Add lint to suggest as_chunks over chunks_exact with constant #16002

rommeld · 2025-11-01T11:10:08Z

Adds a new lint chunks_exact_to_as_chunks that suggests using as_chunks instead of chunks_exact when the chunk size is a compile-time constant.

changelog: [chunks_exact_to_as_chunks]: Suggest using slice.as_chunks::() when chunks_exact(N) is called with a compile-time constant. This provides better ergonomics and type safety.

fixes #15882

rustbot · 2025-11-01T11:10:13Z

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github-actions · 2025-11-01T11:19:44Z

Lintcheck changes for 2731771

Lint	Added	Removed	Changed
`clippy::chunks_exact_with_const_size`	16	0	0

This comment will be updated if you push new changes

ada4a

Good start:) Left a couple suggestions

View changes since this review

ada4a · 2025-11-04T15:15:13Z

tests/ui/chunks_exact_to_as_chunks.stderr

+LL |     let mut it = slice.chunks_exact(CHUNK_SIZE);
+   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider using `as_chunks::<4>()` for better ergonomics


As this example shows, the suggestion replace even named constants with their values. To fix that, build the suggestion using a snippet of arg, to preserve its textual representation:

- format!("consider using `{suggestion}::<{chunk_size}>()` for better ergonomics") + let arg_str = snippet(cx, arg.span, "_"); + format!("consider using `{suggestion}::<{arg_str}>()` for better ergonomics")

Done in 610acda

ada4a · 2025-11-04T15:16:08Z

tests/ui/chunks_exact_to_as_chunks.rs

+
+    // Should trigger lint - literal constant
+    let mut it = slice.chunks_exact(4);
+    //~^ ERROR: using `chunks_exact` with a constant chunk size


nit: you could also use the lint name, which is a bit shorter:

Suggested change

//~^ ERROR: using `chunks_exact` with a constant chunk size

//~^ chunks_exact_to_as_chunks

Done in 610acda

Oh, no, that's not what I meant.. sorry for not articulating myself clearly.

The error message by itself is great, no need to change it. It's just that in the ui-test file, you can specify the error annotation not only as //~^ ERROR: <error message>, but also as //~^ <lint_name> (in your case, //~^ chunks_exact_with_const_size, which makes the test suite much less verbose overall, and so that's the change I was proposing.

I guess I misunderstood, and I hope this hash is correctly implemented and suits your suggestion bc1d010.

Yes, that's exactly what I meant:)

ada4a · 2025-11-04T15:22:14Z

clippy_lints/src/methods/mod.rs

+    /// for chunk in chunks {}
+    /// ```
+    #[clippy::version = "1.93.0"]
+    pub CHUNKS_EXACT_TO_AS_CHUNKS,


lint names usually either point out the "bad" pattern used (e.g. ok_expect), or what the pattern could be replaced with (e.g. manual_map). I think yours could use the former scheme, and be called something like chunks_exact_with_const_size

Done in 610acda

ada4a · 2025-11-04T15:24:56Z

clippy_lints/src/methods/chunks_exact_with_const_size.rs

+            "as_chunks"
+        };
+
+        span_lint_and_help(


This could be pretty effortlessly switched over to give suggestions, which are quite nice to have. https://doc.rust-lang.org/clippy/development/emitting_lints.html#suggestions-automatic-fixes should help you in that, but don't hesitate to ask if something's unclear

ada4a · 2025-11-04T15:29:03Z

clippy_lints/src/methods/chunks_exact_with_const_size.rs

+    // Check receiver is slice or array type
+    let recv_ty = cx.typeck_results().expr_ty(recv).peel_refs();
+    if !recv_ty.is_slice() && !recv_ty.is_array() {
+        return;
+    }


You want to check for types that can be adjusted to slices, which includes e.g. Vec. For that, you can use TyCtxt::expr_ty_adjusted:

Suggested change

// Check receiver is slice or array type

let recv_ty = cx.typeck_results().expr_ty(recv).peel_refs();

if !recv_ty.is_slice() && !recv_ty.is_array() {

return;

