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Diffstat (limited to '')
| -rw-r--r-- | compiler/rustc_lint/src/let_underscore.rs | 175 |
1 files changed, 175 insertions, 0 deletions
diff --git a/compiler/rustc_lint/src/let_underscore.rs b/compiler/rustc_lint/src/let_underscore.rs new file mode 100644 index 000000000..7e885e6c5 --- /dev/null +++ b/compiler/rustc_lint/src/let_underscore.rs @@ -0,0 +1,175 @@ +use crate::{LateContext, LateLintPass, LintContext}; +use rustc_errors::{Applicability, LintDiagnosticBuilder, MultiSpan}; +use rustc_hir as hir; +use rustc_middle::ty; +use rustc_span::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 + /// ``` + /// struct SomeStruct; + /// impl Drop for SomeStruct { + /// fn drop(&mut self) { + /// println!("Dropping SomeStruct"); + /// } + /// } + /// + /// fn main() { + /// #[warn(let_underscore_drop)] + /// // SomeStuct 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 + /// ```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 { + fn check_local(&mut self, cx: &LateContext<'_>, local: &hir::Local<'_>) { + if !matches!(local.pat.kind, hir::PatKind::Wild) { + return; + } + if let Some(init) = local.init { + let init_ty = cx.typeck_results().expr_ty(init); + // If the type has a trivial Drop implementation, then it doesn't + // matter that we drop the value immediately. + if !init_ty.needs_drop(cx.tcx, cx.param_env) { + return; + } + let is_sync_lock = match init_ty.kind() { + ty::Adt(adt, _) => SYNC_GUARD_SYMBOLS + .iter() + .any(|guard_symbol| cx.tcx.is_diagnostic_item(*guard_symbol, adt.did())), + _ => false, + }; + + if is_sync_lock { + let mut span = MultiSpan::from_spans(vec![local.pat.span, init.span]); + span.push_span_label( + local.pat.span, + "this lock is not assigned to a binding and is immediately dropped".to_string(), + ); + span.push_span_label( + init.span, + "this binding will immediately drop the value assigned to it".to_string(), + ); + cx.struct_span_lint(LET_UNDERSCORE_LOCK, span, |lint| { + build_and_emit_lint( + lint, + local, + init.span, + "non-binding let on a synchronization lock", + ) + }) + } else { + cx.struct_span_lint(LET_UNDERSCORE_DROP, local.span, |lint| { + build_and_emit_lint( + lint, + local, + init.span, + "non-binding let on a type that implements `Drop`", + ); + }) + } + } + } +} + +fn build_and_emit_lint( + lint: LintDiagnosticBuilder<'_, ()>, + local: &hir::Local<'_>, + init_span: rustc_span::Span, + msg: &str, +) { + lint.build(msg) + .span_suggestion_verbose( + local.pat.span, + "consider binding to an unused variable to avoid immediately dropping the value", + "_unused", + Applicability::MachineApplicable, + ) + .multipart_suggestion( + "consider immediately dropping the value", + vec![ + (local.span.until(init_span), "drop(".to_string()), + (init_span.shrink_to_hi(), ")".to_string()), + ], + Applicability::MachineApplicable, + ) + .emit(); +} |