`"
);
} else if init_ty.needs_drop(cx.tcx, cx.param_env) {
span_lint_and_help(
cx,
LET_UNDERSCORE_DROP,
local.span,
"non-binding `let` on a type that implements `Drop`",
None,
"consider using an underscore-prefixed named \
binding or dropping explicitly with `std::mem::drop`"
);
2020-07-17 08:47:04 +00:00
} else if is_must_use_ty(cx, cx.typeck_results().expr_ty(init)) {
span_lint_and_help(
cx,
LET_UNDERSCORE_MUST_USE,
2020-04-01 19:18:16 +00:00
local.span,
"non-binding let on an expression with `#[must_use]` type",
None,
"consider explicitly using expression value"
);
} else if is_must_use_func_call(cx, init) {
span_lint_and_help(
cx,
LET_UNDERSCORE_MUST_USE,
2020-04-01 19:18:16 +00:00
local.span,
"non-binding let on a result of a `#[must_use]` function",
None,
"consider explicitly using function result"
);
}
}
}
}
}