Clippy lint · Complexity · checked with Clippy 0.1.99

clippy::only_used_in_recursion: parameter is only used in recursion

What it meansChecks for arguments that are only used in recursion with no side-effects. It is in the complexity group and warns by default.

What you see

warning: parameter is only used in recursion
 --> main.rs:3:16
  |
3 | fn f(a: usize, b: usize) -> usize {
  |                ^ help: if this is intentional, prefix it with an underscore: `_b`
  |
note: parameter used here
 --> main.rs:7:18
  |
7 |         f(a - 1, b + 1)
  |                  ^
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#only_used_in_recursion
  = note: `#[warn(clippy::only_used_in_recursion)]` 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?

It could contain a useless calculation and can make function simpler. The arguments can be involved in calculations and assignments but as long as the calculations have no side-effects (function calls or mutating dereference) and the assigned variables are also only in recursion, it is useless.

Code that does something simple in a complicated way. 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

fn f(a: usize, b: usize) -> usize {
    if a == 0 {
        1
    } else {
        f(a - 1, b + 1)
    }
}

Write this instead

fn f(a: usize) -> usize {
    if a == 0 {
        1
    } else {
        f(a - 1)
    }
}

Turn it off, or on

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

On one item

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

Whole crate (Cargo.toml)

[lints.clippy]
only_used_in_recursion = "allow"

One run

cargo clippy -- -A clippy::only-used-in-recursion

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.

Complexitygroup

warndefault level

nofixed by cargo clippy --fix

1.61.0added in Rust

Clippy's own documentation

What it does

Checks for arguments that are only used in recursion with no side-effects.

Why is this bad?

It could contain a useless calculation and can make function simpler.

The arguments can be involved in calculations and assignments but as long as the calculations have no side-effects (function calls or mutating dereference) and the assigned variables are also only in recursion, it is useless.

Example

fn f(a: usize, b: usize) -> usize {
    if a == 0 {
        1
    } else {
        f(a - 1, b + 1)
    }
}

Use instead:

fn f(a: usize) -> usize {
    if a == 0 {
        1
    } else {
        f(a - 1)
    }
}

Known problems

Too many code paths in the linting code are currently untested and prone to produce false positives or are prone to have performance implications.

In some cases, this would not catch all useless arguments.

fn foo(a: usize, b: usize) -> usize {
    let f = |x| x + 1;

    if a == 0 {
        1
    } else {
        foo(a - 1, f(b))
    }
}

For example, the argument b is only used in recursion, but the lint would not catch it.

List of some examples that can not be caught:

binary operation of non-primitive types
closure usage
some break relative operations
struct pattern binding

Also, when you recurse the function name with path segments, it is not possible to detect.

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

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