Auto merge of #7570 - HKalbasi:derivable-impls, r=camsteffen

Add the `derivable_impls` lint

Fix #7550

changelog: Add new derivable_impls lint. mem_replace_with_default now covers non constructor cases.

CHANGELOG.md

@@ -2613,6 +2613,7 @@ Released 2018-09-13
 [`deprecated_cfg_attr`]: https://rust-lang.github.io/rust-clippy/master/index.html#deprecated_cfg_attr
 [`deprecated_semver`]: https://rust-lang.github.io/rust-clippy/master/index.html#deprecated_semver
 [`deref_addrof`]: https://rust-lang.github.io/rust-clippy/master/index.html#deref_addrof
+[`derivable_impls`]: https://rust-lang.github.io/rust-clippy/master/index.html#derivable_impls
 [`derive_hash_xor_eq`]: https://rust-lang.github.io/rust-clippy/master/index.html#derive_hash_xor_eq
 [`derive_ord_xor_partial_ord`]: https://rust-lang.github.io/rust-clippy/master/index.html#derive_ord_xor_partial_ord
 [`disallowed_method`]: https://rust-lang.github.io/rust-clippy/master/index.html#disallowed_method

clippy_lints/src/derivable_impls.rs

@@ -0,0 +1,108 @@
+use clippy_utils::diagnostics::span_lint_and_help;
+use clippy_utils::{in_macro, is_automatically_derived, is_default_equivalent, remove_blocks};
+use rustc_hir::{
+    def::{DefKind, Res},
+    Body, Expr, ExprKind, Impl, ImplItemKind, Item, ItemKind, Node, QPath,
+};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_middle::ty::TypeFoldable;
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+use rustc_span::sym;
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// Detects manual `std::default::Default` implementations that are identical to a derived implementation.
+    ///
+    /// ### Why is this bad?
+    /// It is less concise.
+    ///
+    /// ### Example
+    /// ```rust
+    /// struct Foo {
+    ///     bar: bool
+    /// }
+    ///
+    /// impl std::default::Default for Foo {
+    ///     fn default() -> Self {
+    ///         Self {
+    ///             bar: false
+    ///         }
+    ///     }
+    /// }
+    /// ```
+    ///
+    /// Could be written as:
+    ///
+    /// ```rust
+    /// #[derive(Default)]
+    /// struct Foo {
+    ///     bar: bool
+    /// }
+    /// ```
+    ///
+    /// ### Known problems
+    /// Derive macros [sometimes use incorrect bounds](https://github.com/rust-lang/rust/issues/26925)
+    /// in generic types and the user defined `impl` maybe is more generalized or
+    /// specialized than what derive will produce. This lint can't detect the manual `impl`
+    /// has exactly equal bounds, and therefore this lint is disabled for types with
+    /// generic parameters.
+    ///
+    pub DERIVABLE_IMPLS,
+    complexity,
+    "manual implementation of the `Default` trait which is equal to a derive"
+}
+
+declare_lint_pass!(DerivableImpls => [DERIVABLE_IMPLS]);
+
+fn is_path_self(e: &Expr<'_>) -> bool {
+    if let ExprKind::Path(QPath::Resolved(_, p)) = e.kind {
+        matches!(p.res, Res::SelfCtor(..) | Res::Def(DefKind::Ctor(..), _))
+    } else {
+        false
+    }
+}
+
+impl<'tcx> LateLintPass<'tcx> for DerivableImpls {
+    fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx Item<'_>) {
+        if_chain! {
+            if let ItemKind::Impl(Impl {
+                of_trait: Some(ref trait_ref),
+                items: [child],
+                ..
+            }) = item.kind;
+            if let attrs = cx.tcx.hir().attrs(item.hir_id());
+            if !is_automatically_derived(attrs);
+            if !in_macro(item.span);
+            if let Some(def_id) = trait_ref.trait_def_id();
+            if cx.tcx.is_diagnostic_item(sym::Default, def_id);
+            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);
+            if let Some(adt_def) = cx.tcx.type_of(item.def_id).ty_adt_def();
+            then {
+                if cx.tcx.type_of(item.def_id).definitely_has_param_types_or_consts(cx.tcx) {
+                    return;
+                }
+                let should_emit = match remove_blocks(func_expr).kind {
+                    ExprKind::Tup(fields) => fields.iter().all(|e| is_default_equivalent(cx, e)),
+                    ExprKind::Call(callee, args)
+                        if is_path_self(callee) => args.iter().all(|e| is_default_equivalent(cx, e)),
+                    ExprKind::Struct(_, fields, _) => fields.iter().all(|ef| is_default_equivalent(cx, ef.expr)),
+                    _ => false,
+                };
+                if should_emit {
+                    let path_string = cx.tcx.def_path_str(adt_def.did);
+                    span_lint_and_help(
+                        cx,
+                        DERIVABLE_IMPLS,
+                        item.span,
+                        "this `impl` can be derived",
+                        None,
+                        &format!("try annotating `{}` with `#[derive(Default)]`", path_string),
+                    );
+                }
+            }
+        }
+    }
+}

