Suggest `Cstr::count_bytes` in `strlen_on_c_strings` #16323

Tigls · 2026-01-01T10:27:34Z

changelog: [strlen_on_c_strings]: changes suggestion to use CStr::count_bytes()

rustbot · 2026-01-01T10:27:39Z

rustbot has assigned @dswij.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

dswij · 2026-01-01T15:23:25Z

@Tigls CI is failing on fmt check

samueltardieu

Could you please add tests below/at the considered MSRV, and inside/outside const contexts?

View changes since this review

samueltardieu · 2026-01-01T21:38:13Z

clippy_lints/src/strlen_on_c_strings.rs

            && let ExprKind::MethodCall(path, self_arg, [], _) = recv.kind
            && !recv.span.from_expansion()
            && path.ident.name == sym::as_ptr
+            && self.msrv.meets(cx, msrvs::CONST_BLOCKS)


What is the link between .count_bytes() and CONST_BLOCKS?

Also:

If the MSRV isn't met, the previous suggestion should still apply.

Different MSRV apply in and outside a const context.

yeah, I didn't know I can add new names to MSRV enum, thought they are kind of fixed code names for versions.

But makes sense, all fixed now.

Don't forget to add tests for all combinations of MSRV/const contexts. You can look at how it's done in other tests by looking for the #[clippy::msrv] attribute.

Looks like I can't call the libc::strlen in the const context. Since it is an FFI function, which is linked at runtime, it just won't compile. Does it mean I can safely remove the msrv branch for const context and keep only the branch for non-const contex, since the user won't be able to compile it anyway?

@samueltardieu let me know if you had time to have a look and close this

rustbot · 2026-01-01T21:40:34Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

Tigls · 2026-01-05T16:57:35Z

@rustbot ready

ada4a · 2026-01-12T14:34:07Z

clippy_lints/src/strlen_on_c_strings.rs


-declare_lint_pass!(StrlenOnCStrings => [STRLEN_ON_C_STRINGS]);
+pub struct StrlenOnCStrings<'a> {
+    msrv: &'a Msrv,


Msrv is Copy, so it should be fine to store it by value

ada4a · 2026-01-12T14:36:01Z

clippy_lints/src/strlen_on_c_strings.rs

+                let ty = cx.typeck_results().expr_ty(self_arg).peel_refs();
+                let method_name = if ty.is_diag_item(cx, sym::cstring_type) {
+                    "as_bytes"
+                } else if ty.is_lang_item(cx, LangItem::CStr) {
+                    "to_bytes"
+                } else {
+                    return;
+                };
+                span_lint_and_sugg(
+                    cx,
+                    STRLEN_ON_C_STRINGS,
+                    span,
+                    "using `libc::strlen` on a `CString` or `CStr` value",
+                    "try",
+                    format!("{val_name}.{method_name}().len()"),
+                    app,
+                );
+            }


The code in the two branches is very similar. Imo it would be better to use span_lint_and_then and put the MSRV-dependent logic inside its closure.

samueltardieu · 2026-01-13T08:33:31Z

@Tigls Your changes should be very easily adaptable to those merged in #16391. Once it's done, I'll review them (+ the msrv change).

r? samueltardieu

rustbot · 2026-01-13T20:14:56Z

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.

samueltardieu

That's looking good. Could you please squash all the commits together?

View changes since this review

Tigls · 2026-01-14T07:33:58Z

@rustbot ready

ada4a

@samueltardieu should we include this last change in this PR? If so, @Tigls, feel free to amend it into your last commit

View changes since this review

ada4a · 2026-01-14T07:42:16Z

clippy_lints/src/strlen_on_c_strings.rs

+                    let suggestion = if self.msrv.meets(cx, msrvs::CSTR_COUNT_BYTES) {
+                        format!("{val_name}.count_bytes()")
+                    } else if ty.is_diag_item(cx, sym::cstring_type) {
+                        format!("{val_name}.as_bytes().len()")
+                    } else if ty.is_lang_item(cx, LangItem::CStr) {
+                        format!("{val_name}.to_bytes().len()")
+                    } else {
+                        return;
+                    };


In #16391, it was decided that we should lint types that dereference to CStr as well, and just recommend to_bytes() to avoid complications with CString in particular (see #16391 (comment) for more context). Therefore it should be safe to simplify this to:

Suggested change

let suggestion = if self.msrv.meets(cx, msrvs::CSTR_COUNT_BYTES) {

format!("{val_name}.count_bytes()")

