clippy::implied_bounds_in_impls: Looks for bounds in impl Trait in return position that are implied by other bounds. This can happen when a trait is specified that another trait already has as a supertrait (e.g

fn f() -> impl Deref<Target = i32> + DerefMut<Target = i32> {
//             ^^^^^^^^^^^^^^^^^^^ unnecessary bound, already implied by the `DerefMut` trait bound
    Box::new(123)
}

fn f() -> impl DerefMut<Target = i32> {
    Box::new(123)
}

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

[lints.clippy]
implied_bounds_in_impls = "allow"

cargo clippy -- -A clippy::implied-bounds-in-impls

Clippy's own documentation

What it does

Looks for bounds in impl Trait in return position that are implied by other bounds. This can happen when a trait is specified that another trait already has as a supertrait (e.g. fn() -> impl Deref + DerefMut<Target = i32> has an unnecessary Deref bound, because Deref is a supertrait of DerefMut)

Why is this bad?

Specifying more bounds than necessary adds needless complexity for the reader.

Limitations

This lint does not check for implied bounds transitively. Meaning that it doesn’t check for implied bounds from supertraits of supertraits (e.g. trait A {} trait B: A {} trait C: B {}, then having an fn() -> impl A + C)

Example

fn f() -> impl Deref<Target = i32> + DerefMut<Target = i32> {
//             ^^^^^^^^^^^^^^^^^^^ unnecessary bound, already implied by the `DerefMut` trait bound
    Box::new(123)
}

Use instead:

fn f() -> impl DerefMut<Target = i32> {
    Box::new(123)
}

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

Related lints

• too_many_argumentsChecks for functions with too many parameters
• clone_on_copyusing `clone` on type `u64` which implements the `Copy` trait
• type_complexityChecks for types used in structs, parameters and let declarations above a certain complexity threshold
• flat_map_identityChecks for usage of flat_map(|x| x)
• get_last_with_lenaccessing last element with `x.get(x.len() - 1)`
• identity_opChecks for identity operations, e.g., x + 0
• inspect_for_eachcalled `inspect(..).for_each(..)` on an `Iterator`
• int_plus_oneunnecessary `>= y + 1` or `x - 1 >=`
• iter_countcalled `.iter().count()` on a `Vec`

All 847 Clippy lints · Rust compiler errors · Cargo.toml checker