| use crate::{ |
| lints::{NonBindingLet, NonBindingLetSub}, |
| LateContext, LateLintPass, LintContext, |
| }; |
| use rustc_errors::MultiSpan; |
| use rustc_hir as hir; |
| use rustc_middle::ty; |
| use rustc_session::{declare_lint, declare_lint_pass}; |
| use rustc_span::{sym, Symbol}; |
| |
| declare_lint! { |
| /// The `let_underscore_drop` lint checks for statements which don't bind |
| /// an expression which has a non-trivial Drop implementation to anything, |
| /// causing the expression to be dropped immediately instead of at end of |
| /// scope. |
| /// |
| /// ### Example |
| /// |
| /// ```rust |
| /// struct SomeStruct; |
| /// impl Drop for SomeStruct { |
| /// fn drop(&mut self) { |
| /// println!("Dropping SomeStruct"); |
| /// } |
| /// } |
| /// |
| /// fn main() { |
| /// #[warn(let_underscore_drop)] |
| /// // SomeStruct is dropped immediately instead of at end of scope, |
| /// // so "Dropping SomeStruct" is printed before "end of main". |
| /// // The order of prints would be reversed if SomeStruct was bound to |
| /// // a name (such as "_foo"). |
| /// let _ = SomeStruct; |
| /// println!("end of main"); |
| /// } |
| /// ``` |
| /// |
| /// {{produces}} |
| /// |
| /// ### Explanation |
| /// |
| /// Statements which assign an expression to an underscore causes the |
| /// expression to immediately drop instead of extending the expression's |
| /// lifetime to the end of the scope. This is usually unintended, |
| /// especially for types like `MutexGuard`, which are typically used to |
| /// lock a mutex for the duration of an entire scope. |
| /// |
| /// If you want to extend the expression's lifetime to the end of the scope, |
| /// assign an underscore-prefixed name (such as `_foo`) to the expression. |
| /// If you do actually want to drop the expression immediately, then |
| /// calling `std::mem::drop` on the expression is clearer and helps convey |
| /// intent. |
| pub LET_UNDERSCORE_DROP, |
| Allow, |
| "non-binding let on a type that implements `Drop`" |
| } |
| |
| declare_lint! { |
| /// The `let_underscore_lock` lint checks for statements which don't bind |
| /// a mutex to anything, causing the lock to be released immediately instead |
| /// of at end of scope, which is typically incorrect. |
| /// |
| /// ### Example |
| /// ```rust,compile_fail |
| /// use std::sync::{Arc, Mutex}; |
| /// use std::thread; |
| /// let data = Arc::new(Mutex::new(0)); |
| /// |
| /// thread::spawn(move || { |
| /// // The lock is immediately released instead of at the end of the |
| /// // scope, which is probably not intended. |
| /// let _ = data.lock().unwrap(); |
| /// println!("doing some work"); |
| /// let mut lock = data.lock().unwrap(); |
| /// *lock += 1; |
| /// }); |
| /// ``` |
| /// |
| /// {{produces}} |
| /// |
| /// ### Explanation |
| /// |
| /// Statements which assign an expression to an underscore causes the |
| /// expression to immediately drop instead of extending the expression's |
| /// lifetime to the end of the scope. This is usually unintended, |
| /// especially for types like `MutexGuard`, which are typically used to |
| /// lock a mutex for the duration of an entire scope. |
| /// |
| /// If you want to extend the expression's lifetime to the end of the scope, |
| /// assign an underscore-prefixed name (such as `_foo`) to the expression. |
| /// If you do actually want to drop the expression immediately, then |
| /// calling `std::mem::drop` on the expression is clearer and helps convey |
| /// intent. |
| pub LET_UNDERSCORE_LOCK, |
| Deny, |
| "non-binding let on a synchronization lock" |
| } |
| |
| declare_lint_pass!(LetUnderscore => [LET_UNDERSCORE_DROP, LET_UNDERSCORE_LOCK]); |
| |
| const SYNC_GUARD_SYMBOLS: [Symbol; 3] = [ |
| rustc_span::sym::MutexGuard, |
| rustc_span::sym::RwLockReadGuard, |
| rustc_span::sym::RwLockWriteGuard, |
| ]; |
| |
| impl<'tcx> LateLintPass<'tcx> for LetUnderscore { |
| #[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable |
| fn check_local(&mut self, cx: &LateContext<'_>, local: &hir::LetStmt<'_>) { |
| if matches!(local.source, rustc_hir::LocalSource::AsyncFn) { |
| return; |
| } |
| |
| let mut top_level = true; |
| |
| // We recursively walk through all patterns, so that we can catch cases where the lock is |
| // nested in a pattern. For the basic `let_underscore_drop` lint, we only look at the top |
| // level, since there are many legitimate reasons to bind a sub-pattern to an `_`, if we're |
| // only interested in the rest. But with locks, we prefer having the chance of "false |
| // positives" over missing cases, since the effects can be quite catastrophic. |
| local.pat.walk_always(|pat| { |
| let is_top_level = top_level; |
| top_level = false; |
| |
| if !matches!(pat.kind, hir::PatKind::Wild) { |
| return; |
| } |
| |
| let ty = cx.typeck_results().pat_ty(pat); |
| |
| // If the type has a trivial Drop implementation, then it doesn't |
| // matter that we drop the value immediately. |
| if !ty.needs_drop(cx.tcx, cx.param_env) { |
| return; |
| } |
| // Lint for patterns like `mutex.lock()`, which returns `Result<MutexGuard, _>` as well. |
| let potential_lock_type = match ty.kind() { |
| ty::Adt(adt, args) if cx.tcx.is_diagnostic_item(sym::Result, adt.did()) => { |
| args.type_at(0) |
| } |
| _ => ty, |
| }; |
| let is_sync_lock = match potential_lock_type.kind() { |
| ty::Adt(adt, _) => SYNC_GUARD_SYMBOLS |
| .iter() |
| .any(|guard_symbol| cx.tcx.is_diagnostic_item(*guard_symbol, adt.did())), |
| _ => false, |
| }; |
| |
| let can_use_init = is_top_level.then_some(local.init).flatten(); |
| |
| let sub = NonBindingLetSub { |
| suggestion: pat.span, |
| // We can't suggest `drop()` when we're on the top level. |
| drop_fn_start_end: can_use_init |
| .map(|init| (local.span.until(init.span), init.span.shrink_to_hi())), |
| is_assign_desugar: matches!(local.source, rustc_hir::LocalSource::AssignDesugar(_)), |
| }; |
| if is_sync_lock { |
| let mut span = MultiSpan::from_span(pat.span); |
| span.push_span_label( |
| pat.span, |
| "this lock is not assigned to a binding and is immediately dropped".to_string(), |
| ); |
| cx.emit_span_lint(LET_UNDERSCORE_LOCK, span, NonBindingLet::SyncLock { sub }); |
| // Only emit let_underscore_drop for top-level `_` patterns. |
| } else if can_use_init.is_some() { |
| cx.emit_span_lint(LET_UNDERSCORE_DROP, local.span, NonBindingLet::DropType { sub }); |
| } |
| }); |
| } |
| } |