Clippy lint · Suspicious · checked with Clippy 0.1.99

clippy::needless_maybe_sized: `?Sized` bound is ignored because of a `Sized` requirement

What it meansLints ?Sized bounds applied to type parameters that cannot be unsized It is in the suspicious group and warns by default.

What you see

warning: `?Sized` bound is ignored because of a `Sized` requirement
 --> main.rs:4:17
  |
4 | fn f<T: Clone + ?Sized>(t: &T) {}
  |                 ^^^^^^
  |
note: `T` cannot be unsized because of the bound
 --> main.rs:4:9
  |
4 | fn f<T: Clone + ?Sized>(t: &T) {}
  |         ^^^^^
  = note: ...because `Clone` has the bound `Sized`
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#needless_maybe_sized
  = note: `#[warn(clippy::needless_maybe_sized)]` on by default
help: change the bounds that require `Sized`, or remove the `?Sized` bound
  |
4 - fn f<T: Clone + ?Sized>(t: &T) {}
4 + fn f<T: Clone>(t: &T) {}
  |

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?

The ?Sized bound is misleading because it cannot be satisfied by an unsized type

Code that is most likely wrong or useless. Warns by default.

How to fix it

Clippy does not fix this one automatically (its suggestion may change behaviour, so it only shows it). Change the code by hand:

Flagged

// `T` cannot be unsized because `Clone` requires it to be `Sized`
fn f<T: Clone + ?Sized>(t: &T) {}

Write this instead

fn f<T: Clone>(t: &T) {}

// or choose alternative bounds for `T` so that it can be unsized

Turn it off, or on

Allow it where it does not fit your code, as narrowly as you can:

On one item

#[allow(clippy::needless_maybe_sized)]
fn my_function() { /* ... */ }

Whole crate (Cargo.toml)

[lints.clippy]
needless_maybe_sized = "allow"

One run

cargo clippy -- -A clippy::needless-maybe-sized

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.

Suspiciousgroup

warndefault level

nofixed by cargo clippy --fix

1.81.0added in Rust

Clippy's own documentation

What it does

Lints ?Sized bounds applied to type parameters that cannot be unsized

Why is this bad?

The ?Sized bound is misleading because it cannot be satisfied by an unsized type

Example

// `T` cannot be unsized because `Clone` requires it to be `Sized`
fn f<T: Clone + ?Sized>(t: &T) {}

Use instead:

fn f<T: Clone>(t: &T) {}

// or choose alternative bounds for `T` so that it can be unsized

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

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