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| 1 | +use crate::{LateContext, LateLintPass, LintContext}; |
| 2 | +use rustc_errors::{Applicability, LintDiagnosticBuilder, MultiSpan}; |
| 3 | +use rustc_hir as hir; |
| 4 | +use rustc_middle::ty; |
| 5 | +use rustc_span::Symbol; |
| 6 | + |
| 7 | +declare_lint! { |
| 8 | + /// The `let_underscore_drop` lint checks for statements which don't bind |
| 9 | + /// an expression which has a non-trivial Drop implementation to anything, |
| 10 | + /// causing the expression to be dropped immediately instead of at end of |
| 11 | + /// scope. |
| 12 | + /// |
| 13 | + /// ### Example |
| 14 | + /// ``` |
| 15 | + /// struct SomeStruct; |
| 16 | + /// impl Drop for SomeStruct { |
| 17 | + /// fn drop(&mut self) { |
| 18 | + /// println!("Dropping SomeStruct"); |
| 19 | + /// } |
| 20 | + /// } |
| 21 | + /// |
| 22 | + /// fn main() { |
| 23 | + /// #[warn(let_underscore_drop)] |
| 24 | + /// // SomeStuct is dropped immediately instead of at end of scope, |
| 25 | + /// // so "Dropping SomeStruct" is printed before "end of main". |
| 26 | + /// // The order of prints would be reversed if SomeStruct was bound to |
| 27 | + /// // a name (such as "_foo"). |
| 28 | + /// let _ = SomeStruct; |
| 29 | + /// println!("end of main"); |
| 30 | + /// } |
| 31 | + /// ``` |
| 32 | + /// |
| 33 | + /// {{produces}} |
| 34 | + /// |
| 35 | + /// ### Explanation |
| 36 | + /// |
| 37 | + /// Statements which assign an expression to an underscore causes the |
| 38 | + /// expression to immediately drop instead of extending the expression's |
| 39 | + /// lifetime to the end of the scope. This is usually unintended, |
| 40 | + /// especially for types like `MutexGuard`, which are typically used to |
| 41 | + /// lock a mutex for the duration of an entire scope. |
| 42 | + /// |
| 43 | + /// If you want to extend the expression's lifetime to the end of the scope, |
| 44 | + /// assign an underscore-prefixed name (such as `_foo`) to the expression. |
| 45 | + /// If you do actually want to drop the expression immediately, then |
| 46 | + /// calling `std::mem::drop` on the expression is clearer and helps convey |
| 47 | + /// intent. |
| 48 | + pub LET_UNDERSCORE_DROP, |
| 49 | + Allow, |
| 50 | + "non-binding let on a type that implements `Drop`" |
| 51 | +} |
| 52 | + |
| 53 | +declare_lint! { |
| 54 | + /// The `let_underscore_lock` lint checks for statements which don't bind |
| 55 | + /// a mutex to anything, causing the lock to be released immediately instead |
| 56 | + /// of at end of scope, which is typically incorrect. |
| 57 | + /// |
| 58 | + /// ### Example |
| 59 | + /// ```compile_fail |
| 60 | + /// use std::sync::{Arc, Mutex}; |
| 61 | + /// use std::thread; |
| 62 | + /// let data = Arc::new(Mutex::new(0)); |
| 63 | + /// |
| 64 | + /// thread::spawn(move || { |
| 65 | + /// // The lock is immediately released instead of at the end of the |
| 66 | + /// // scope, which is probably not intended. |
| 67 | + /// let _ = data.lock().unwrap(); |
| 68 | + /// println!("doing some work"); |
| 69 | + /// let mut lock = data.lock().unwrap(); |
| 70 | + /// *lock += 1; |
| 71 | + /// }); |
| 72 | + /// ``` |
| 73 | + /// |
| 74 | + /// {{produces}} |
| 75 | + /// |
| 76 | + /// ### Explanation |
| 77 | + /// |
| 78 | + /// Statements which assign an expression to an underscore causes the |
| 79 | + /// expression to immediately drop instead of extending the expression's |
| 80 | + /// lifetime to the end of the scope. This is usually unintended, |
| 81 | + /// especially for types like `MutexGuard`, which are typically used to |