} else if ty.is_diag_item(cx, sym::cstring_type) {

format!("{val_name}.as_bytes().len()")

} else if ty.is_lang_item(cx, LangItem::CStr) {

format!("{val_name}.to_bytes().len()")

} else {

return;

};

let suggestion = if self.msrv.meets(cx, msrvs::CSTR_COUNT_BYTES) {

format!("{val_name}.count_bytes()")

} else {

format!("{val_name}.to_bytes().len()")

};

ada4a · 2026-01-14T07:44:10Z

clippy_lints/src/strlen_on_c_strings.rs

    /// ### What it does
    /// Checks for usage of `libc::strlen` on a `CString` or `CStr` value,
-    /// and suggest calling `as_bytes().len()` or `to_bytes().len()` respectively instead.
+    /// and suggest calling `count_bytes()`instead.


small typo:

Suggested change

/// and suggest calling `count_bytes()`instead.

/// and suggest calling `count_bytes()` instead.

samueltardieu · 2026-01-14T07:50:57Z

@samueltardieu should we include this last change in this PR? If so, @Tigls, feel free to amend it into your last commit

View changes since this review

Yes, that would be better indeed.

rustbot assigned dswij Jan 1, 2026

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties label Jan 1, 2026

dswij added S-waiting-on-author Status: This is awaiting some action from the author. (Use `@rustbot ready` to update this status) and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties labels Jan 1, 2026

samueltardieu requested changes Jan 1, 2026

View reviewed changes

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties and removed S-waiting-on-author Status: This is awaiting some action from the author. (Use `@rustbot ready` to update this status) labels Jan 5, 2026

ada4a suggested changes Jan 12, 2026

View reviewed changes

This comment has been minimized.

Sign in to view

rustbot assigned samueltardieu and unassigned dswij Jan 13, 2026

samueltardieu mentioned this pull request Jan 13, 2026

feat(strlen_on_c_strings): mention the specific type (CString or CStr) #16391

Merged

This comment has been minimized.

Sign in to view

rustbot added has-merge-commits PR has merge commits, merge with caution. S-waiting-on-author Status: This is awaiting some action from the author. (Use `@rustbot ready` to update this status) labels Jan 13, 2026

Tigls force-pushed the issue16308 branch from e7e389b to 4b211a3 Compare January 13, 2026 20:27

rustbot removed S-waiting-on-author Status: This is awaiting some action from the author. (Use `@rustbot ready` to update this status) has-merge-commits PR has merge commits, merge with caution. labels Jan 13, 2026

samueltardieu requested changes Jan 13, 2026

View reviewed changes

rustbot added S-waiting-on-author Status: This is awaiting some action from the author. (Use `@rustbot ready` to update this status) and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties labels Jan 13, 2026

feat(strlen_on_c_strings): suggest .count_bytes()

f4c69be

Tigls force-pushed the issue16308 branch from 4b211a3 to f4c69be Compare January 13, 2026 23:32

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties and removed S-waiting-on-author Status: This is awaiting some action from the author. (Use `@rustbot ready` to update this status) labels Jan 14, 2026

ada4a reviewed Jan 14, 2026

View reviewed changes

	/// and suggest calling `count_bytes()`instead.
	/// and suggest calling `count_bytes()` instead.

Suggest Cstr::count_bytes in strlen_on_c_strings #16323

Are you sure you want to change the base?

Suggest Cstr::count_bytes in strlen_on_c_strings #16323

Conversation

Tigls commented Jan 1, 2026 • edited by rustbot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

rustbot commented Jan 1, 2026

Uh oh!

dswij commented Jan 1, 2026

Uh oh!

samueltardieu left a comment • edited by rustbot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

rustbot commented Jan 1, 2026

Uh oh!

Tigls commented Jan 5, 2026

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

This comment has been minimized.

samueltardieu commented Jan 13, 2026

Uh oh!

rustbot commented Jan 13, 2026

Uh oh!

This comment has been minimized.

samueltardieu left a comment • edited by rustbot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Tigls commented Jan 14, 2026

Uh oh!

ada4a left a comment • edited by rustbot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

samueltardieu commented Jan 14, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

5 participants

Suggest `Cstr::count_bytes` in `strlen_on_c_strings` #16323

Suggest `Cstr::count_bytes` in `strlen_on_c_strings` #16323

Tigls commented Jan 1, 2026 •

edited by rustbot

Loading

samueltardieu left a comment •

edited by rustbot

Loading

samueltardieu left a comment •

edited by rustbot

Loading

ada4a left a comment •

edited by rustbot

Loading