Use `mem::take` instead of `mem::replace` when applicable

`std::mem::take` can be used to replace a value of type `T`
with `T::default()` instead of `std::mem::replace`.

7 files changed, 189 insertions, 69 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 50a7f44ad8e..05d437d3598 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1173,6 +1173,7 @@ Released 2018-09-13
 [`mem_discriminant_non_enum`]: https://rust-lang.github.io/rust-clippy/master/index.html#mem_discriminant_non_enum
 [`mem_forget`]: https://rust-lang.github.io/rust-clippy/master/index.html#mem_forget
 [`mem_replace_option_with_none`]: https://rust-lang.github.io/rust-clippy/master/index.html#mem_replace_option_with_none
+[`mem_replace_with_default`]: https://rust-lang.github.io/rust-clippy/master/index.html#mem_replace_with_default
 [`mem_replace_with_uninit`]: https://rust-lang.github.io/rust-clippy/master/index.html#mem_replace_with_uninit
 [`min_max`]: https://rust-lang.github.io/rust-clippy/master/index.html#min_max
 [`misaligned_transmute`]: https://rust-lang.github.io/rust-clippy/master/index.html#misaligned_transmute
diff --git a/clippy_lints/src/lib.rs b/clippy_lints/src/lib.rs
index 49101695a58..abcddc88527 100644
--- a/clippy_lints/src/lib.rs
+++ b/clippy_lints/src/lib.rs
@@ -599,6 +599,7 @@ pub fn register_plugins(store: &mut lint::LintStore, sess: &Session, conf: &Conf
         &mem_discriminant::MEM_DISCRIMINANT_NON_ENUM,
         &mem_forget::MEM_FORGET,
         &mem_replace::MEM_REPLACE_OPTION_WITH_NONE,
+        &mem_replace::MEM_REPLACE_WITH_DEFAULT,
         &mem_replace::MEM_REPLACE_WITH_UNINIT,
         &methods::CHARS_LAST_CMP,
         &methods::CHARS_NEXT_CMP,
@@ -1594,6 +1595,7 @@ pub fn register_plugins(store: &mut lint::LintStore, sess: &Session, conf: &Conf
     store.register_group(true, "clippy::nursery", Some("clippy_nursery"), vec![
         LintId::of(&attrs::EMPTY_LINE_AFTER_OUTER_ATTR),
         LintId::of(&fallible_impl_from::FALLIBLE_IMPL_FROM),
+        LintId::of(&mem_replace::MEM_REPLACE_WITH_DEFAULT),
         LintId::of(&missing_const_for_fn::MISSING_CONST_FOR_FN),
         LintId::of(&mul_add::MANUAL_MUL_ADD),
         LintId::of(&mutex_atomic::MUTEX_INTEGER),
diff --git a/clippy_lints/src/mem_replace.rs b/clippy_lints/src/mem_replace.rs
index b7b0538cbae..5ad41a53b89 100644
--- a/clippy_lints/src/mem_replace.rs
+++ b/clippy_lints/src/mem_replace.rs
@@ -3,7 +3,7 @@ use crate::utils::{
 };
 use if_chain::if_chain;
 use rustc::declare_lint_pass;
-use rustc::hir::{BorrowKind, Expr, ExprKind, Mutability, QPath};
+use rustc::hir::{BorrowKind, Expr, ExprKind, HirVec, Mutability, QPath};
 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
 use rustc_errors::Applicability;
 use rustc_session::declare_tool_lint;
@@ -67,8 +67,127 @@ declare_clippy_lint! {
     "`mem::replace(&mut _, mem::uninitialized())` or `mem::replace(&mut _, mem::zeroed())`"
 }
 
