Lint mem_forget if fields are Drop

Author: Centri3

clippy_lints/src/mem_forget.rs

@@ -7,7 +7,7 @@ use rustc_span::sym;
 declare_clippy_lint! {
     /// ### What it does
     /// Checks for usage of `std::mem::forget(t)` where `t` is
-    /// `Drop`.
+    /// `Drop` or has a field that implements `Drop`.
     ///
     /// ### Why is this bad?
     /// `std::mem::forget(t)` prevents `t` from running its
@@ -29,18 +29,26 @@ declare_lint_pass!(MemForget => [MEM_FORGET]);
 
 impl<'tcx> LateLintPass<'tcx> for MemForget {
     fn check_expr(&mut self, cx: &LateContext<'tcx>, e: &'tcx Expr<'_>) {
-        if let ExprKind::Call(path_expr, [ref first_arg, ..]) = e.kind {
-            if let ExprKind::Path(ref qpath) = path_expr.kind {
-                if let Some(def_id) = cx.qpath_res(qpath, path_expr.hir_id).opt_def_id() {
-                    if cx.tcx.is_diagnostic_item(sym::mem_forget, def_id) {
-                        let forgot_ty = cx.typeck_results().expr_ty(first_arg);
-
-                        if forgot_ty.ty_adt_def().map_or(false, |def| def.has_dtor(cx.tcx)) {
-                            span_lint(cx, MEM_FORGET, e.span, "usage of `mem::forget` on `Drop` type");
-                        }
+        if let ExprKind::Call(path_expr, [ref first_arg, ..]) = e.kind
+            && let ExprKind::Path(ref qpath) = path_expr.kind
+            && let Some(def_id) = cx.qpath_res(qpath, path_expr.hir_id).opt_def_id()
+            && cx.tcx.is_diagnostic_item(sym::mem_forget, def_id)
+            && let forgot_ty = cx.typeck_results().expr_ty(first_arg)
+            && forgot_ty.needs_drop(cx.tcx, cx.param_env)
+        {
+            span_lint(
+                cx,
+                MEM_FORGET,
+                e.span,
+                &format!(
+                    "usage of `mem::forget` on {}",
+                    if forgot_ty.ty_adt_def().map_or(false, |def| def.has_dtor(cx.tcx)) {
+                        "`Drop` type"
+                    } else {
+                        "type with `Drop` fields"
                     }
-                }
-            }
+                ),
+            );
         }
     }
 }

tests/ui/mem_forget.rs

@@ -19,5 +19,8 @@ fn main() {
     let eight: Vec<i32> = vec![8];
     forgetSomething(eight);
 
+    let string = String::new();
+    std::mem::forget(string);
+
     std::mem::forget(7);
 }

tests/ui/mem_forget.stderr

@@ -18,5 +18,11 @@ error: usage of `mem::forget` on `Drop` type
 LL |     forgetSomething(eight);
    |     ^^^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 3 previous errors
+error: usage of `mem::forget` on type with `Drop` fields
+  --> $DIR/mem_forget.rs:23:5
+   |
+LL |     std::mem::forget(string);
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 4 previous errors