rust-lang · samueltardieu · Jan 16, 2026 · Jan 13, 2026
diff --git a/clippy_lints/src/lib.rs b/clippy_lints/src/lib.rs
@@ -718,7 +718,7 @@ pub fn register_lint_passes(store: &mut rustc_lint::LintStore, conf: &'static Co
         Box::new(|_| Box::<unused_async::UnusedAsync>::default()),
         Box::new(move |tcx| Box::new(disallowed_types::DisallowedTypes::new(tcx, conf))),
         Box::new(move |tcx| Box::new(missing_enforced_import_rename::ImportRename::new(tcx, conf))),
-        Box::new(|_| Box::new(strlen_on_c_strings::StrlenOnCStrings)),
+        Box::new(move |_| Box::new(strlen_on_c_strings::StrlenOnCStrings::new(conf))),
         Box::new(move |_| Box::new(self_named_constructors::SelfNamedConstructors)),
         Box::new(move |_| Box::new(iter_not_returning_iterator::IterNotReturningIterator)),
         Box::new(move |_| Box::new(manual_assert::ManualAssert)),

diff --git a/clippy_lints/src/strlen_on_c_strings.rs b/clippy_lints/src/strlen_on_c_strings.rs
@@ -1,17 +1,19 @@
+use clippy_config::Conf;
 use clippy_utils::diagnostics::span_lint_and_then;
+use clippy_utils::msrvs::{self, Msrv};
 use clippy_utils::res::MaybeDef;
 use clippy_utils::source::snippet_with_context;
 use clippy_utils::visitors::is_expr_unsafe;
 use clippy_utils::{match_libc_symbol, sym};
 use rustc_errors::Applicability;
 use rustc_hir::{Block, BlockCheckMode, Expr, ExprKind, LangItem, Node, UnsafeSource};
 use rustc_lint::{LateContext, LateLintPass};
-use rustc_session::declare_lint_pass;
+use rustc_session::impl_lint_pass;
 
 declare_clippy_lint! {
     /// ### 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.
     ///
     /// ### Why is this bad?
     /// libc::strlen is an unsafe function, which we don't need to call
@@ -27,15 +29,25 @@ declare_clippy_lint! {
     /// ```rust, no_run
     /// use std::ffi::CString;
     /// let cstring = CString::new("foo").expect("CString::new failed");
-    /// let len = cstring.as_bytes().len();
+    /// let len = cstring.count_bytes();
     /// ```
     #[clippy::version = "1.55.0"]
     pub STRLEN_ON_C_STRINGS,
     complexity,
-    "using `libc::strlen` on a `CString` or `CStr` value, while `as_bytes().len()` or `to_bytes().len()` respectively can be used instead"
+    "using `libc::strlen` on a `CString` or `CStr` value, while `count_bytes()` can be used instead"
 }
 
-declare_lint_pass!(StrlenOnCStrings => [STRLEN_ON_C_STRINGS]);
+pub struct StrlenOnCStrings {
+    msrv: Msrv,
+}
+
+impl StrlenOnCStrings {
+    pub fn new(conf: &Conf) -> Self {
+        Self { msrv: conf.msrv }
+    }
+}
+
+impl_lint_pass!(StrlenOnCStrings => [STRLEN_ON_C_STRINGS]);
 
 impl<'tcx> LateLintPass<'tcx> for StrlenOnCStrings {
     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
@@ -80,7 +92,14 @@ impl<'tcx> LateLintPass<'tcx> for StrlenOnCStrings {
                 |diag| {
                     let mut app = Applicability::MachineApplicable;
                     let val_name = snippet_with_context(cx, self_arg.span, ctxt, "_", &mut app).0;
-                    diag.span_suggestion(span, "use", format!("{val_name}.to_bytes().len()"), app);
+
+                    let suggestion = if self.msrv.meets(cx, msrvs::CSTR_COUNT_BYTES) {
+                        format!("{val_name}.count_bytes()")
+                    } else {
+                        format!("{val_name}.to_bytes().len()")
+                    };
+
+                    diag.span_suggestion(span, "use", suggestion, app);
                 },
             );
         }

diff --git a/clippy_utils/src/msrvs.rs b/clippy_utils/src/msrvs.rs
@@ -32,7 +32,7 @@ msrv_aliases! {
     1,82,0 { IS_NONE_OR, REPEAT_N, RAW_REF_OP, SPECIALIZED_TO_STRING_FOR_REFS }
     1,81,0 { LINT_REASONS_STABILIZATION, ERROR_IN_CORE, EXPLICIT_SELF_TYPE_ELISION, DURATION_ABS_DIFF }
     1,80,0 { BOX_INTO_ITER, LAZY_CELL }
-    1,79,0 { CONST_BLOCKS }
+    1,79,0 { CONST_BLOCKS, CSTR_COUNT_BYTES }
     1,77,0 { C_STR_LITERALS }
     1,76,0 { PTR_FROM_REF, OPTION_RESULT_INSPECT }
     1,75,0 { OPTION_AS_SLICE }

diff --git a/tests/ui/strlen_on_c_strings.fixed b/tests/ui/strlen_on_c_strings.fixed
@@ -7,40 +7,62 @@ use std::ffi::{CStr, CString};
 fn main() {
     // CString
     let cstring = CString::new("foo").expect("CString::new failed");
-    let _ = cstring.to_bytes().len();
+    let _ = cstring.count_bytes();
     //~^ ERROR: using `libc::strlen` on a `CString` value
 
