Clippy lints / Style
Clippy lint · Style · checked with Clippy 0.1.99
clippy::box_default: `Box::new(_)` of default value
What it meanschecks for Box::new(Default::default()), which can be written as Box::default(). It is in the style group and warns by default.
What you see
warning: `Box::new(_)` of default value --> main.rs:3:22 | 3 | let x: Box<String> = Box::new(Default::default()); | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `Box::default()` | = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#box_default = note: `#[warn(clippy::box_default)]` on by default
Real output of Clippy 0.1.99 (Rust 1.99) on the example below. Names and line numbers in yours will differ.
Does it matter?
Box::default() is equivalent and more concise.
Code that works but is not idiomatic Rust. Warns by default.
How to fix it
Clippy can fix this one for you: run cargo clippy --fix (add --allow-dirty if you have uncommitted changes). Review the diff before you commit.
Flagged
let x: Box<String> = Box::new(Default::default());
Write this instead
let x: Box<String> = Box::default();
Turn it off, or on
Allow it where it does not fit your code, as narrowly as you can:
On one item
#[allow(clippy::box_default)]
fn my_function() { /* ... */ }Whole crate (Cargo.toml)
[lints.clippy]
box_default = "allow"One run
cargo clippy -- -A clippy::box-defaultThe [lints] table needs Rust 1.74 or newer. In a workspace, put it under [workspace.lints.clippy] and add [lints] workspace = true to each member.
Clippy's own documentation
What it does
checks for Box::new(Default::default()), which can be written as
Box::default().
Why is this bad?
Box::default() is equivalent and more concise.
Example
let x: Box<String> = Box::new(Default::default());
Use instead:
let x: Box<String> = Box::default();
From the Clippy lint list (0.1.99), part of the Rust project, MIT or Apache-2.0. Official entry.
Related lints
- needless_returnunneeded `return` statement
- new_without_defaultChecks for public types with a pub fn new() -> Self method and no implementation of Default
- len_zeroChecks for getting the length of something via .len() just to compare to zero, and suggests using .is_empty() where applicable
- blocks_in_conditionsChecks for if and match conditions that use blocks containing an expression, statements or conditions that use closures with blocks
- bool_assert_comparisonused `assert_eq!` with a literal bool
- borrow_interior_mutable_constborrow of a named constant with interior mutability
- builtin_type_shadowthis generic shadows the built-in type `u32`
- byte_char_slicescan be more succinctly written as a byte str
- bytes_nthcalled `.bytes().nth()` on a `str`
All 847 Clippy lints · Rust compiler errors · Cargo.toml checker
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