70 lines
2.7 KiB
Rust
70 lines
2.7 KiB
Rust
use crate::utils::{match_def_path, paths, span_lint};
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use if_chain::if_chain;
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use rustc_hir::{GenericBound, GenericParam, WhereBoundPredicate, WherePredicate};
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use rustc_lint::LateLintPass;
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use rustc_session::{declare_lint_pass, declare_tool_lint};
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declare_clippy_lint! {
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/// **What it does:** Checks for generics with `std::ops::Drop` as bounds.
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///
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/// **Why is this bad?** `Drop` bounds do not really accomplish anything.
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/// A type may have compiler-generated drop glue without implementing the
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/// `Drop` trait itself. The `Drop` trait also only has one method,
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/// `Drop::drop`, and that function is by fiat not callable in user code.
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/// So there is really no use case for using `Drop` in trait bounds.
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///
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/// The most likely use case of a drop bound is to distinguish between types
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/// that have destructors and types that don't. Combined with specialization,
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/// a naive coder would write an implementation that assumed a type could be
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/// trivially dropped, then write a specialization for `T: Drop` that actually
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/// calls the destructor. Except that doing so is not correct; String, for
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/// example, doesn't actually implement Drop, but because String contains a
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/// Vec, assuming it can be trivially dropped will leak memory.
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///
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/// **Known problems:** None.
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///
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/// **Example:**
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/// ```rust
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/// fn foo<T: Drop>() {}
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/// ```
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pub DROP_BOUNDS,
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correctness,
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"Bounds of the form `T: Drop` are useless"
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}
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const DROP_BOUNDS_SUMMARY: &str = "Bounds of the form `T: Drop` are useless. \
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Use `std::mem::needs_drop` to detect if a type has drop glue.";
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declare_lint_pass!(DropBounds => [DROP_BOUNDS]);
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impl<'a, 'tcx> LateLintPass<'a, 'tcx> for DropBounds {
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fn check_generic_param(&mut self, cx: &rustc_lint::LateContext<'a, 'tcx>, p: &'tcx GenericParam<'_>) {
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for bound in p.bounds.iter() {
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lint_bound(cx, bound);
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}
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}
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fn check_where_predicate(&mut self, cx: &rustc_lint::LateContext<'a, 'tcx>, p: &'tcx WherePredicate<'_>) {
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if let WherePredicate::BoundPredicate(WhereBoundPredicate { bounds, .. }) = p {
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for bound in *bounds {
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lint_bound(cx, bound);
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}
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}
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}
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}
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fn lint_bound<'a, 'tcx>(cx: &rustc_lint::LateContext<'a, 'tcx>, bound: &'tcx GenericBound<'_>) {
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if_chain! {
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if let GenericBound::Trait(t, _) = bound;
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if let Some(def_id) = t.trait_ref.path.res.opt_def_id();
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if match_def_path(cx, def_id, &paths::DROP_TRAIT);
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then {
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span_lint(
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cx,
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DROP_BOUNDS,
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t.span,
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DROP_BOUNDS_SUMMARY
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);
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}
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}
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}
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