New lint: `manual_is_multiple_of` #14292

samueltardieu · 2025-02-25T12:28:47Z

I've added a min_divisor configuration option, default to 4, to not trigger on smaller divisibility operations. I would prefer not to lint if a & 1 == 0 as if a.is_multiple_of(2) by default because the former is very idiomatic in systems (and embedded) programming.

A min_and_mask_size option, defaulting to 3, sets the default bits to be and-masked before this lint triggers; that would be n in v & ((1 << n) - 1) == 0. The form v % 10 == 0 is always linted.

~~This PR will stay in draft mode until the next rustup which will mark unsigned_is_multiple_of stable for Rust 1.87.0, and the feature flags will be removed.~~

What should its category be? I've used "complexity", but "pedantic" might be suitable as well.

Close #14289
changelog: [manual_is_multiple_of]: new lint
r? ghost

joshtriplett · 2025-02-25T17:05:31Z

Having a min_divisor configuration option seems reasonable, but I think that option should default to flagging every instance of this. Large codebases that want to keep the & 1 == 0 pattern can set that option.

samueltardieu · 2025-02-25T17:28:59Z

Having a min_divisor configuration option seems reasonable, but I think that option should default to flagging every instance of this. Large codebases that want to keep the & 1 == 0 pattern can set that option.

I've added an extra commit to test this with lintcheck. There was only one hit in Clippy sources (except for tests).

samueltardieu · 2025-02-25T17:43:37Z

@joshtriplett The lintcheck output seems quite reasonable indeed.

y21 · 2025-02-25T17:48:56Z

I agree with linting the a % b == 0 pattern, but I'm unsure about the & one... Something like (x & 0b111) == 0 feels like a very general expression.
I'm a bit concerned that linting here has the opposite effect: emitting warnings on code that has a totally different semantic meaning where x.is_multiple_of(8) would be a very obfuscated way of checking that the last 3 bits aren't set.

joshtriplett · 2025-02-25T17:59:49Z

First of all, please don't treat that as a blocker; it's not as important as the % N == 0 case.

That said: I've seen many, many codebases which use & (PAGE_SIZE - 1) == 0 and similar instead of % PAGE_SIZE == 0, when they know the value is a power of two.

We may need to tune the heuristic to make sure it doesn't produce false positives, and for instance we may want a different threshold for & vs % (& 1 == 0 might be a flag test), but I think things like & 4095 == 0 or & (4096 - 1) == 0 or & (CONST_EQUAL_TO_4096 - 1) == 0 are pretty clear.

Looking at the lintcheck output, I think I agree with not flagging & 1 == 0 by default.

samueltardieu · 2025-02-25T20:46:17Z

What about applying the min-divisor option (maybe using another name, like min-one-bits to represent the number of ones) only to the &(2^n-1) form of the lint, with a sensible default such as 5 bits? The % form would be linted unconditionally.

joshtriplett · 2025-02-25T20:55:24Z

@samueltardieu Sounds reasonable. I would say the default should flag 7 and 15 by default, and maybe 3, but not 1. Because 1 might be a flag, but 3 is definitely two bits.

samueltardieu · 2025-02-25T21:56:31Z

Done, see the toplevel comment for the updated option.

joshtriplett · 2025-02-26T10:00:40Z

Looking at the lintcheck output, this looks great!

samueltardieu · 2025-02-27T22:44:12Z

rustup happened, PR ready.
r? clippy

samueltardieu · 2025-02-28T17:35:53Z

Rebased.

samueltardieu · 2025-02-28T23:02:09Z

Rebased

blyxyas

Very good lint and initial implementation, some nits in this initial reviewing round (some slip-ups)

blyxyas · 2025-03-20T23:54:54Z

clippy_lints/src/operators/identity_op.rs

@@ -170,9 +170,7 @@ fn is_allowed(cx: &LateContext<'_>, cmp: BinOpKind, left: &Expr<'_>, right: &Exp
    cx.typeck_results().expr_ty(left).peel_refs().is_integral()
        && cx.typeck_results().expr_ty(right).peel_refs().is_integral()
        // `1 << 0` is a common pattern in bit manipulation code
-        && !(cmp == BinOpKind::Shl
-            && ConstEvalCtxt::new(cx).eval_simple(right) == Some(Constant::Int(0))
-            && ConstEvalCtxt::new(cx).eval_simple(left) == Some(Constant::Int(1)))