+declare_clippy_lint! {
+    /// **What it does:** Checks for `std::mem::replace` on a value of type
+    /// `T` with `T::default()`.
+    ///
+    /// **Why is this bad?** `std::mem` module already has the method `take` to
+    /// take the current value and replace it with the default value of that type.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    /// ```rust
+    /// let mut text = String::from("foo");
+    /// let replaced = std::mem::replace(&mut text, String::default());
+    /// ```
+    /// Is better expressed with:
+    /// ```rust
+    /// let mut text = String::from("foo");
+    /// let taken = std::mem::take(&mut text);
+    /// ```
+    pub MEM_REPLACE_WITH_DEFAULT,
+    nursery,
+    "replacing a value of type `T` with `T::default()` instead of using `std::mem::take`"
+}
+
 declare_lint_pass!(MemReplace =>
-    [MEM_REPLACE_OPTION_WITH_NONE, MEM_REPLACE_WITH_UNINIT]);
+    [MEM_REPLACE_OPTION_WITH_NONE, MEM_REPLACE_WITH_UNINIT, MEM_REPLACE_WITH_DEFAULT]);
+
+fn check_replace_option_with_none(cx: &LateContext<'_, '_>, expr: &'_ Expr, args: &HirVec<Expr>) {
+    if let ExprKind::Path(ref replacement_qpath) = args[1].kind {
+        // Check that second argument is `Option::None`
+        if match_qpath(replacement_qpath, &paths::OPTION_NONE) {
+            // Since this is a late pass (already type-checked),
+            // and we already know that the second argument is an
+            // `Option`, we do not need to check the first
+            // argument's type. All that's left is to get
+            // replacee's path.
+            let replaced_path = match args[0].kind {
+                ExprKind::AddrOf(BorrowKind::Ref, Mutability::Mut, ref replaced) => {
+                    if let ExprKind::Path(QPath::Resolved(None, ref replaced_path)) = replaced.kind {
+                        replaced_path
+                    } else {
+                        return;
+                    }
+                },
+                ExprKind::Path(QPath::Resolved(None, ref replaced_path)) => replaced_path,
+                _ => return,
+            };
+
+            let mut applicability = Applicability::MachineApplicable;
+            span_lint_and_sugg(
+                cx,
+                MEM_REPLACE_OPTION_WITH_NONE,
+                expr.span,
+                "replacing an `Option` with `None`",
+                "consider `Option::take()` instead",
+                format!(
+                    "{}.take()",
+                    snippet_with_applicability(cx, replaced_path.span, "", &mut applicability)
+                ),
+                applicability,
+            );
+        }
+    }
+}
+
+fn check_replace_with_uninit(cx: &LateContext<'_, '_>, expr: &'_ Expr, args: &HirVec<Expr>) {
+    if let ExprKind::Call(ref repl_func, ref repl_args) = args[1].kind {
+        if_chain! {
+            if repl_args.is_empty();
+            if let ExprKind::Path(ref repl_func_qpath) = repl_func.kind;
+            if let Some(repl_def_id) = cx.tables.qpath_res(repl_func_qpath, repl_func.hir_id).opt_def_id();
+            then {
+                if match_def_path(cx, repl_def_id, &paths::MEM_UNINITIALIZED) {
+                    span_help_and_lint(
+                        cx,
+                        MEM_REPLACE_WITH_UNINIT,
+                        expr.span,
+                        "replacing with `mem::uninitialized()`",
+                        "consider using the `take_mut` crate instead",
+                    );
+                } else if match_def_path(cx, repl_def_id, &paths::MEM_ZEROED) &&
+                        !cx.tables.expr_ty(&args[1]).is_primitive() {
+                    span_help_and_lint(
+                        cx,
+                        MEM_REPLACE_WITH_UNINIT,
+                        expr.span,
+                        "replacing with `mem::zeroed()`",
+                        "consider using a default value or the `take_mut` crate instead",
+                    );
+                }
+            }
+        }
+    }
+}
+
+fn check_replace_with_default(cx: &LateContext<'_, '_>, expr: &'_ Expr, args: &HirVec<Expr>) {
+    if let ExprKind::Call(ref repl_func, ref repl_args) = args[1].kind {
+        if_chain! {
+            if repl_args.is_empty();
+            if let ExprKind::Path(ref repl_func_qpath) = repl_func.kind;
+            if let Some(repl_def_id) = cx.tables.qpath_res(repl_func_qpath, repl_func.hir_id).opt_def_id();
+            if match_def_path(cx, repl_def_id, &paths::DEFAULT_TRAIT_METHOD);
+            then {
+                let mut applicability = Applicability::MachineApplicable;
+
+                span_lint_and_sugg(
+                    cx,
+                    MEM_REPLACE_WITH_DEFAULT,
+                    expr.span,
+                    "replacing a value of type `T` with `T::default()` is better expressed using `std::mem::take`",
+                    "consider using",
+                    format!(
+                        "std::mem::take({})",
+                        snippet_with_applicability(cx, args[0].span, "", &mut applicability)
+                    ),
+                    applicability,
+                );
+            }
+        }
+    }
+}
 
