Clippy lint · Pedantic · checked with Clippy 0.1.99

clippy::ip_constant: hand-coded well-known IP address

What it meansChecks for IP addresses that could be replaced with predefined constants such as Ipv4Addr::new(127, 0, 0, 1) instead of using the appropriate constants. It is in the pedantic group and off by default.

What you see

warning: hand-coded well-known IP address
 --> main.rs:6:15
  |
6 | let addr_v4 = Ipv4Addr::new(127, 0, 0, 1);
  |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#ip_constant
  = note: `-W clippy::ip-constant` implied by `-W clippy::pedantic`
help: use
  |
6 - let addr_v4 = Ipv4Addr::new(127, 0, 0, 1);
6 + let addr_v4 = Ipv4Addr::LOCALHOST;
  |

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?

Using specific IP addresses like 127.0.0.1 or ::1 is less clear and less maintainable than using the predefined constants Ipv4Addr::LOCALHOST or Ipv6Addr::LOCALHOST. These constants improve code readability, make the intent explicit, and are less error-prone.

Stricter lints with some false positives. Off by default; opt in with clippy::pedantic.

How to fix it

Clippy can fix this one for you: run cargo clippy --fix (add --allow-dirty if you have uncommitted changes). Review the diff before you commit.

Flagged

use std::net::{Ipv4Addr, Ipv6Addr};

// IPv4 loopback
let addr_v4 = Ipv4Addr::new(127, 0, 0, 1);

// IPv6 loopback
let addr_v6 = Ipv6Addr::new(0, 0, 0, 0, 0, 0, 0, 1);

Write this instead

use std::net::{Ipv4Addr, Ipv6Addr};

// IPv4 loopback
let addr_v4 = Ipv4Addr::LOCALHOST;

// IPv6 loopback
let addr_v6 = Ipv6Addr::LOCALHOST;

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]
ip_constant = "warn"

In code

#![warn(clippy::ip_constant)]

One run

cargo clippy -- -W clippy::ip-constant

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.

Pedanticgroup

allowdefault level

yesfixed by cargo clippy --fix

1.89.0added in Rust

Clippy's own documentation

What it does

Checks for IP addresses that could be replaced with predefined constants such as Ipv4Addr::new(127, 0, 0, 1) instead of using the appropriate constants.

Why is this bad?

Using specific IP addresses like 127.0.0.1 or ::1 is less clear and less maintainable than using the predefined constants Ipv4Addr::LOCALHOST or Ipv6Addr::LOCALHOST. These constants improve code readability, make the intent explicit, and are less error-prone.

Example

use std::net::{Ipv4Addr, Ipv6Addr};

// IPv4 loopback
let addr_v4 = Ipv4Addr::new(127, 0, 0, 1);

// IPv6 loopback
let addr_v6 = Ipv6Addr::new(0, 0, 0, 0, 0, 0, 0, 1);

Use instead:

use std::net::{Ipv4Addr, Ipv6Addr};

// IPv4 loopback
let addr_v4 = Ipv4Addr::LOCALHOST;

// IPv6 loopback
let addr_v6 = Ipv6Addr::LOCALHOST;

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

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