1 use crate::utils::{match_def_path, paths, span_lint};
2 use if_chain::if_chain;
4 use rustc::lint::{LateLintPass, LintArray, LintPass};
5 use rustc::{declare_tool_lint, lint_array};
7 /// **What it does:** Checks for generics with `std::ops::Drop` as bounds.
9 /// **Why is this bad?** `Drop` bounds do not really accomplish anything.
10 /// A type may have compiler-generated drop glue without implementing the
11 /// `Drop` trait itself. The `Drop` trait also only has one method,
12 /// `Drop::drop`, and that function is by fiat not callable in user code.
13 /// So there is really no use case for using `Drop` in trait bounds.
15 /// The most likely use case of a drop bound is to distinguish between types
16 /// that have destructors and types that don't. Combined with specialization,
17 /// a naive coder would write an implementation that assumed a type could be
18 /// trivially dropped, then write a specialization for `T: Drop` that actually
19 /// calls the destructor. Except that doing so is not correct; String, for
20 /// example, doesn't actually implement Drop, but because String contains a
21 /// Vec, assuming it can be trivially dropped will leak memory.
23 /// **Known problems:** None.
27 /// fn foo<T: Drop>() {}
29 declare_clippy_lint! {
32 "Bounds of the form `T: Drop` are useless"
35 const DROP_BOUNDS_SUMMARY: &str = "Bounds of the form `T: Drop` are useless. \
36 Use `std::mem::needs_drop` to detect if a type has drop glue.";
40 impl LintPass for Pass {
41 fn get_lints(&self) -> LintArray {
42 lint_array!(DROP_BOUNDS)
45 fn name(&self) -> &'static str {
50 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Pass {
51 fn check_generic_param(&mut self, cx: &rustc::lint::LateContext<'a, 'tcx>, p: &'tcx GenericParam) {
52 for bound in &p.bounds {
53 lint_bound(cx, bound);
56 fn check_where_predicate(&mut self, cx: &rustc::lint::LateContext<'a, 'tcx>, p: &'tcx WherePredicate) {
57 if let WherePredicate::BoundPredicate(WhereBoundPredicate { bounds, .. }) = p {
59 lint_bound(cx, bound);
65 fn lint_bound<'a, 'tcx>(cx: &rustc::lint::LateContext<'a, 'tcx>, bound: &'tcx GenericBound) {
67 if let GenericBound::Trait(t, _) = bound;
68 if let Some(def_id) = t.trait_ref.path.def.opt_def_id();
69 if match_def_path(cx.tcx, def_id, &paths::DROP_TRAIT);