Couldn't these lines be replaced with the functions you moved into clippy_utils? 👀

Here also, this comment points at code which already does exactly what you are suggesting.

clippy_lints/src/if_not_else.rs

clippy_utils/src/consts.rs

clippy_lints/src/operators/manual_is_multiple_of.rs

clippy_lints/src/operators/mod.rs

samueltardieu · 2025-04-16T13:57:52Z

@blyxyas Should I reassign?

blyxyas · 2025-04-16T22:28:25Z

Oh, I got here I review I never submitted ;(

blyxyas · 2025-03-24T00:43:36Z

clippy_lints/src/operators/manual_is_multiple_of.rs

+                Sugg::hir_with_applicability(cx, lhs_right, "_", &mut app).into_string(),
+            )
+        } else if lhs_op.node == BinOpKind::BitAnd {
+            let min_divisor = 1 << u128::from(min_and_mask_size);


Is there a reason why this is an u128, it will never be greater than 256 because it's parsed from conf as a u8.
Testing, seems that changing it to u8 doesn't throw Clippy into an eternal pit of sadness. Is it u128 because of possible future compatibility?

It's because min_divisor needs to be a u128 (someone might want to not trigger the lint except for constants greater than 0x100 for example — I'll add a test). I could write it as 1u128 << min_and_mask_size though.

Or did I not understand your remark?

blyxyas · 2025-03-24T01:03:12Z

clippy_lints/src/operators/manual_is_multiple_of.rs

+}
+
+/// If `expr` is made of all ones, return the representation of `expr+1` if it is no smaller than
+/// `min_divisor`.


Suggested change

/// `min_divisor`.

/// `min_divisor`. Made to catch ((1 << A) - 1) where A is either not a constant, or >= 2^min_divisor

I've used the more complete:

/// If `expr` is provably made of all ones, return the representation of `expr+1` if it is no smaller than /// `min_divisor`. This will catch expressions of the following forms: /// - `(1 << A) - 1` where `A` is a constant /// - integer literals — if it uses hexadecimal, the return value will as well /// /// The function will not attempt to evaluate non-literal constant expressions, as those may depend on /// conditional compilation.

blyxyas · 2025-04-19T23:19:26Z

I've opened the FCP
https://rust-lang.zulipchat.com/#narrow/channel/257328-clippy/topic/FCP.20manual_is_multiple_of

samueltardieu · 2025-05-05T23:14:35Z

Removed the & part as well as the min-and-mask-size option, as discussed in the FCP.

Also updated the clippy version to 1.88.

@rustbot label +S-final-comment-period

samueltardieu · 2025-05-05T23:22:30Z

Rebased to fix UI test error in iter_kv_map.rs.

samueltardieu · 2025-05-06T06:07:33Z

Rebased

This prevents triggering the new `manual_is_multiple_of` lint on unrelated lint tests.

samueltardieu · 2025-05-10T21:07:03Z

Rebased and set Clippy version to 1.89.

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties label Feb 25, 2025

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samueltardieu marked this pull request as ready for review February 27, 2025 22:43

rustbot assigned blyxyas Feb 27, 2025

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blyxyas reviewed Mar 21, 2025

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samueltardieu mentioned this pull request Apr 16, 2025

Suggest using next_multiple_of() instead of ((x + N-1) / N) * N or (x + N-1) & !(N-1) (for powers of two) #14543

Open

blyxyas reviewed Apr 16, 2025

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samueltardieu added 3 commits May 10, 2025 23:06

Add integer_const() and is_zero_integer_const() utility functions

6e21c0a

Use .is_multiple_of() in Clippy tests sources

3b641c2

This prevents triggering the new `manual_is_multiple_of` lint on unrelated lint tests.

New lint: manual_is_multiple_of

313ec7d

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New lint: `manual_is_multiple_of` #14292

New lint: `manual_is_multiple_of` #14292

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samueltardieu commented May 10, 2025

	/// `min_divisor`.
	/// `min_divisor`. Made to catch ((1 << A) - 1) where A is either not a constant, or >= 2^min_divisor

New lint: manual_is_multiple_of #14292

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New lint: manual_is_multiple_of #14292

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New lint: `manual_is_multiple_of` #14292

New lint: `manual_is_multiple_of` #14292

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