`manual_div_ceil` should warn on manual implementations using `next_multiple_of`

[mnemonikr]

Summary

This should be applicable for both unsigned and signed types (the latter with feature int_roundings):

#![feature(int_roundings)]
let a: i32 = 8;
let b = 3;

assert_eq!(a.div_ceil(b), a.next_multiple_of(b) / b);
assert_eq!(a.div_ceil(-b), a.next_multiple_of(-b) / -b);
assert_eq!((-a).div_ceil(b), (-a).next_multiple_of(b) / b);
assert_eq!((-a).div_ceil(-b), (-a).next_multiple_of(-b) / -b);

Playground

Using div_ceil in this case has the advantage of avoiding the subtle issue that can occur if a.next_multiple_of(b) overflows.

Lint Name

manual_div_ceil

Reproducer

I tried this code:

let a: u32 = 8;
let b = 3;
let c = a.next_multiple_of(b) / b;

I expected to see this happen:

clippy: manually reimplementing `div_ceil`
...
clippy: consider using `.div_ceil()`: `a.div_ceil(b)` [manual_div_ceil]

Instead, this happened: The lint did not fire.

Version

rustc 1.92.0 (ded5c06cf 2025-12-08)
binary: rustc
commit-hash: ded5c06cf21d2b93bffd5d884aa6e96934ee4234
commit-date: 2025-12-08
host: x86_64-unknown-linux-gnu
release: 1.92.0
LLVM version: 21.1.3

[mnemonikr]

I can pick up implementing a fix for this unless there are any objections. Will post a PR shortly.

[mnemonikr]

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