Lint mem_forget if any fields are Drop

clippy_lints/src/declared_lints.rs

@@ -140,6 +140,7 @@ pub(crate) static LINTS: &[&crate::LintInfo] = &[
     crate::double_parens::DOUBLE_PARENS_INFO,
     crate::drop_forget_ref::DROP_NON_DROP_INFO,
     crate::drop_forget_ref::FORGET_NON_DROP_INFO,
+    crate::drop_forget_ref::MEM_FORGET_INFO,
     crate::drop_forget_ref::UNDROPPED_MANUALLY_DROPS_INFO,
     crate::duplicate_mod::DUPLICATE_MOD_INFO,
     crate::else_if_without_else::ELSE_IF_WITHOUT_ELSE_INFO,
@@ -310,7 +311,6 @@ pub(crate) static LINTS: &[&crate::LintInfo] = &[
     crate::matches::TRY_ERR_INFO,
     crate::matches::WILDCARD_ENUM_MATCH_ARM_INFO,
     crate::matches::WILDCARD_IN_OR_PATTERNS_INFO,
-    crate::mem_forget::MEM_FORGET_INFO,
     crate::mem_replace::MEM_REPLACE_OPTION_WITH_NONE_INFO,
     crate::mem_replace::MEM_REPLACE_WITH_DEFAULT_INFO,
     crate::mem_replace::MEM_REPLACE_WITH_UNINIT_INFO,

clippy_lints/src/drop_forget_ref.rs

@@ -6,6 +6,7 @@ use rustc_hir::{Arm, Expr, ExprKind, LangItem, Node};
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_session::{declare_lint_pass, declare_tool_lint};
 use rustc_span::sym;
+use std::borrow::Cow;
 
 declare_clippy_lint! {
     /// ### What it does
@@ -76,6 +77,27 @@ declare_clippy_lint! {
     "use of safe `std::mem::drop` function to drop a std::mem::ManuallyDrop, which will not drop the inner value"
 }
 
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for usage of `std::mem::forget(t)` where `t` is
+    /// `Drop` or has a field that implements `Drop`.
+    ///
+    /// ### Why is this bad?
+    /// `std::mem::forget(t)` prevents `t` from running its
+    /// destructor, possibly causing leaks.
+    ///
+    /// ### Example
+    /// ```rust
+    /// # use std::mem;
+    /// # use std::rc::Rc;
+    /// mem::forget(Rc::new(55))
+    /// ```
+    #[clippy::version = "pre 1.29.0"]
+    pub MEM_FORGET,
+    restriction,
+    "`mem::forget` usage on `Drop` types, likely to cause memory leaks"
+}
+
 const DROP_NON_DROP_SUMMARY: &str = "call to `std::mem::drop` with a value that does not implement `Drop`. \
                                  Dropping such a type only extends its contained lifetimes";
 const FORGET_NON_DROP_SUMMARY: &str = "call to `std::mem::forget` with a value that does not implement `Drop`. \
@@ -84,7 +106,8 @@ const FORGET_NON_DROP_SUMMARY: &str = "call to `std::mem::forget` with a value t
 declare_lint_pass!(DropForgetRef => [
     DROP_NON_DROP,
     FORGET_NON_DROP,
-    UNDROPPED_MANUALLY_DROPS
+    UNDROPPED_MANUALLY_DROPS,
+    MEM_FORGET,
 ]);
 