 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for MemReplace {
     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr<'_>) {
@@ -80,67 +199,10 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for MemReplace {
             if let Some(def_id) = cx.tables.qpath_res(func_qpath, func.hir_id).opt_def_id();
             if match_def_path(cx, def_id, &paths::MEM_REPLACE);
 
-            // Check that second argument is `Option::None`
             then {
-                if let ExprKind::Path(ref replacement_qpath) = func_args[1].kind {
-                    if match_qpath(replacement_qpath, &paths::OPTION_NONE) {
-
-                        // Since this is a late pass (already type-checked),
-                        // and we already know that the second argument is an
-                        // `Option`, we do not need to check the first
-                        // argument's type. All that's left is to get
-                        // replacee's path.
-                        let replaced_path = match func_args[0].kind {
-                            ExprKind::AddrOf(BorrowKind::Ref, Mutability::Mut, ref replaced) => {
-                                if let ExprKind::Path(QPath::Resolved(None, ref replaced_path)) = replaced.kind {
-                                    replaced_path
-                                } else {
-                                    return
-                                }
-                            },
-                            ExprKind::Path(QPath::Resolved(None, ref replaced_path)) => replaced_path,
-                            _ => return,
-                        };
-
-                        let mut applicability = Applicability::MachineApplicable;
-                        span_lint_and_sugg(
-                            cx,
-                            MEM_REPLACE_OPTION_WITH_NONE,
-                            expr.span,
-                            "replacing an `Option` with `None`",
-                            "consider `Option::take()` instead",
-                            format!("{}.take()", snippet_with_applicability(cx, replaced_path.span, "", &mut applicability)),
-                            applicability,
-                        );
-                    }
-                }
-                if let ExprKind::Call(ref repl_func, ref repl_args) = func_args[1].kind {
-                    if_chain! {
-                        if repl_args.is_empty();
-                        if let ExprKind::Path(ref repl_func_qpath) = repl_func.kind;
-                        if let Some(repl_def_id) = cx.tables.qpath_res(repl_func_qpath, repl_func.hir_id).opt_def_id();
-                        then {
-                            if match_def_path(cx, repl_def_id, &paths::MEM_UNINITIALIZED) {
-                                span_help_and_lint(
-                                    cx,
-                                    MEM_REPLACE_WITH_UNINIT,
-                                    expr.span,
-                                    "replacing with `mem::uninitialized()`",
-                                    "consider using the `take_mut` crate instead",
-                                );
-                            } else if match_def_path(cx, repl_def_id, &paths::MEM_ZEROED) &&
-                                    !cx.tables.expr_ty(&func_args[1]).is_primitive() {
-                                span_help_and_lint(
-                                    cx,
-                                    MEM_REPLACE_WITH_UNINIT,
-                                    expr.span,
-                                    "replacing with `mem::zeroed()`",
-                                    "consider using a default value or the `take_mut` crate instead",
-                                );
-                            }
-                        }
-                    }
-                }
+                check_replace_option_with_none(cx, expr, &func_args);
+                check_replace_with_uninit(cx, expr, &func_args);
+                check_replace_with_default(cx, expr, &func_args);
             }
         }
     }
diff --git a/src/lintlist/mod.rs b/src/lintlist/mod.rs
index 08cbff40444..4b820250c8d 100644
--- a/src/lintlist/mod.rs
+++ b/src/lintlist/mod.rs
@@ -1100,6 +1100,13 @@ pub const ALL_LINTS: [Lint; 342] = [
         module: "mem_replace",
     },
     Lint {
+        name: "mem_replace_with_default",
+        group: "nursery",
+        desc: "replacing a value of type `T` with `T::default()` instead of using `std::mem::take`",
+        deprecation: None,
+        module: "mem_replace",
+    },
+    Lint {
         name: "mem_replace_with_uninit",
         group: "correctness",
         desc: "`mem::replace(&mut _, mem::uninitialized())` or `mem::replace(&mut _, mem::zeroed())`",
diff --git a/tests/ui/mem_replace.fixed b/tests/ui/mem_replace.fixed
index 4e47ac95d82..19205dc3822 100644
--- a/tests/ui/mem_replace.fixed
+++ b/tests/ui/mem_replace.fixed
@@ -9,13 +9,30 @@
 