clippy_lints/src/lib.rs

@@ -187,6 +187,7 @@ mod dbg_macro;
 mod default;
 mod default_numeric_fallback;
 mod dereference;
+mod derivable_impls;
 mod derive;
 mod disallowed_method;
 mod disallowed_script_idents;
@@ -586,6 +587,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         default::FIELD_REASSIGN_WITH_DEFAULT,
         default_numeric_fallback::DEFAULT_NUMERIC_FALLBACK,
         dereference::EXPLICIT_DEREF_METHODS,
+        derivable_impls::DERIVABLE_IMPLS,
         derive::DERIVE_HASH_XOR_EQ,
         derive::DERIVE_ORD_XOR_PARTIAL_ORD,
         derive::EXPL_IMPL_CLONE_ON_COPY,
@@ -1204,6 +1206,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(copies::IFS_SAME_COND),
         LintId::of(copies::IF_SAME_THEN_ELSE),
         LintId::of(default::FIELD_REASSIGN_WITH_DEFAULT),
+        LintId::of(derivable_impls::DERIVABLE_IMPLS),
         LintId::of(derive::DERIVE_HASH_XOR_EQ),
         LintId::of(derive::DERIVE_ORD_XOR_PARTIAL_ORD),
         LintId::of(doc::MISSING_SAFETY_DOC),
@@ -1589,6 +1592,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(booleans::NONMINIMAL_BOOL),
         LintId::of(casts::CHAR_LIT_AS_U8),
         LintId::of(casts::UNNECESSARY_CAST),
+        LintId::of(derivable_impls::DERIVABLE_IMPLS),
         LintId::of(double_comparison::DOUBLE_COMPARISONS),
         LintId::of(double_parens::DOUBLE_PARENS),
         LintId::of(duration_subsec::DURATION_SUBSEC),
@@ -1937,6 +1941,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_late_pass(|| Box::new(panic_unimplemented::PanicUnimplemented));
     store.register_late_pass(|| Box::new(strings::StringLitAsBytes));
     store.register_late_pass(|| Box::new(derive::Derive));
+    store.register_late_pass(|| Box::new(derivable_impls::DerivableImpls));
     store.register_late_pass(|| Box::new(get_last_with_len::GetLastWithLen));
     store.register_late_pass(|| Box::new(drop_forget_ref::DropForgetRef));
     store.register_late_pass(|| Box::new(empty_enum::EmptyEnum));

clippy_lints/src/mem_replace.rs

@@ -1,9 +1,9 @@
 use clippy_utils::diagnostics::{span_lint_and_help, span_lint_and_sugg, span_lint_and_then};
 use clippy_utils::source::{snippet, snippet_with_applicability};
-use clippy_utils::{in_macro, is_diag_trait_item, is_lang_ctor, match_def_path, meets_msrv, msrvs, paths};
+use clippy_utils::ty::is_non_aggregate_primitive_type;
+use clippy_utils::{in_macro, is_default_equivalent, is_lang_ctor, match_def_path, meets_msrv, msrvs, paths};
 use if_chain::if_chain;
 use rustc_errors::Applicability;
-use rustc_hir::def_id::DefId;
 use rustc_hir::LangItem::OptionNone;
 use rustc_hir::{BorrowKind, Expr, ExprKind, Mutability, QPath};
 use rustc_lint::{LateContext, LateLintPass, LintContext};
@@ -194,64 +194,37 @@ fn check_replace_with_uninit(cx: &LateContext<'_>, src: &Expr<'_>, dest: &Expr<'
     }
 }
 
