New lint: decimal_bit_mask

[Artur-Sulej]

changelog: [decimal_bit_mask]: add a new lint that detects the use of decimal literals as bit masks in bitwise operations. Using decimal literals for bit masks can obscure the intended bit pattern and reduce code readability. This lint encourages the use of binary (0b...) or hexadecimal (0x...) notation to make bit patterns explicit and easier to understand at a glance.

Addresses issue: #1775

---

Example:

let masked = x & 15; // Bad: decimal literal as bit mask
let masked = x & 0b1111; // Good: bit pattern is explicit

Summary Notes

• Feature-freeze by github-actions[bot]

Managed by @rustbot—see help for details

[github-actions]

Lintcheck changes for 12ab9f6

Lint	Added	Removed	Changed
clippy::decimal_bit_mask	238	0	0

This comment will be updated if you push new changes

[Artur-Sulej]

Although I annotated my test with //~ decimal_bit_mask, Clippy still reports the lint as an unmatched diagnostic and the test fails.
Any idea what I might be missing?

[rustbot]

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[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[ada4a]

Although I annotated my test with //~ decimal_bit_mask, Clippy still reports the lint as an unmatched diagnostic and the test fails.
Any idea what I might be missing?

That's because in this case, you're firing the lint twice, once for each operand, so you'll need to add a second error mark. That's the easiest to do by bringing them to under the linted expression:

Suggested change

	88 & 99; //~ decimal_bit_mask
	88 & 99;
	//~^ decimal_bit_mask
	//~| decimal_bit_mask

• ^ means the lint should happen on the line above the comment
• | "attaches" the error mark to another one on a neighbouring line (in this case the //~^ decimal_bit_mask directly above), and will expect a lint at the same location (in this case, the line containing 88 & 99;)

[ada4a]

It would be a bit more concise to have these simply as left and right, and then use left.span, right.kind etc. throughout the code

[ada4a]

You can improve the output by pointing the lint at the exact operand that is decimal -- for example, here you would use span1 (or left.span, as per previous comment)

[ada4a]

The second sentence should be emitted separately, as a help message -- see https://doc.rust-lang.org/clippy/development/emitting_lints.html#emitting-a-lint-1 for more info.

Try using span_lint_and_help here

[ada4a]

You can use SpanRangeExt::get_source_code here to avoid allocating a String (which is what snippet_opt does)

                && let Some(snippet) = span2.get_source_text(cx)

In fact, since you're only using the snippet to check its contents, you can get away with SpanRangeExt::check_source_code:

Suggested change

	&& let Some(snippet) = snippet_opt(cx, *span2)
	&& !snippet.starts_with("0b")
	&& !snippet.starts_with("0x")
	&& span2.check_source_text(cx, |src| !src.starts_with("0b") && !src.starts_with("0x"))