diff options
Diffstat (limited to 'clippy_lints/src/let_underscore.rs')
| -rw-r--r-- | clippy_lints/src/let_underscore.rs | 157 |
1 files changed, 68 insertions, 89 deletions
diff --git a/clippy_lints/src/let_underscore.rs b/clippy_lints/src/let_underscore.rs index b7798b1c1d7..61f87b91400 100644 --- a/clippy_lints/src/let_underscore.rs +++ b/clippy_lints/src/let_underscore.rs @@ -1,13 +1,11 @@ use clippy_utils::diagnostics::span_lint_and_help; -use clippy_utils::ty::{is_must_use_ty, is_type_diagnostic_item, match_type}; +use clippy_utils::ty::{implements_trait, is_must_use_ty, match_type}; use clippy_utils::{is_must_use_func_call, paths}; -use if_chain::if_chain; use rustc_hir::{Local, PatKind}; use rustc_lint::{LateContext, LateLintPass}; use rustc_middle::lint::in_external_macro; use rustc_middle::ty::subst::GenericArgKind; use rustc_session::{declare_lint_pass, declare_tool_lint}; -use rustc_span::{sym, Symbol}; declare_clippy_lint! { /// ### What it does @@ -30,13 +28,14 @@ declare_clippy_lint! { #[clippy::version = "1.42.0"] pub LET_UNDERSCORE_MUST_USE, restriction, - "non-binding let on a `#[must_use]` expression" + "non-binding `let` on a `#[must_use]` expression" } declare_clippy_lint! { /// ### What it does - /// Checks for `let _ = sync_lock`. - /// This supports `mutex` and `rwlock` in `std::sync` and `parking_lot`. + /// Checks for `let _ = sync_lock`. This supports `mutex` and `rwlock` in + /// `parking_lot`. For `std` locks see the `rustc` lint + /// [`let_underscore_lock`](https://doc.rust-lang.org/nightly/rustc/lints/listing/deny-by-default.html#let-underscore-lock) /// /// ### Why is this bad? /// This statement immediately drops the lock instead of @@ -57,50 +56,41 @@ declare_clippy_lint! { #[clippy::version = "1.43.0"] pub LET_UNDERSCORE_LOCK, correctness, - "non-binding let on a synchronization lock" + "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` + /// Checks for `let _ = <expr>` where the resulting type of expr implements `Future` /// /// ### 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. + /// Futures must be polled for work to be done. The original intention was most likely to await the future + /// and ignore the resulting value. /// /// ### Example /// ```rust - /// # struct DroppableItem; - /// { - /// let _ = DroppableItem; - /// // ^ dropped here - /// /* more code */ + /// async fn foo() -> Result<(), ()> { + /// Ok(()) /// } + /// let _ = foo(); /// ``` /// /// Use instead: /// ```rust - /// # struct DroppableItem; - /// { - /// let _droppable = DroppableItem; - /// /* more code */ - /// // dropped at end of scope + /// # async fn context() { + /// async fn foo() -> Result<(), ()> { + /// Ok(()) /// } + /// let _ = foo().await; + /// # } /// ``` - #[clippy::version = "1.50.0"] - pub LET_UNDERSCORE_DROP, - pedantic, - "non-binding let on a type that implements `Drop`" + #[clippy::version = "1.66"] + pub LET_UNDERSCORE_FUTURE, + suspicious, + "non-binding `let` on a future" } -declare_lint_pass!(LetUnderscore => [LET_UNDERSCORE_MUST_USE, LET_UNDERSCORE_LOCK, LET_UNDERSCORE_DROP]); - -const SYNC_GUARD_SYMS: [Symbol; 3] = [sym::MutexGuard, sym::RwLockReadGuard, sym::RwLockWriteGuard]; +declare_lint_pass!(LetUnderscore => [LET_UNDERSCORE_MUST_USE, LET_UNDERSCORE_LOCK, LET_UNDERSCORE_FUTURE]); const SYNC_GUARD_PATHS: [&[&str]; 3] = [ &paths::PARKING_LOT_MUTEX_GUARD, @@ -110,64 +100,53 @@ const SYNC_GUARD_PATHS: [&[&str]; 3] = [ impl<'tcx> LateLintPass<'tcx> for LetUnderscore { fn check_local(&mut self, cx: &LateContext<'_>, local: &Local<'_>) { - 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 { - let init_ty = cx.typeck_results().expr_ty(init); - let contains_sync_guard = init_ty.walk().any(|inner| match inner.unpack() { - GenericArgKind::Type(inner_ty) => { - SYNC_GUARD_SYMS - .iter() - .any(|&sym| is_type_diagnostic_item(cx, inner_ty, sym)) - || 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, - 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 \ + if !in_external_macro(cx.tcx.sess, local.span) + && let PatKind::Wild = local.pat.kind + && let Some(init) = local.init + { + let init_ty = cx.typeck_results().expr_ty(init); + 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, + 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 is_must_use_ty(cx, cx.typeck_results().expr_ty(init)) { - span_lint_and_help( - cx, - LET_UNDERSCORE_MUST_USE, - 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, - local.span, - "non-binding let on a result of a `#[must_use]` function", - None, - "consider explicitly using function result", - ); - } + ); + } else if let Some(future_trait_def_id) = cx.tcx.lang_items().future_trait() + && implements_trait(cx, cx.typeck_results().expr_ty(init), future_trait_def_id, &[]) { + span_lint_and_help( + cx, + LET_UNDERSCORE_FUTURE, + local.span, + "non-binding `let` on a future", + None, + "consider awaiting the future or dropping explicitly with `std::mem::drop`" + ); + } else if is_must_use_ty(cx, cx.typeck_results().expr_ty(init)) { + span_lint_and_help( + cx, + LET_UNDERSCORE_MUST_USE, + 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, + local.span, + "non-binding `let` on a result of a `#[must_use]` function", + None, + "consider explicitly using function result", + ); } } } |