Uplift drop-bounds lint from clippy

1 files changed, 79 insertions, 0 deletions

diff --git a/compiler/rustc_lint/src/traits.rs b/compiler/rustc_lint/src/traits.rs
new file mode 100644
index 00000000000..d4f79036e5a
--- /dev/null
+++ b/compiler/rustc_lint/src/traits.rs
@@ -0,0 +1,79 @@
+use crate::LateContext;
+use crate::LateLintPass;
+use crate::LintContext;
+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
+    ///
+    /// `Drop` bounds do not really accomplish anything. A type may have
+    /// compiler-generated drop glue without implementing the `Drop` trait
+    /// itself. The `Drop` trait also only has one method, `Drop::drop`, and
+    /// that function is by fiat not callable in user code. So there is really
+    /// no use case for using `Drop` in trait bounds.
+    ///
+    /// The most likely use case of a drop bound is to distinguish between
+    /// types that have destructors and types that don't. Combined with
+    /// specialization, a naive coder would write an implementation that
+    /// assumed a type could be trivially dropped, then write a specialization
+    /// for `T: Drop` that actually calls the destructor. Except that doing so
+    /// is not correct; String, for example, doesn't actually implement Drop,
+    /// but because String contains a Vec, assuming it can be trivially dropped
+    /// will leak memory.
+    pub DROP_BOUNDS,
+    Warn,
+    "bounds of the form `T: 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]
+);
+
+impl<'tcx> LateLintPass<'tcx> for DropTraitConstraints {
+    fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx hir::Item<'tcx>) {
+        use rustc_middle::ty::PredicateAtom::*;
+
+        let def_id = cx.tcx.hir().local_def_id(item.hir_id);
+        let predicates = cx.tcx.explicit_predicates_of(def_id);
+        for &(predicate, span) in predicates.predicates {
+            let trait_predicate = match predicate.skip_binders() {
+                Trait(trait_predicate, _constness) => trait_predicate,
+                _ => 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 needs_drop = match cx.tcx.get_diagnostic_item(sym::needs_drop) {
+                        Some(needs_drop) => needs_drop,
+                        None => return,
+                    };
+                    let msg = format!(
+                        "bounds on `{}` are useless, consider instead \
+                         using `{}` to detect if a type has a destructor",
+                        predicate,
+                        cx.tcx.def_path_str(needs_drop)
+                    );
+                    lint.build(&msg).emit()
+                });
+            }
+        }
+    }
+}