-/// Returns true if the `def_id` associated with the `path` is recognized as a "default-equivalent"
-/// constructor from the std library
-fn is_default_equivalent_ctor(cx: &LateContext<'_>, def_id: DefId, path: &QPath<'_>) -> bool {
-    let std_types_symbols = &[
-        sym::string_type,
-        sym::vec_type,
-        sym::vecdeque_type,
-        sym::LinkedList,
-        sym::hashmap_type,
-        sym::BTreeMap,
-        sym::hashset_type,
-        sym::BTreeSet,
-        sym::BinaryHeap,
-    ];
-
-    if let QPath::TypeRelative(_, method) = path {
-        if method.ident.name == sym::new {
-            if let Some(impl_did) = cx.tcx.impl_of_method(def_id) {
-                if let Some(adt) = cx.tcx.type_of(impl_did).ty_adt_def() {
-                    return std_types_symbols
-                        .iter()
-                        .any(|&symbol| cx.tcx.is_diagnostic_item(symbol, adt.did));
-                }
-            }
+fn check_replace_with_default(cx: &LateContext<'_>, src: &Expr<'_>, dest: &Expr<'_>, expr_span: Span) {
+    // disable lint for primitives
+    let expr_type = cx.typeck_results().expr_ty_adjusted(src);
+    if is_non_aggregate_primitive_type(expr_type) {
+        return;
+    }
+    // disable lint for Option since it is covered in another lint
+    if let ExprKind::Path(q) = &src.kind {
+        if is_lang_ctor(cx, q, OptionNone) {
+            return;
         }
     }
-    false
-}
+    if is_default_equivalent(cx, src) && !in_external_macro(cx.tcx.sess, expr_span) {
+        span_lint_and_then(
+            cx,
+            MEM_REPLACE_WITH_DEFAULT,
+            expr_span,
+            "replacing a value of type `T` with `T::default()` is better expressed using `std::mem::take`",
+            |diag| {
+                if !in_macro(expr_span) {
+                    let suggestion = format!("std::mem::take({})", snippet(cx, dest.span, ""));
 
-fn check_replace_with_default(cx: &LateContext<'_>, src: &Expr<'_>, dest: &Expr<'_>, expr_span: Span) {
-    if_chain! {
-        if let ExprKind::Call(repl_func, _) = src.kind;
-        if !in_external_macro(cx.tcx.sess, expr_span);
-        if let ExprKind::Path(ref repl_func_qpath) = repl_func.kind;
-        if let Some(repl_def_id) = cx.qpath_res(repl_func_qpath, repl_func.hir_id).opt_def_id();
-        if is_diag_trait_item(cx, repl_def_id, sym::Default)
-            || is_default_equivalent_ctor(cx, repl_def_id, repl_func_qpath);
-
-        then {
-            span_lint_and_then(
-                cx,
-                MEM_REPLACE_WITH_DEFAULT,
-                expr_span,
-                "replacing a value of type `T` with `T::default()` is better expressed using `std::mem::take`",
-                |diag| {
-                    if !in_macro(expr_span) {
-                        let suggestion = format!("std::mem::take({})", snippet(cx, dest.span, ""));
-
-                        diag.span_suggestion(
-                            expr_span,
-                            "consider using",
-                            suggestion,
-                            Applicability::MachineApplicable
-                        );
-                    }
+                    diag.span_suggestion(
+                        expr_span,
+                        "consider using",
+                        suggestion,
+                        Applicability::MachineApplicable,
+                    );
                 }
-            );
-        }
+            },
+        );
     }
 }
 

clippy_utils/src/lib.rs

@@ -70,7 +70,7 @@ use rustc_hir::def::{DefKind, Res};
 use rustc_hir::def_id::DefId;
 use rustc_hir::hir_id::{HirIdMap, HirIdSet};
 use rustc_hir::intravisit::{self, walk_expr, ErasedMap, FnKind, NestedVisitorMap, Visitor};
-use rustc_hir::LangItem::{ResultErr, ResultOk};
+use rustc_hir::LangItem::{OptionNone, ResultErr, ResultOk};
 use rustc_hir::{
     def, Arm, BindingAnnotation, Block, Body, Constness, Destination, Expr, ExprKind, FnDecl, GenericArgs, HirId, Impl,
     ImplItem, ImplItemKind, IsAsync, Item, ItemKind, LangItem, Local, MatchSource, Mutability, Node, Param, Pat,
@@ -644,6 +644,65 @@ pub fn can_mut_borrow_both(cx: &LateContext<'_>, e1: &Expr<'_>, e2: &Expr<'_>) -
     false
 }
 
