Clippy lints / Pedantic
Clippy lint · Pedantic · checked with Clippy 0.1.99
clippy::mismatching_type_param_order: `Foo` has a similarly named generic type parameter `B` in its declaration, but in a different order
What it meansChecks for type parameters which are positioned inconsistently between a type definition and impl block. Specifically, a parameter in an impl block which has the same name as a parameter in the type def, but is in a different place. It is in the pedantic group and off by default.
What you see
warning: `Foo` has a similarly named generic type parameter `B` in its declaration, but in a different order --> main.rs:8:16 | 8 | impl<B, A> Foo<B, A> {} | ^ | = help: try `A`, or a name that does not conflict with `Foo`'s generic params = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#mismatching_type_param_order = note: `-W clippy::mismatching-type-param-order` implied by `-W clippy::pedantic`
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?
Type parameters are determined by their position rather than name. Naming type parameters inconsistently may cause you to refer to the wrong type parameter.
Stricter lints with some false positives. Off by default; opt in with clippy::pedantic.
How to fix it
Clippy does not fix this one automatically. Change the code by hand:
Flagged
struct Foo<A, B> {
x: A,
y: B,
}
// inside the impl, B refers to Foo::A
impl<B, A> Foo<B, A> {}
Write this instead
struct Foo<A, B> {
x: A,
y: B,
}
impl<A, B> Foo<A, B> {}
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]
mismatching_type_param_order = "warn"In code
#![warn(clippy::mismatching_type_param_order)]One run
cargo clippy -- -W clippy::mismatching-type-param-orderThe [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 type parameters which are positioned inconsistently between a type definition and impl block. Specifically, a parameter in an impl block which has the same name as a parameter in the type def, but is in a different place.
Why is this bad?
Type parameters are determined by their position rather than name. Naming type parameters inconsistently may cause you to refer to the wrong type parameter.
Limitations
This lint only applies to impl blocks with simple generic params, e.g.
A. If there is anything more complicated, such as a tuple, it will be
ignored.
Example
struct Foo<A, B> {
x: A,
y: B,
}
// inside the impl, B refers to Foo::A
impl<B, A> Foo<B, A> {}
Use instead:
struct Foo<A, B> {
x: A,
y: B,
}
impl<A, B> Foo<A, B> {}
From the Clippy lint list (0.1.99), part of the Rust project, MIT or Apache-2.0. Official entry.
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All 847 Clippy lints · Rust compiler errors · Cargo.toml checker
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