}

// Check if receiver is slice-like

if !cx.typeck_results().expr_ty_adjusted(recv).peel_refs().is_slice() {

return;

}

Done in 1d99b91

ada4a · 2025-11-04T15:29:29Z

clippy_lints/src/methods/chunks_exact_with_const_size.rs

+    // Check for Rust version
+    if !msrv.meets(cx, msrvs::AS_CHUNKS) {
+        return;
+    }


This check is somewhat expensive, so it's best to perform it towards the end, e.g. after the constant check

Done in 2731771

Suggest using slice.as_chunks::<N>() when chunks_exact(N) is called with a compile-time constant. This provides better ergonomics and type safety. Fixes rust-lang#15882

Changed example suggestion from const to snippet Shortend error message Followed clippy naming conventions

rustbot · 2025-11-04T18:38:05Z

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.

rustbot · 2025-11-04T18:38:07Z

⚠️ Warning ⚠️

There are issue links (such as #123) in the commit messages of the following commits.
Please move them to the PR description, to avoid spamming the issues with references to the commit, and so this bot can automatically canonicalize them to avoid issues with subtree.
- 3d3d2be

ada4a · 2025-11-04T20:57:09Z

clippy_lints/src/methods/mod.rs

                    );
                },
+                (name @ (sym::chunks_exact | sym::chunks_exact_mut), [arg]) => {
+                    chunks_exact_with_const_size::check(cx, expr, recv, arg, name, self.msrv);


Currently, the span that the lint points at is the whole method call expression (expr.span) -- as the last result in Lintcheck (https://github.com/rust-lang/rust-clippy/actions/runs/19080004877#user-content-chunks-exact-with-const-size) shows, that might lead to a somewhat confusing diagnostics:

warning: chunks_exact_with_const_size --> target/lintcheck/sources/rustc-demangle-0.1.24/src/v0.rs:299:25 | 299 | let mut bytes = self | _________________________^ 300 | | .nibbles 301 | | .as_bytes() 302 | | .chunks_exact(2) | |____________________________^ | = help: consider using `as_chunks::<2>()` for better ergonomics = note: `--force-warn clippy::chunks-exact-with-const-size` implied by `--force-warn clippy::all`

You can instead only highlight the method call (in this case, chunks_exact(2), by using call_span, which comes from the call to method_call(expr) above (line 13) -- see filter_map_bool_then for a (somewhat) simple example

rustbot assigned Jarcho Nov 1, 2025

rustbot added needs-fcp S-waiting-on-review Status: Awaiting review from the assignee but also interested parties labels Nov 1, 2025

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ada4a reviewed Nov 4, 2025

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rommeld added 4 commits November 4, 2025 18:51

Add lint to suggest as_chunks over chunks_exact with constant

3d3d2be

Suggest using slice.as_chunks::<N>() when chunks_exact(N) is called with a compile-time constant. This provides better ergonomics and type safety. Fixes rust-lang#15882

Fix pass value instead of reference

4a4c955

Switched to symbols instead of using strings

d332ec5

Added suggestions and changes from review

610acda

Changed example suggestion from const to snippet Shortend error message Followed clippy naming conventions

rommeld force-pushed the suggest-as-chunks-over-chunks-exact branch from 849dbe8 to 610acda Compare November 4, 2025 18:38

Updated CHANGELOG.md via

632e37b

ada4a reviewed Nov 4, 2025

View reviewed changes

rommeld added 3 commits November 4, 2025 22:05

Reduced error message verbosity

bc1d010

Added check for types that can be adjusted as slices

1d99b91

Moved Rust version check and removed

2731771

	//~^ ERROR: using `chunks_exact` with a constant chunk size
	//~^ chunks_exact_to_as_chunks

Add lint to suggest as_chunks over chunks_exact with constant #16002

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Add lint to suggest as_chunks over chunks_exact with constant #16002

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