67 lines
2.6 KiB
Rust
67 lines
2.6 KiB
Rust
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use crate::{LateContext, LateLintPass, LintContext};
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use rustc_hir as hir;
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declare_lint! {
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/// The `let_underscore_drop` lint checks for statements which don't bind
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/// an expression which has a non-trivial Drop implementation to anything,
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/// causing the expression to be dropped immediately instead of at end of
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/// scope.
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///
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/// ### Example
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/// ```rust
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/// struct SomeStruct;
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/// impl Drop for SomeStruct {
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/// fn drop(&mut self) {
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/// println!("Dropping SomeStruct");
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/// }
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/// }
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///
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/// fn main() {
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/// // SomeStuct is dropped immediately instead of at end of scope,
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/// // so "Dropping SomeStruct" is printed before "end of main".
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/// // The order of prints would be reversed if SomeStruct was bound to
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/// // a name (such as "_foo").
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/// let _ = SomeStruct;
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/// println!("end of main");
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/// }
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/// ```
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/// ### Explanation
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///
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/// Statements which assign an expression to an underscore causes the
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/// expression to immediately drop instead of extending the expression's
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/// lifetime to the end of the scope. This is usually unintended,
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/// especially for types like `MutexGuard`, which are typically used to
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/// lock a mutex for the duration of an entire scope.
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///
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/// If you want to extend the expression's lifetime to the end of the scope,
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/// assign an underscore-prefixed name (such as `_foo`) to the expression.
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/// If you do actually want to drop the expression immediately, then
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/// calling `std::mem::drop` on the expression is clearer and helps convey
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/// intent.
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pub LET_UNDERSCORE_DROP,
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Warn,
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"non-binding let on a type that implements `Drop`"
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}
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declare_lint_pass!(LetUnderscore => [LET_UNDERSCORE_DROP]);
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impl<'tcx> LateLintPass<'tcx> for LetUnderscore {
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fn check_local(&mut self, cx: &LateContext<'_>, local: &hir::Local<'_>) {
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if !matches!(local.pat.kind, hir::PatKind::Wild) {
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return;
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}
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if let Some(init) = local.init {
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let init_ty = cx.typeck_results().expr_ty(init);
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let needs_drop = init_ty.needs_drop(cx.tcx, cx.param_env);
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if needs_drop {
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cx.struct_span_lint(LET_UNDERSCORE_DROP, local.span, |lint| {
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lint.build("non-binding let on a type that implements `Drop`")
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.help("consider binding to an unused variable")
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.help("consider explicitly droping with `std::mem::drop`")
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.emit();
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})
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}
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}
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}
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}
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