     // CStr
     let cstr = CStr::from_bytes_with_nul(b"foo\0").expect("CStr::from_bytes_with_nul failed");
-    let _ = cstr.to_bytes().len();
+    let _ = cstr.count_bytes();
     //~^ ERROR: using `libc::strlen` on a `CStr` value
 
-    let _ = cstr.to_bytes().len();
+    let _ = cstr.count_bytes();
     //~^ ERROR: using `libc::strlen` on a `CStr` value
 
     let pcstr: *const &CStr = &cstr;
-    let _ = unsafe { (*pcstr).to_bytes().len() };
+    let _ = unsafe { (*pcstr).count_bytes() };
     //~^ ERROR: using `libc::strlen` on a `CStr` value
 
     unsafe fn unsafe_identity<T>(x: T) -> T {
         x
     }
-    let _ = unsafe { unsafe_identity(cstr).to_bytes().len() };
+    let _ = unsafe { unsafe_identity(cstr).count_bytes() };
     //~^ ERROR: using `libc::strlen` on a `CStr` value
-    let _ = unsafe { unsafe_identity(cstr) }.to_bytes().len();
+    let _ = unsafe { unsafe_identity(cstr) }.count_bytes();
     //~^ ERROR: using `libc::strlen` on a `CStr` value
 
     let f: unsafe fn(_) -> _ = unsafe_identity;
-    let _ = unsafe { f(cstr).to_bytes().len() };
+    let _ = unsafe { f(cstr).count_bytes() };
     //~^ ERROR: using `libc::strlen` on a `CStr` value
 }
 
 // make sure we lint types that _adjust_ to `CStr`
 fn adjusted(box_cstring: Box<CString>, box_cstr: Box<CStr>, arc_cstring: std::sync::Arc<CStr>) {
-    let _ = box_cstring.to_bytes().len();
+    let _ = box_cstring.count_bytes();
     //~^ ERROR: using `libc::strlen` on a type that dereferences to `CStr`
-    let _ = box_cstr.to_bytes().len();
+    let _ = box_cstr.count_bytes();
     //~^ ERROR: using `libc::strlen` on a type that dereferences to `CStr`
-    let _ = arc_cstring.to_bytes().len();
+    let _ = arc_cstring.count_bytes();
     //~^ ERROR: using `libc::strlen` on a type that dereferences to `CStr`
 }
+
+#[clippy::msrv = "1.78"]
+fn msrv_1_78() {
+    let cstr = CStr::from_bytes_with_nul(b"foo\0").expect("CStr::from_bytes_with_nul failed");
+    let _ = cstr.to_bytes().len();
+    //~^ strlen_on_c_strings
+
+    let cstring = CString::new("foo").expect("CString::new failed");
+    let _ = cstring.to_bytes().len();
+    //~^ strlen_on_c_strings
+}
+
+#[clippy::msrv = "1.79"]
+fn msrv_1_79() {
+    let cstr = CStr::from_bytes_with_nul(b"foo\0").expect("CStr::from_bytes_with_nul failed");
+    let _ = cstr.count_bytes();
+    //~^ strlen_on_c_strings
+
+    let cstring = CString::new("foo").expect("CString::new failed");
+    let _ = cstring.count_bytes();
+    //~^ strlen_on_c_strings
+}
diff --git a/tests/ui/strlen_on_c_strings.rs b/tests/ui/strlen_on_c_strings.rs
@@ -44,3 +44,25 @@ fn adjusted(box_cstring: Box<CString>, box_cstr: Box<CStr>, arc_cstring: std::sy
     let _ = unsafe { libc::strlen(arc_cstring.as_ptr()) };
     //~^ ERROR: using `libc::strlen` on a type that dereferences to `CStr`
 }
+
+#[clippy::msrv = "1.78"]
+fn msrv_1_78() {
+    let cstr = CStr::from_bytes_with_nul(b"foo\0").expect("CStr::from_bytes_with_nul failed");
+    let _ = unsafe { libc::strlen(cstr.as_ptr()) };
+    //~^ strlen_on_c_strings
+
+    let cstring = CString::new("foo").expect("CString::new failed");
+    let _ = unsafe { libc::strlen(cstring.as_ptr()) };
+    //~^ strlen_on_c_strings
+}
+
+#[clippy::msrv = "1.79"]
+fn msrv_1_79() {
+    let cstr = CStr::from_bytes_with_nul(b"foo\0").expect("CStr::from_bytes_with_nul failed");
+    let _ = unsafe { libc::strlen(cstr.as_ptr()) };
+    //~^ strlen_on_c_strings
+
+    let cstring = CString::new("foo").expect("CString::new failed");
+    let _ = unsafe { libc::strlen(cstring.as_ptr()) };
+    //~^ strlen_on_c_strings
+}
diff --git a/tests/ui/strlen_on_c_strings.stderr b/tests/ui/strlen_on_c_strings.stderr
@@ -2,7 +2,7 @@ error: using `libc::strlen` on a `CString` value
   --> tests/ui/strlen_on_c_strings.rs:10:13
    |
 LL |     let _ = unsafe { libc::strlen(cstring.as_ptr()) };
-   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use: `cstring.to_bytes().len()`
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use: `cstring.count_bytes()`
    |
    = note: `-D clippy::strlen-on-c-strings` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::strlen_on_c_strings)]`
@@ -11,55 +11,79 @@ error: using `libc::strlen` on a `CStr` value
   --> tests/ui/strlen_on_c_strings.rs:15:13
    |
 LL |     let _ = unsafe { libc::strlen(cstr.as_ptr()) };
-   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use: `cstr.to_bytes().len()`
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use: `cstr.count_bytes()`
 