+/// Returns true if the `def_id` associated with the `path` is recognized as a "default-equivalent"
+/// constructor from the std library
+fn is_default_equivalent_ctor(cx: &LateContext<'_>, def_id: DefId, path: &QPath<'_>) -> bool {
+    let std_types_symbols = &[
+        sym::string_type,
+        sym::vec_type,
+        sym::vecdeque_type,
+        sym::LinkedList,
+        sym::hashmap_type,
+        sym::BTreeMap,
+        sym::hashset_type,
+        sym::BTreeSet,
+        sym::BinaryHeap,
+    ];
+
+    if let QPath::TypeRelative(_, method) = path {
+        if method.ident.name == sym::new {
+            if let Some(impl_did) = cx.tcx.impl_of_method(def_id) {
+                if let Some(adt) = cx.tcx.type_of(impl_did).ty_adt_def() {
+                    return std_types_symbols
+                        .iter()
+                        .any(|&symbol| cx.tcx.is_diagnostic_item(symbol, adt.did));
+                }
+            }
+        }
+    }
+    false
+}
+
+/// Returns true if the expr is equal to `Default::default()` of it's type when evaluated.
+/// It doesn't cover all cases, for example indirect function calls (some of std
+/// functions are supported) but it is the best we have.
+pub fn is_default_equivalent(cx: &LateContext<'_>, e: &Expr<'_>) -> bool {
+    match &e.kind {
+        ExprKind::Lit(lit) => match lit.node {
+            LitKind::Bool(false) | LitKind::Int(0, _) => true,
+            LitKind::Str(s, _) => s.is_empty(),
+            _ => false,
+        },
+        ExprKind::Tup(items) | ExprKind::Array(items) => items.iter().all(|x| is_default_equivalent(cx, x)),
+        ExprKind::Repeat(x, _) => is_default_equivalent(cx, x),
+        ExprKind::Call(repl_func, _) => if_chain! {
+            if let ExprKind::Path(ref repl_func_qpath) = repl_func.kind;
+            if let Some(repl_def_id) = cx.qpath_res(repl_func_qpath, repl_func.hir_id).opt_def_id();
+            if is_diag_trait_item(cx, repl_def_id, sym::Default)
+                || is_default_equivalent_ctor(cx, repl_def_id, repl_func_qpath);
+            then {
+                true
+            }
+            else {
+                false
+            }
+        },
+        ExprKind::Path(qpath) => is_lang_ctor(cx, qpath, OptionNone),
+        ExprKind::AddrOf(rustc_hir::BorrowKind::Ref, _, expr) => matches!(expr.kind, ExprKind::Array([])),
+        _ => false,
+    }
+}
+
 /// Checks if the top level expression can be moved into a closure as is.
 /// Currently checks for:
 /// * Break/Continue outside the given loop HIR ids.

clippy_utils/src/ty.rs

@@ -224,6 +224,13 @@ fn is_normalizable_helper<'tcx>(
     result
 }
 
+/// Returns true iff the given type is a non aggregate primitive (a bool or char, any integer or
+/// floating-point number type). For checking aggregation of primitive types (e.g. tuples and slices
+/// of primitive type) see `is_recursively_primitive_type`
+pub fn is_non_aggregate_primitive_type(ty: Ty<'_>) -> bool {
+    matches!(ty.kind(), ty::Bool | ty::Char | ty::Int(_) | ty::Uint(_) | ty::Float(_))
+}
+
 /// Returns true iff the given type is a primitive (a bool or char, any integer or floating-point
 /// number type, a str, or an array, slice, or tuple of those types).
 pub fn is_recursively_primitive_type(ty: Ty<'_>) -> bool {

