Add empty_drop lint

CHANGELOG.md

@@ -3365,6 +3365,7 @@ Released 2018-09-13
 [`duplicate_underscore_argument`]: https://rust-lang.github.io/rust-clippy/master/index.html#duplicate_underscore_argument
 [`duration_subsec`]: https://rust-lang.github.io/rust-clippy/master/index.html#duration_subsec
 [`else_if_without_else`]: https://rust-lang.github.io/rust-clippy/master/index.html#else_if_without_else
+[`empty_drop`]: https://rust-lang.github.io/rust-clippy/master/index.html#empty_drop
 [`empty_enum`]: https://rust-lang.github.io/rust-clippy/master/index.html#empty_enum
 [`empty_line_after_outer_attr`]: https://rust-lang.github.io/rust-clippy/master/index.html#empty_line_after_outer_attr
 [`empty_loop`]: https://rust-lang.github.io/rust-clippy/master/index.html#empty_loop

clippy_lints/src/empty_drop.rs

@@ -0,0 +1,65 @@
+use clippy_utils::{diagnostics::span_lint_and_sugg, peel_blocks};
+use if_chain::if_chain;
+use rustc_errors::Applicability;
+use rustc_hir::{Body, ExprKind, Impl, ImplItemKind, Item, ItemKind, Node};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for empty `Drop` implementations.
+    ///
+    /// ### Why is this bad?
+    /// Empty `Drop` implementations have no effect when dropping an instance of the type. They are
+    /// most likely useless. However, an empty `Drop` implementation prevents a type from being
+    /// destructured, which might be the intention behind adding the implementation as a marker.
+    ///
+    /// ### Example
+    /// ```rust
+    /// struct S;
+    ///
+    /// impl Drop for S {
+    ///     fn drop(&mut self) {}
+    /// }
+    /// ```
+    /// Use instead:
+    /// ```rust
+    /// struct S;
+    /// ```
+    #[clippy::version = "1.61.0"]
+    pub EMPTY_DROP,
+    restriction,
+    "empty `Drop` implementations"
+}
+declare_lint_pass!(EmptyDrop => [EMPTY_DROP]);
+
+impl LateLintPass<'_> for EmptyDrop {
+    fn check_item(&mut self, cx: &LateContext<'_>, item: &Item<'_>) {
+        if_chain! {
+            if let ItemKind::Impl(Impl {
+                of_trait: Some(ref trait_ref),
+                items: [child],
+                ..
+            }) = item.kind;
+            if trait_ref.trait_def_id() == cx.tcx.lang_items().drop_trait();
+            if let impl_item_hir = child.id.hir_id();
+            if let Some(Node::ImplItem(impl_item)) = cx.tcx.hir().find(impl_item_hir);
+            if let ImplItemKind::Fn(_, b) = &impl_item.kind;
+            if let Body { value: func_expr, .. } = cx.tcx.hir().body(*b);
+            let func_expr = peel_blocks(func_expr);
+            if let ExprKind::Block(block, _) = func_expr.kind;
+            if block.stmts.is_empty() && block.expr.is_none();
+            then {
+                span_lint_and_sugg(
+                    cx,
+                    EMPTY_DROP,
+                    item.span,
+                    "empty drop implementation",
+                    "try removing this impl",
+                    String::new(),
+                    Applicability::MaybeIncorrect
+                );
+            }
+        }
+    }
+}

clippy_lints/src/lib.register_lints.rs

@@ -134,6 +134,7 @@ store.register_lints(&[
     drop_forget_ref::UNDROPPED_MANUALLY_DROPS,
     duration_subsec::DURATION_SUBSEC,
     else_if_without_else::ELSE_IF_WITHOUT_ELSE,
+    empty_drop::EMPTY_DROP,
     empty_enum::EMPTY_ENUM,
     empty_structs_with_brackets::EMPTY_STRUCTS_WITH_BRACKETS,
     entry::MAP_ENTRY,

clippy_lints/src/lib.register_restriction.rs

@@ -16,6 +16,7 @@ store.register_group(true, "clippy::restriction", Some("clippy_restriction"), ve
     LintId::of(default_union_representation::DEFAULT_UNION_REPRESENTATION),
     LintId::of(disallowed_script_idents::DISALLOWED_SCRIPT_IDENTS),
     LintId::of(else_if_without_else::ELSE_IF_WITHOUT_ELSE),
+    LintId::of(empty_drop::EMPTY_DROP),
     LintId::of(empty_structs_with_brackets::EMPTY_STRUCTS_WITH_BRACKETS),
     LintId::of(exhaustive_items::EXHAUSTIVE_ENUMS),
     LintId::of(exhaustive_items::EXHAUSTIVE_STRUCTS),

clippy_lints/src/lib.rs

@@ -210,6 +210,7 @@ mod double_parens;
 mod drop_forget_ref;
 mod duration_subsec;
 mod else_if_without_else;
+mod empty_drop;
 mod empty_enum;
 mod empty_structs_with_brackets;
 mod entry;
@@ -822,6 +823,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_late_pass(move || Box::new(disallowed_methods::DisallowedMethods::new(disallowed_methods.clone())));
     store.register_early_pass(|| Box::new(asm_syntax::InlineAsmX86AttSyntax));
     store.register_early_pass(|| Box::new(asm_syntax::InlineAsmX86IntelSyntax));
+    store.register_late_pass(|| Box::new(empty_drop::EmptyDrop));
     store.register_late_pass(|| Box::new(strings::StrToString));
     store.register_late_pass(|| Box::new(strings::StringToString));
     store.register_late_pass(|| Box::new(zero_sized_map_values::ZeroSizedMapValues));

tests/ui/empty_drop.fixed

@@ -0,0 +1,24 @@
+// run-rustfix
+#![warn(clippy::empty_drop)]
+#![allow(unused)]
+
+// should cause an error
+struct Foo;
+
+
+
+// shouldn't cause an error
+struct Bar;
+
+impl Drop for Bar {
+    fn drop(&mut self) {
+        println!("dropping bar!");
+    }
+}
+
+// should error
+struct Baz;
+
+
+
+fn main() {}

tests/ui/empty_drop.rs

@@ -0,0 +1,30 @@
+// run-rustfix
+#![warn(clippy::empty_drop)]
+#![allow(unused)]
+
+// should cause an error
+struct Foo;
+
+impl Drop for Foo {
+    fn drop(&mut self) {}
+}
+
+// shouldn't cause an error
+struct Bar;
+
+impl Drop for Bar {
+    fn drop(&mut self) {
+        println!("dropping bar!");
+    }
+}
+
+// should error
+struct Baz;
+
+impl Drop for Baz {
+    fn drop(&mut self) {
+        {}
+    }
+}
+
+fn main() {}

tests/ui/empty_drop.stderr

@@ -0,0 +1,22 @@
+error: empty drop implementation
+  --> $DIR/empty_drop.rs:8:1
+   |
+LL | / impl Drop for Foo {
+LL | |     fn drop(&mut self) {}
+LL | | }
+   | |_^ help: try removing this impl
+   |
+   = note: `-D clippy::empty-drop` implied by `-D warnings`
+
+error: empty drop implementation
+  --> $DIR/empty_drop.rs:24:1
+   |
+LL | / impl Drop for Baz {
+LL | |     fn drop(&mut self) {
+LL | |         {}
+LL | |     }
+LL | | }
+   | |_^ help: try removing this impl
+
+error: aborting due to 2 previous errors
+