Clippy lint · Correctness · checked with Clippy 0.1.99

clippy::recursive_format_impl: using `self.to_string` in `fmt::Display` implementation will cause infinite recursion

What it meansChecks for format trait implementations (e.g. Display) with a recursive call to itself which uses self as a parameter. This is typically done indirectly with the write! macro or with to_string(). It is in the correctness group and denied by default (the build fails).

What you see

error: using `self.to_string` in `fmt::Display` implementation will cause infinite recursion
 --> main.rs:8:25
  |
8 |         write!(f, "{}", self.to_string())
  |                         ^^^^^^^^^^^^^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#recursive_format_impl
  = note: `#[deny(clippy::recursive_format_impl)]` 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?

This will lead to infinite recursion and a stack overflow.

Code that is almost certainly wrong. Denied by default: the build fails until you fix it or allow it.

How to fix it

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

Flagged

use std::fmt;

struct Structure(i32);
impl fmt::Display for Structure {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "{}", self.to_string())
    }
}

Write this instead

use std::fmt;

struct Structure(i32);
impl fmt::Display for Structure {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "{}", self.0)
    }
}

Turn it off, or on

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

On one item

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

Whole crate (Cargo.toml)

[lints.clippy]
recursive_format_impl = "allow"

One run

cargo clippy -- -A clippy::recursive-format-impl

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.

Correctnessgroup

denydefault level

nofixed by cargo clippy --fix

1.48.0added in Rust

Clippy's own documentation

What it does

Checks for format trait implementations (e.g. Display) with a recursive call to itself which uses self as a parameter. This is typically done indirectly with the write! macro or with to_string().

Why is this bad?

This will lead to infinite recursion and a stack overflow.

Example

use std::fmt;

struct Structure(i32);
impl fmt::Display for Structure {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "{}", self.to_string())
    }
}

Use instead:

use std::fmt;

struct Structure(i32);
impl fmt::Display for Structure {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "{}", self.0)
    }
}

Past names

to_string_in_display

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

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