| 82 | + /// lock a mutex for the duration of an entire scope. |
| 83 | + /// |
| 84 | + /// If you want to extend the expression's lifetime to the end of the scope, |
| 85 | + /// assign an underscore-prefixed name (such as `_foo`) to the expression. |
| 86 | + /// If you do actually want to drop the expression immediately, then |
| 87 | + /// calling `std::mem::drop` on the expression is clearer and helps convey |
| 88 | + /// intent. |
| 89 | + pub LET_UNDERSCORE_LOCK, |
| 90 | + Deny, |
| 91 | + "non-binding let on a synchronization lock" |
| 92 | +} |
| 93 | + |
| 94 | +declare_lint_pass!(LetUnderscore => [LET_UNDERSCORE_DROP, LET_UNDERSCORE_LOCK]); |
| 95 | + |
| 96 | +const SYNC_GUARD_SYMBOLS: [Symbol; 3] = [ |
| 97 | + rustc_span::sym::MutexGuard, |
| 98 | + rustc_span::sym::RwLockReadGuard, |
| 99 | + rustc_span::sym::RwLockWriteGuard, |
| 100 | +]; |
| 101 | + |
| 102 | +impl<'tcx> LateLintPass<'tcx> for LetUnderscore { |
| 103 | + fn check_local(&mut self, cx: &LateContext<'_>, local: &hir::Local<'_>) { |
| 104 | + if !matches!(local.pat.kind, hir::PatKind::Wild) { |
| 105 | + return; |
| 106 | + } |
| 107 | + if let Some(init) = local.init { |
| 108 | + let init_ty = cx.typeck_results().expr_ty(init); |
| 109 | + // If the type has a trivial Drop implementation, then it doesn't |
| 110 | + // matter that we drop the value immediately. |
| 111 | + if !init_ty.needs_drop(cx.tcx, cx.param_env) { |
| 112 | + return; |
| 113 | + } |
| 114 | + let is_sync_lock = match init_ty.kind() { |
| 115 | + ty::Adt(adt, _) => SYNC_GUARD_SYMBOLS |
| 116 | + .iter() |
| 117 | + .any(|guard_symbol| cx.tcx.is_diagnostic_item(*guard_symbol, adt.did())), |
| 118 | + _ => false, |
| 119 | + }; |
| 120 | + |
| 121 | + if is_sync_lock { |
| 122 | + let mut span = MultiSpan::from_spans(vec![local.pat.span, init.span]); |
| 123 | + span.push_span_label( |
| 124 | + local.pat.span, |
| 125 | + "this lock is not assigned to a binding and is immediately dropped".to_string(), |
| 126 | + ); |
| 127 | + span.push_span_label( |
| 128 | + init.span, |
| 129 | + "this binding will immediately drop the value assigned to it".to_string(), |
| 130 | + ); |
| 131 | + cx.struct_span_lint(LET_UNDERSCORE_LOCK, span, |lint| { |
| 132 | + build_and_emit_lint( |
| 133 | + lint, |
| 134 | + local, |
| 135 | + init.span, |
| 136 | + "non-binding let on a synchronization lock", |
| 137 | + ) |
| 138 | + }) |
| 139 | + } else { |
| 140 | + cx.struct_span_lint(LET_UNDERSCORE_DROP, local.span, |lint| { |
| 141 | + build_and_emit_lint( |
| 142 | + lint, |
| 143 | + local, |
| 144 | + init.span, |
| 145 | + "non-binding let on a type that implements `Drop`", |
| 146 | + ); |
| 147 | + }) |
| 148 | + } |
| 149 | + } |
| 150 | + } |
| 151 | +} |
| 152 | + |
| 153 | +fn build_and_emit_lint( |
| 154 | + lint: LintDiagnosticBuilder<'_, ()>, |
| 155 | + local: &hir::Local<'_>, |
| 156 | + init_span: rustc_span::Span, |
| 157 | + msg: &str, |
| 158 | +) { |
| 159 | + lint.build(msg) |
| 160 | + .span_suggestion_verbose( |
| 161 | + local.pat.span, |
| 162 | + "consider binding to an unused variable to avoid immediately dropping the value", |
| 163 | + "_unused", |
| 164 | + Applicability::MachineApplicable, |
| 165 | + ) |
| 166 | + .multipart_suggestion( |
| 167 | + "consider immediately dropping the value", |
| 168 | + vec![ |
| 169 | + (local.span.until(init_span), "drop(".to_string()), |
| 170 | + (init_span.shrink_to_hi(), ")".to_string()), |
| 171 | + ], |
| 172 | + Applicability::MachineApplicable, |
| 173 | + ) |
| 174 | + .emit(); |
| 175 | +} |
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