 impl<'tcx> LateLintPass<'tcx> for DropForgetRef {
@@ -97,7 +120,7 @@ impl<'tcx> LateLintPass<'tcx> for DropForgetRef {
             let arg_ty = cx.typeck_results().expr_ty(arg);
             let is_copy = is_copy(cx, arg_ty);
             let drop_is_single_call_in_arm = is_single_call_in_arm(cx, arg, expr);
-            let (lint, msg) = match fn_name {
+            let (lint, msg, note_span) = match fn_name {
                 // early return for uplifted lints: dropping_references, dropping_copy_types, forgetting_references, forgetting_copy_types
                 sym::mem_drop if arg_ty.is_ref() && !drop_is_single_call_in_arm => return,
                 sym::mem_forget if arg_ty.is_ref() => return,
@@ -121,19 +144,34 @@ impl<'tcx> LateLintPass<'tcx> for DropForgetRef {
                         || drop_is_single_call_in_arm
                         ) =>
                 {
-                    (DROP_NON_DROP, DROP_NON_DROP_SUMMARY)
-                },
-                sym::mem_forget if !arg_ty.needs_drop(cx.tcx, cx.param_env) => {
-                    (FORGET_NON_DROP, FORGET_NON_DROP_SUMMARY)
+                    (DROP_NON_DROP, DROP_NON_DROP_SUMMARY.into(), Some(arg.span))
                 },
+                sym::mem_forget => {
+                    if arg_ty.needs_drop(cx.tcx, cx.param_env) {
+                        (
+                            MEM_FORGET,
+                            Cow::Owned(format!(
+                                "usage of `mem::forget` on {}",
+                                if arg_ty.ty_adt_def().map_or(false, |def| def.has_dtor(cx.tcx)) {
+                                    "`Drop` type"
+                                } else {
+                                    "type with `Drop` fields"
+                                }
+                            )),
+                            None,
+                        )
+                    } else {
+                        (FORGET_NON_DROP, FORGET_NON_DROP_SUMMARY.into(), Some(arg.span))
+                    }
+                }
                 _ => return,
             };
             span_lint_and_note(
                 cx,
                 lint,
                 expr.span,
-                msg,
-                Some(arg.span),
+                &msg,
+                note_span,
                 &format!("argument has type `{arg_ty}`"),
             );
         }

clippy_lints/src/lib.rs

@@ -196,7 +196,6 @@ mod manual_strip;
 mod map_unit_fn;
 mod match_result_ok;
 mod matches;
-mod mem_forget;
 mod mem_replace;
 mod methods;
 mod min_ident_chars;
@@ -744,7 +743,6 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     let missing_docs_in_crate_items = conf.missing_docs_in_crate_items;
     store.register_late_pass(move |_| Box::new(doc::DocMarkdown::new(doc_valid_idents.clone())));
     store.register_late_pass(|_| Box::new(neg_multiply::NegMultiply));
-    store.register_late_pass(|_| Box::new(mem_forget::MemForget));
     store.register_late_pass(|_| Box::new(let_if_seq::LetIfSeq));
     store.register_late_pass(|_| Box::new(mixed_read_write_in_expression::EvalOrderDependence));
     store.register_late_pass(move |_| Box::new(missing_doc::MissingDoc::new(missing_docs_in_crate_items)));

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

@@ -4,19 +4,32 @@ error: usage of `mem::forget` on `Drop` type
 LL |     memstuff::forget(six);
    |     ^^^^^^^^^^^^^^^^^^^^^
    |
+   = note: argument has type `std::sync::Arc<i32>`
    = note: `-D clippy::mem-forget` implied by `-D warnings`
 
 error: usage of `mem::forget` on `Drop` type
   --> $DIR/mem_forget.rs:17:5
    |
 LL |     std::mem::forget(seven);
    |     ^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: argument has type `std::rc::Rc<i32>`
 
 error: usage of `mem::forget` on `Drop` type
   --> $DIR/mem_forget.rs:20:5
    |
 LL |     forgetSomething(eight);
    |     ^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: argument has type `std::vec::Vec<i32>`
+
+error: usage of `mem::forget` on type with `Drop` fields
+  --> $DIR/mem_forget.rs:23:5
+   |
+LL |     std::mem::forget(string);
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: argument has type `std::string::String`
 
-error: aborting due to 3 previous errors
+error: aborting due to 4 previous errors