Clippy lint · Nursery · checked with Clippy 0.1.99

clippy::unnecessary_struct_initialization: unnecessary struct building

What it meansChecks for initialization of an identical struct from another instance of the type, either by copying a base without setting any field or by moving all fields individually. It is in the nursery group and off by default.

What you see

warning: unnecessary struct building
 --> main.rs:6:9
  |
6 | let b = S { ..a };
  |         ^^^^^^^^^ help: replace with: `a`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#unnecessary_struct_initialization
  = note: `-W clippy::unnecessary-struct-initialization` implied by `-W clippy::nursery`

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?

Readability suffers from unnecessary struct building.

New lints still being finished. Off by default and may have false positives.

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

struct S { s: String }

let a = S { s: String::from("Hello, world!") };
let b = S { ..a };

Write this instead

struct S { s: String }

let a = S { s: String::from("Hello, world!") };
let b = a;

Turn it off, or on

It is off by default. To use it, set it to warn or deny:

Whole crate (Cargo.toml)

[lints.clippy]
unnecessary_struct_initialization = "warn"

In code

#![warn(clippy::unnecessary_struct_initialization)]

One run

cargo clippy -- -W clippy::unnecessary-struct-initialization

The [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.

Nurserygroup

allowdefault level

yesfixed by cargo clippy --fix

1.70.0added in Rust

Clippy's own documentation

What it does

Checks for initialization of an identical struct from another instance of the type, either by copying a base without setting any field or by moving all fields individually.

Why is this bad?

Readability suffers from unnecessary struct building.

Example

struct S { s: String }

let a = S { s: String::from("Hello, world!") };
let b = S { ..a };

Use instead:

struct S { s: String }

let a = S { s: String::from("Hello, world!") };
let b = a;

The struct literal S { ..a } in the assignment to b could be replaced with just a.

Known Problems

Has false positives when the base is a place expression that cannot be moved out of, see #10547.

Empty structs are ignored by the lint.

From the Clippy lint list (0.1.99), part of the Rust project, MIT or Apache-2.0. Official entry.

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