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Auto merge of #9511 - rust-lang:box-default, r=Alexendoo
add `box-default` lint This adds a `box-default` lint to suggest using `Box::default()` instead of `Box::new(Default::default())`, which offers less moving parts and potentially better performance according to [the perf book](https://nnethercote.github.io/perf-book/standard-library-types.html#box). --- changelog: add [`box_default`] lint
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commit
9aa85dc35b
11 changed files with 197 additions and 16 deletions
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@ -3759,6 +3759,7 @@ Released 2018-09-13
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[`borrow_interior_mutable_const`]: https://rust-lang.github.io/rust-clippy/master/index.html#borrow_interior_mutable_const
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[`borrowed_box`]: https://rust-lang.github.io/rust-clippy/master/index.html#borrowed_box
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[`box_collection`]: https://rust-lang.github.io/rust-clippy/master/index.html#box_collection
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[`box_default`]: https://rust-lang.github.io/rust-clippy/master/index.html#box_default
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[`box_vec`]: https://rust-lang.github.io/rust-clippy/master/index.html#box_vec
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[`boxed_local`]: https://rust-lang.github.io/rust-clippy/master/index.html#boxed_local
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[`branches_sharing_code`]: https://rust-lang.github.io/rust-clippy/master/index.html#branches_sharing_code
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61
clippy_lints/src/box_default.rs
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61
clippy_lints/src/box_default.rs
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@ -0,0 +1,61 @@
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use clippy_utils::{diagnostics::span_lint_and_help, is_default_equivalent, path_def_id};
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use rustc_hir::{Expr, ExprKind, QPath};
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use rustc_lint::{LateContext, LateLintPass, LintContext};
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use rustc_middle::lint::in_external_macro;
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use rustc_session::{declare_lint_pass, declare_tool_lint};
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use rustc_span::sym;
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declare_clippy_lint! {
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/// ### What it does
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/// checks for `Box::new(T::default())`, which is better written as
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/// `Box::<T>::default()`.
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///
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/// ### Why is this bad?
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/// First, it's more complex, involving two calls instead of one.
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/// Second, `Box::default()` can be faster
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/// [in certain cases](https://nnethercote.github.io/perf-book/standard-library-types.html#box).
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///
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/// ### Known problems
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/// The lint may miss some cases (e.g. Box::new(String::from(""))).
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/// On the other hand, it will trigger on cases where the `default`
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/// code comes from a macro that does something different based on
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/// e.g. target operating system.
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///
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/// ### Example
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/// ```rust
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/// let x: Box<String> = Box::new(Default::default());
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/// ```
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/// Use instead:
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/// ```rust
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/// let x: Box<String> = Box::default();
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/// ```
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#[clippy::version = "1.65.0"]
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pub BOX_DEFAULT,
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perf,
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"Using Box::new(T::default()) instead of Box::default()"
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}
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declare_lint_pass!(BoxDefault => [BOX_DEFAULT]);
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impl LateLintPass<'_> for BoxDefault {
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fn check_expr(&mut self, cx: &LateContext<'_>, expr: &Expr<'_>) {
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if let ExprKind::Call(box_new, [arg]) = expr.kind
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&& let ExprKind::Path(QPath::TypeRelative(ty, seg)) = box_new.kind
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&& let ExprKind::Call(..) = arg.kind
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&& !in_external_macro(cx.sess(), expr.span)
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&& expr.span.eq_ctxt(arg.span)
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&& seg.ident.name == sym::new
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&& path_def_id(cx, ty) == cx.tcx.lang_items().owned_box()
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&& is_default_equivalent(cx, arg)
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{
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span_lint_and_help(
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cx,
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BOX_DEFAULT,
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expr.span,
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"`Box::new(_)` of default value",
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None,
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"use `Box::default()` instead",
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);
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}
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}
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}
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@ -21,6 +21,7 @@ store.register_group(true, "clippy::all", Some("clippy_all"), vec.
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### Known problems
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The lint may miss some cases (e.g. Box::new(String::from(""))).
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On the other hand, it will trigger on cases where the `default`
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code comes from a macro that does something different based on
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e.g. target operating system.
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### Example
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```
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let x: Box<String> = Box::new(Default::default());
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```
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Use instead:
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```
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let x: Box<String> = Box::default();
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```
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@ -15,7 +15,7 @@ macro_rules! boxit {
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}
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fn test_macro() {
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boxit!(Vec::new(), Vec<u8>);
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boxit!(vec![1], Vec<u8>);
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}
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fn test1(foo: Box<Vec<bool>>) {}
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@ -50,7 +50,7 @@ fn test_local_not_linted() {
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pub fn pub_test(foo: Box<Vec<bool>>) {}
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pub fn pub_test_ret() -> Box<Vec<bool>> {
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Box::new(Vec::new())
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Box::default()
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}
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fn main() {}
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31
tests/ui/box_default.rs
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31
tests/ui/box_default.rs
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@ -0,0 +1,31 @@
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#![warn(clippy::box_default)]
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#[derive(Default)]
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struct ImplementsDefault;
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struct OwnDefault;
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impl OwnDefault {
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fn default() -> Self {
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Self
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}
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}
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macro_rules! outer {
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($e: expr) => {
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$e
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};
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}
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fn main() {
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let _string: Box<String> = Box::new(Default::default());
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let _byte = Box::new(u8::default());
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let _vec = Box::new(Vec::<u8>::new());
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let _impl = Box::new(ImplementsDefault::default());
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let _impl2 = Box::new(<ImplementsDefault as Default>::default());
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let _impl3: Box<ImplementsDefault> = Box::new(Default::default());
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let _own = Box::new(OwnDefault::default()); // should not lint
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let _in_macro = outer!(Box::new(String::new()));
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// false negative: default is from different expansion
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let _vec2: Box<Vec<ImplementsDefault>> = Box::new(vec![]);
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}
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59
tests/ui/box_default.stderr
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59
tests/ui/box_default.stderr
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@ -0,0 +1,59 @@
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error: `Box::new(_)` of default value
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--> $DIR/box_default.rs:21:32
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LL | let _string: Box<String> = Box::new(Default::default());
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| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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= note: `-D clippy::box-default` implied by `-D warnings`
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= help: use `Box::default()` instead
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error: `Box::new(_)` of default value
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--> $DIR/box_default.rs:22:17
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LL | let _byte = Box::new(u8::default());
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| ^^^^^^^^^^^^^^^^^^^^^^^
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= help: use `Box::default()` instead
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error: `Box::new(_)` of default value
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--> $DIR/box_default.rs:23:16
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LL | let _vec = Box::new(Vec::<u8>::new());
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| ^^^^^^^^^^^^^^^^^^^^^^^^^^
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= help: use `Box::default()` instead
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error: `Box::new(_)` of default value
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--> $DIR/box_default.rs:24:17
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LL | let _impl = Box::new(ImplementsDefault::default());
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| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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= help: use `Box::default()` instead
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error: `Box::new(_)` of default value
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--> $DIR/box_default.rs:25:18
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LL | let _impl2 = Box::new(<ImplementsDefault as Default>::default());
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| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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= help: use `Box::default()` instead
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error: `Box::new(_)` of default value
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--> $DIR/box_default.rs:26:42
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LL | let _impl3: Box<ImplementsDefault> = Box::new(Default::default());
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| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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= help: use `Box::default()` instead
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error: `Box::new(_)` of default value
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--> $DIR/box_default.rs:28:28
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LL | let _in_macro = outer!(Box::new(String::new()));
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| ^^^^^^^^^^^^^^^^^^^^^^^
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= help: use `Box::default()` instead
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error: aborting due to 7 previous errors
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