1 files changed, 134 insertions, 0 deletions
diff --git a/compiler/rustc_lint/src/traits.rs b/compiler/rustc_lint/src/traits.rs
new file mode 100644
index 000000000..df1587c59
--- /dev/null
+++ b/compiler/rustc_lint/src/traits.rs
@@ -0,0 +1,134 @@
+use crate::LateContext;
+use crate::LateLintPass;
+use crate::LintContext;
+use rustc_errors::fluent;
+use rustc_hir as hir;
+use rustc_span::symbol::sym;
+
+declare_lint! {
+    /// The `drop_bounds` lint checks for generics with `std::ops::Drop` as
+    /// bounds.
+    ///
+    /// ### Example
+    ///
+    /// ```rust
+    /// fn foo<T: Drop>() {}
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// A generic trait bound of the form `T: Drop` is most likely misleading
+    /// and not what the programmer intended (they probably should have used
+    /// `std::mem::needs_drop` instead).
+    ///
+    /// `Drop` bounds do not actually indicate whether a type can be trivially
+    /// dropped or not, because a composite type containing `Drop` types does
+    /// not necessarily implement `Drop` itself. Naïvely, one might be tempted
+    /// to write an implementation that assumes that a type can be trivially
+    /// dropped while also supplying a specialization for `T: Drop` that
+    /// actually calls the destructor. However, this breaks down e.g. when `T`
+    /// is `String`, which does not implement `Drop` itself but contains a
+    /// `Vec`, which does implement `Drop`, so assuming `T` can be trivially
+    /// dropped would lead to a memory leak here.
+    ///
+    /// Furthermore, the `Drop` trait only contains one method, `Drop::drop`,
+    /// which may not be called explicitly in user code (`E0040`), so there is
+    /// really no use case for using `Drop` in trait bounds, save perhaps for
+    /// some obscure corner cases, which can use `#[allow(drop_bounds)]`.
+    pub DROP_BOUNDS,
+    Warn,
+    "bounds of the form `T: Drop` are most likely incorrect"
+}
+
+declare_lint! {
+    /// The `dyn_drop` lint checks for trait objects with `std::ops::Drop`.
+    ///
+    /// ### Example
+    ///
+    /// ```rust
+    /// fn foo(_x: Box<dyn Drop>) {}
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// A trait object bound of the form `dyn Drop` is most likely misleading
+    /// and not what the programmer intended.
+    ///
+    /// `Drop` bounds do not actually indicate whether a type can be trivially
+    /// dropped or not, because a composite type containing `Drop` types does
+    /// not necessarily implement `Drop` itself. Naïvely, one might be tempted
+    /// to write a deferred drop system, to pull cleaning up memory out of a
+    /// latency-sensitive code path, using `dyn Drop` trait objects. However,
+    /// this breaks down e.g. when `T` is `String`, which does not implement
+    /// `Drop`, but should probably be accepted.
+    ///
+    /// To write a trait object bound that accepts anything, use a placeholder
+    /// trait with a blanket implementation.
+    ///
+    /// ```rust
+    /// trait Placeholder {}
+    /// impl<T> Placeholder for T {}
+    /// fn foo(_x: Box<dyn Placeholder>) {}
+    /// ```
+    pub DYN_DROP,
+    Warn,
+    "trait objects of the form `dyn Drop` are useless"
+}
+
+declare_lint_pass!(
+    /// Lint for bounds of the form `T: Drop`, which usually
+    /// indicate an attempt to emulate `std::mem::needs_drop`.
+    DropTraitConstraints => [DROP_BOUNDS, DYN_DROP]
+);
+
+impl<'tcx> LateLintPass<'tcx> for DropTraitConstraints {
+    fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx hir::Item<'tcx>) {
+        use rustc_middle::ty::PredicateKind::*;
+
+        let predicates = cx.tcx.explicit_predicates_of(item.def_id);
+        for &(predicate, span) in predicates.predicates {
+            let Trait(trait_predicate) = predicate.kind().skip_binder() else {
+                continue
+            };
+            let def_id = trait_predicate.trait_ref.def_id;
+            if cx.tcx.lang_items().drop_trait() == Some(def_id) {
+                // Explicitly allow `impl Drop`, a drop-guards-as-Voldemort-type pattern.
+                if trait_predicate.trait_ref.self_ty().is_impl_trait() {
+                    continue;
+                }
+                cx.struct_span_lint(DROP_BOUNDS, span, |lint| {
+                    let Some(needs_drop) = cx.tcx.get_diagnostic_item(sym::needs_drop) else {
+                        return
+                    };
+                    lint.build(fluent::lint::drop_trait_constraints)
+                        .set_arg("predicate", predicate)
+                        .set_arg("needs_drop", cx.tcx.def_path_str(needs_drop))
+                        .emit();
+                });
+            }
+        }
+    }
+
+    fn check_ty(&mut self, cx: &LateContext<'_>, ty: &'tcx hir::Ty<'tcx>) {
+        let hir::TyKind::TraitObject(bounds, _lifetime, _syntax) = &ty.kind else {
+            return
+        };
+        for bound in &bounds[..] {
+            let def_id = bound.trait_ref.trait_def_id();
+            if cx.tcx.lang_items().drop_trait() == def_id {
+                cx.struct_span_lint(DYN_DROP, bound.span, |lint| {
+                    let Some(needs_drop) = cx.tcx.get_diagnostic_item(sym::needs_drop) else {
+                        return
+                    };
+                    lint.build(fluent::lint::drop_glue)
+                        .set_arg("needs_drop", cx.tcx.def_path_str(needs_drop))
+                        .emit();
+                });
+            }
+        }
+    }
+}