 // run-rustfix
 #![allow(unused_imports)]
-#![warn(clippy::all, clippy::style, clippy::mem_replace_option_with_none)]
+#![warn(
+    clippy::all,
+    clippy::style,
+    clippy::mem_replace_option_with_none,
+    clippy::mem_replace_with_default
+)]
 
 use std::mem;
 
-fn main() {
+fn replace_option_with_none() {
     let mut an_option = Some(1);
     let _ = an_option.take();
     let an_option = &mut Some(1);
     let _ = an_option.take();
 }
+
+fn replace_with_default() {
+    let mut s = String::from("foo");
+    let _ = std::mem::take(&mut s);
+    let s = &mut String::from("foo");
+    let _ = std::mem::take(s);
+}
+
+fn main() {
+    replace_option_with_none();
+    replace_with_default();
+}
diff --git a/tests/ui/mem_replace.rs b/tests/ui/mem_replace.rs
index 6824ab18e7f..97ac283abc6 100644
--- a/tests/ui/mem_replace.rs
+++ b/tests/ui/mem_replace.rs
@@ -9,13 +9,30 @@
 
 // run-rustfix
 #![allow(unused_imports)]
-#![warn(clippy::all, clippy::style, clippy::mem_replace_option_with_none)]
+#![warn(
+    clippy::all,
+    clippy::style,
+    clippy::mem_replace_option_with_none,
+    clippy::mem_replace_with_default
+)]
 
 use std::mem;
 
-fn main() {
+fn replace_option_with_none() {
     let mut an_option = Some(1);
     let _ = mem::replace(&mut an_option, None);
     let an_option = &mut Some(1);
     let _ = mem::replace(an_option, None);
 }
+
+fn replace_with_default() {
+    let mut s = String::from("foo");
+    let _ = std::mem::replace(&mut s, String::default());
+    let s = &mut String::from("foo");
+    let _ = std::mem::replace(s, String::default());
+}
+
+fn main() {
+    replace_option_with_none();
+    replace_with_default();
+}
diff --git a/tests/ui/mem_replace.stderr b/tests/ui/mem_replace.stderr
index 791c4d71dbf..44495a973c8 100644
--- a/tests/ui/mem_replace.stderr
+++ b/tests/ui/mem_replace.stderr
@@ -1,5 +1,5 @@
 error: replacing an `Option` with `None`
-  --> $DIR/mem_replace.rs:18:13
+  --> $DIR/mem_replace.rs:23:13
    |
 LL |     let _ = mem::replace(&mut an_option, None);
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider `Option::take()` instead: `an_option.take()`
@@ -7,10 +7,24 @@ LL |     let _ = mem::replace(&mut an_option, None);
    = note: `-D clippy::mem-replace-option-with-none` implied by `-D warnings`
 
 error: replacing an `Option` with `None`
-  --> $DIR/mem_replace.rs:20:13
+  --> $DIR/mem_replace.rs:25:13
    |
 LL |     let _ = mem::replace(an_option, None);
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider `Option::take()` instead: `an_option.take()`
 
-error: aborting due to 2 previous errors
+error: replacing a value of type `T` with `T::default()` is better expressed using `std::mem::take`
+  --> $DIR/mem_replace.rs:30:13
+   |
+LL |     let _ = std::mem::replace(&mut s, String::default());
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using: `std::mem::take(&mut s)`
+   |
+   = note: `-D clippy::mem-replace-with-default` implied by `-D warnings`
+
+error: replacing a value of type `T` with `T::default()` is better expressed using `std::mem::take`
+  --> $DIR/mem_replace.rs:32:13
+   |
+LL |     let _ = std::mem::replace(s, String::default());
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using: `std::mem::take(s)`
+
+error: aborting due to 4 previous errors