tests/ui/derivable_impls.rs

@@ -0,0 +1,170 @@
+use std::collections::HashMap;
+
+struct FooDefault<'a> {
+    a: bool,
+    b: i32,
+    c: u64,
+    d: Vec<i32>,
+    e: FooND1,
+    f: FooND2,
+    g: HashMap<i32, i32>,
+    h: (i32, Vec<i32>),
+    i: [Vec<i32>; 3],
+    j: [i32; 5],
+    k: Option<i32>,
+    l: &'a [i32],
+}
+
+impl std::default::Default for FooDefault<'_> {
+    fn default() -> Self {
+        Self {
+            a: false,
+            b: 0,
+            c: 0u64,
+            d: vec![],
+            e: Default::default(),
+            f: FooND2::default(),
+            g: HashMap::new(),
+            h: (0, vec![]),
+            i: [vec![], vec![], vec![]],
+            j: [0; 5],
+            k: None,
+            l: &[],
+        }
+    }
+}
+
+struct TupleDefault(bool, i32, u64);
+
+impl std::default::Default for TupleDefault {
+    fn default() -> Self {
+        Self(false, 0, 0u64)
+    }
+}
+
+struct FooND1 {
+    a: bool,
+}
+
+impl std::default::Default for FooND1 {
+    fn default() -> Self {
+        Self { a: true }
+    }
+}
+
+struct FooND2 {
+    a: i32,
+}
+
+impl std::default::Default for FooND2 {
+    fn default() -> Self {
+        Self { a: 5 }
+    }
+}
+
+struct FooNDNew {
+    a: bool,
+}
+
+impl FooNDNew {
+    fn new() -> Self {
+        Self { a: true }
+    }
+}
+
+impl Default for FooNDNew {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+struct FooNDVec(Vec<i32>);
+
+impl Default for FooNDVec {
+    fn default() -> Self {
+        Self(vec![5, 12])
+    }
+}
+
+struct StrDefault<'a>(&'a str);
+
+impl Default for StrDefault<'_> {
+    fn default() -> Self {
+        Self("")
+    }
+}
+
+#[derive(Default)]
+struct AlreadyDerived(i32, bool);
+
+macro_rules! mac {
+    () => {
+        0
+    };
+    ($e:expr) => {
+        struct X(u32);
+        impl Default for X {
+            fn default() -> Self {
+                Self($e)
+            }
+        }
+    };
+}
+
+mac!(0);
+
+struct Y(u32);
+impl Default for Y {
+    fn default() -> Self {
+        Self(mac!())
+    }
+}
+
+struct RustIssue26925<T> {
+    a: Option<T>,
+}
+
+// We should watch out for cases where a manual impl is needed because a
+// derive adds different type bounds (https://github.com/rust-lang/rust/issues/26925).
+// For example, a struct with Option<T> does not require T: Default, but a derive adds
+// that type bound anyways. So until #26925 get fixed we should disable lint
+// for the following case
+impl<T> Default for RustIssue26925<T> {
+    fn default() -> Self {
+        Self { a: None }
+    }
+}
+
+struct SpecializedImpl<A, B> {
+    a: A,
+    b: B,
+}
+
+impl<T: Default> Default for SpecializedImpl<T, T> {
+    fn default() -> Self {
+        Self {
+            a: T::default(),
+            b: T::default(),
+        }
+    }
+}
+
+struct WithoutSelfCurly {
+    a: bool,
+}
+
+impl Default for WithoutSelfCurly {
+    fn default() -> Self {
+        WithoutSelfCurly { a: false }
+    }
+}
+
+struct WithoutSelfParan(bool);
+
+impl Default for WithoutSelfParan {
+    fn default() -> Self {
+        WithoutSelfParan(false)
+    }
+}
+
+fn main() {}

tests/ui/derivable_impls.stderr

@@ -0,0 +1,77 @@
+error: this `impl` can be derived
+  --> $DIR/derivable_impls.rs:18:1
+   |
+LL | / impl std::default::Default for FooDefault<'_> {
+LL | |     fn default() -> Self {
+LL | |         Self {
+LL | |             a: false,
+...  |
+LL | |     }
+LL | | }
+   | |_^
+   |
+   = note: `-D clippy::derivable-impls` implied by `-D warnings`
+   = help: try annotating `FooDefault` with `#[derive(Default)]`
+
+error: this `impl` can be derived
+  --> $DIR/derivable_impls.rs:39:1
+   |
+LL | / impl std::default::Default for TupleDefault {
+LL | |     fn default() -> Self {
+LL | |         Self(false, 0, 0u64)
+LL | |     }
+LL | | }
+   | |_^
+   |
+   = help: try annotating `TupleDefault` with `#[derive(Default)]`
+
+error: this `impl` can be derived
+  --> $DIR/derivable_impls.rs:91:1
+   |
+LL | / impl Default for StrDefault<'_> {
+LL | |     fn default() -> Self {
+LL | |         Self("")
+LL | |     }
+LL | | }
+   | |_^
+   |
+   = help: try annotating `StrDefault` with `#[derive(Default)]`
+
+error: this `impl` can be derived
+  --> $DIR/derivable_impls.rs:117:1
+   |
+LL | / impl Default for Y {
+LL | |     fn default() -> Self {
+LL | |         Self(mac!())
+LL | |     }
+LL | | }
+   | |_^
+   |
+   = help: try annotating `Y` with `#[derive(Default)]`
+
+error: this `impl` can be derived
+  --> $DIR/derivable_impls.rs:156:1
+   |
+LL | / impl Default for WithoutSelfCurly {
+LL | |     fn default() -> Self {
+LL | |         WithoutSelfCurly { a: false }
+LL | |     }
+LL | | }
+   | |_^
+   |
+   = help: try annotating `WithoutSelfCurly` with `#[derive(Default)]`
+
+error: this `impl` can be derived
+  --> $DIR/derivable_impls.rs:164:1
+   |
+LL | / impl Default for WithoutSelfParan {
+LL | |     fn default() -> Self {
+LL | |         WithoutSelfParan(false)
+LL | |     }
+LL | | }
+   | |_^
+   |
+   = help: try annotating `WithoutSelfParan` with `#[derive(Default)]`
+
+error: aborting due to 6 previous errors
+

