Clippy lint · Suspicious · checked with Clippy 0.1.99

clippy::unconditional_recursion: function cannot return without recursing

What it meansChecks that there isn’t an infinite recursion in trait implementations. It is in the suspicious group and warns by default.

What you see

warning: function cannot return without recursing
  --> main.rs:9:5
   |
 9 | /     fn eq(&self, other: &Self) -> bool {
10 | |         self == other // bad!
11 | |     }
   | |_____^
   |
note: recursive call site
  --> main.rs:10:9
   |
10 |         self == other // bad!
   |         ^^^^^^^^^^^^^
   = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#unconditional_recursion
   = note: `#[warn(clippy::unconditional_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?

Infinite recursion in trait implementation will either cause crashes or result in an infinite loop, and it is hard to detect.

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

How to fix it

Clippy does not fix this one automatically. Change the code by hand:

Flagged

enum Foo {
    A,
    B,
}

impl PartialEq for Foo {
    fn eq(&self, other: &Self) -> bool {
        self == other // bad!
    }
}

Write this instead

#[derive(PartialEq)]
enum Foo {
    A,
    B,
}

Turn it off, or on

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

On one item

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

Whole crate (Cargo.toml)

[lints.clippy]
unconditional_recursion = "allow"

One run

cargo clippy -- -A clippy::unconditional-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.

Suspiciousgroup

warndefault level

nofixed by cargo clippy --fix

1.77.0added in Rust

Clippy's own documentation

What it does

Checks that there isn’t an infinite recursion in trait implementations.

Why is this bad?

Infinite recursion in trait implementation will either cause crashes or result in an infinite loop, and it is hard to detect.

Example

enum Foo {
    A,
    B,
}

impl PartialEq for Foo {
    fn eq(&self, other: &Self) -> bool {
        self == other // bad!
    }
}

Use instead:

#[derive(PartialEq)]
enum Foo {
    A,
    B,
}

As an alternative, rewrite the logic without recursion:

enum Foo {
    A,
    B,
}

impl PartialEq for Foo {
    fn eq(&self, other: &Self) -> bool {
        matches!((self, other), (Foo::A, Foo::A) | (Foo::B, Foo::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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