 error: using `libc::strlen` on a `CStr` value
   --> tests/ui/strlen_on_c_strings.rs:18:13
    |
 LL |     let _ = unsafe { strlen(cstr.as_ptr()) };
-   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use: `cstr.to_bytes().len()`
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use: `cstr.count_bytes()`
 
 error: using `libc::strlen` on a `CStr` value
   --> tests/ui/strlen_on_c_strings.rs:22:22
    |
 LL |     let _ = unsafe { strlen((*pcstr).as_ptr()) };
-   |                      ^^^^^^^^^^^^^^^^^^^^^^^^^ help: use: `(*pcstr).to_bytes().len()`
+   |                      ^^^^^^^^^^^^^^^^^^^^^^^^^ help: use: `(*pcstr).count_bytes()`
 
 error: using `libc::strlen` on a `CStr` value
   --> tests/ui/strlen_on_c_strings.rs:28:22
    |
 LL |     let _ = unsafe { strlen(unsafe_identity(cstr).as_ptr()) };
-   |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use: `unsafe_identity(cstr).to_bytes().len()`
+   |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use: `unsafe_identity(cstr).count_bytes()`
 
 error: using `libc::strlen` on a `CStr` value
   --> tests/ui/strlen_on_c_strings.rs:30:13
    |
 LL |     let _ = unsafe { strlen(unsafe { unsafe_identity(cstr) }.as_ptr()) };
-   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use: `unsafe { unsafe_identity(cstr) }.to_bytes().len()`
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use: `unsafe { unsafe_identity(cstr) }.count_bytes()`
 
 error: using `libc::strlen` on a `CStr` value
   --> tests/ui/strlen_on_c_strings.rs:34:22
    |
 LL |     let _ = unsafe { strlen(f(cstr).as_ptr()) };
-   |                      ^^^^^^^^^^^^^^^^^^^^^^^^ help: use: `f(cstr).to_bytes().len()`
+   |                      ^^^^^^^^^^^^^^^^^^^^^^^^ help: use: `f(cstr).count_bytes()`
 
 error: using `libc::strlen` on a type that dereferences to `CStr`
   --> tests/ui/strlen_on_c_strings.rs:40:13
    |
 LL |     let _ = unsafe { libc::strlen(box_cstring.as_ptr()) };
-   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use: `box_cstring.to_bytes().len()`
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use: `box_cstring.count_bytes()`
 
 error: using `libc::strlen` on a type that dereferences to `CStr`
   --> tests/ui/strlen_on_c_strings.rs:42:13
    |
 LL |     let _ = unsafe { libc::strlen(box_cstr.as_ptr()) };
-   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use: `box_cstr.to_bytes().len()`
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use: `box_cstr.count_bytes()`
 
 error: using `libc::strlen` on a type that dereferences to `CStr`
   --> tests/ui/strlen_on_c_strings.rs:44:13
    |
 LL |     let _ = unsafe { libc::strlen(arc_cstring.as_ptr()) };
-   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use: `arc_cstring.to_bytes().len()`
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use: `arc_cstring.count_bytes()`
+
+error: using `libc::strlen` on a `CStr` value
+  --> tests/ui/strlen_on_c_strings.rs:51:13
+   |
+LL |     let _ = unsafe { libc::strlen(cstr.as_ptr()) };
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use: `cstr.to_bytes().len()`
+
+error: using `libc::strlen` on a `CString` value
+  --> tests/ui/strlen_on_c_strings.rs:55:13
+   |
+LL |     let _ = unsafe { libc::strlen(cstring.as_ptr()) };
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use: `cstring.to_bytes().len()`
+
+error: using `libc::strlen` on a `CStr` value
+  --> tests/ui/strlen_on_c_strings.rs:62:13
+   |
+LL |     let _ = unsafe { libc::strlen(cstr.as_ptr()) };
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use: `cstr.count_bytes()`
+
+error: using `libc::strlen` on a `CString` value
+  --> tests/ui/strlen_on_c_strings.rs:66:13
+   |
+LL |     let _ = unsafe { libc::strlen(cstring.as_ptr()) };
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use: `cstring.count_bytes()`
 
-error: aborting due to 10 previous errors
+error: aborting due to 14 previous errors