tests/ui/mem_replace.fixed

@@ -51,9 +51,29 @@ fn replace_with_default() {
 
     let mut binary_heap: BinaryHeap<i32> = BinaryHeap::new();
     let _ = std::mem::take(&mut binary_heap);
+
+    let mut tuple = (vec![1, 2], BinaryHeap::<i32>::new());
+    let _ = std::mem::take(&mut tuple);
+
+    let mut refstr = "hello";
+    let _ = std::mem::take(&mut refstr);
+
+    let mut slice: &[i32] = &[1, 2, 3];
+    let _ = std::mem::take(&mut slice);
+}
+
+// lint is disabled for primitives because in this case `take`
+// has no clear benefit over `replace` and sometimes is harder to read
+fn dont_lint_primitive() {
+    let mut pbool = true;
+    let _ = std::mem::replace(&mut pbool, false);
+
+    let mut pint = 5;
+    let _ = std::mem::replace(&mut pint, 0);
 }
 
 fn main() {
     replace_option_with_none();
     replace_with_default();
+    dont_lint_primitive();
 }

tests/ui/mem_replace.rs

@@ -51,9 +51,29 @@ fn replace_with_default() {
 
     let mut binary_heap: BinaryHeap<i32> = BinaryHeap::new();
     let _ = std::mem::replace(&mut binary_heap, BinaryHeap::new());
+
+    let mut tuple = (vec![1, 2], BinaryHeap::<i32>::new());
+    let _ = std::mem::replace(&mut tuple, (vec![], BinaryHeap::new()));
+
+    let mut refstr = "hello";
+    let _ = std::mem::replace(&mut refstr, "");
+
+    let mut slice: &[i32] = &[1, 2, 3];
+    let _ = std::mem::replace(&mut slice, &[]);
+}
+
+// lint is disabled for primitives because in this case `take`
+// has no clear benefit over `replace` and sometimes is harder to read
+fn dont_lint_primitive() {
+    let mut pbool = true;
+    let _ = std::mem::replace(&mut pbool, false);
+
+    let mut pint = 5;
+    let _ = std::mem::replace(&mut pint, 0);
 }
 
 fn main() {
     replace_option_with_none();
     replace_with_default();
+    dont_lint_primitive();
 }

tests/ui/mem_replace.stderr

@@ -98,5 +98,23 @@ error: replacing a value of type `T` with `T::default()` is better expressed usi
 LL |     let _ = std::mem::replace(&mut binary_heap, BinaryHeap::new());
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using: `std::mem::take(&mut binary_heap)`
 
-error: aborting due to 16 previous errors
+error: replacing a value of type `T` with `T::default()` is better expressed using `std::mem::take`
+  --> $DIR/mem_replace.rs:56:13
+   |
+LL |     let _ = std::mem::replace(&mut tuple, (vec![], BinaryHeap::new()));
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using: `std::mem::take(&mut tuple)`
+
+error: replacing a value of type `T` with `T::default()` is better expressed using `std::mem::take`
+  --> $DIR/mem_replace.rs:59:13
+   |
+LL |     let _ = std::mem::replace(&mut refstr, "");
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using: `std::mem::take(&mut refstr)`
+
+error: replacing a value of type `T` with `T::default()` is better expressed using `std::mem::take`
+  --> $DIR/mem_replace.rs:62:13
+   |
+LL |     let _ = std::mem::replace(&mut slice, &[]);
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using: `std::mem::take(&mut slice)`
+
+error: aborting due to 19 previous errors