feat: also lint types that dereference to CStr

For simplicity's sake, this also changes the lint to always suggest
`to_bytes`, as it is somewhat hard to find out whether a type could
dereference to `CString` (which is what `as_bytes` would require)

Author: ada4a

clippy_lints/src/strlen_on_c_strings.rs

@@ -47,14 +47,19 @@ impl<'tcx> LateLintPass<'tcx> for StrlenOnCStrings {
             && let ExprKind::MethodCall(path, self_arg, [], _) = recv.kind
             && !recv.span.from_expansion()
             && path.ident.name == sym::as_ptr
+            && let typeck = cx.typeck_results()
+            && typeck
+                .expr_ty_adjusted(self_arg)
+                .peel_refs()
+                .is_lang_item(cx, LangItem::CStr)
         {
-            let ty = cx.typeck_results().expr_ty(self_arg).peel_refs();
-            let (ty_name, method_name) = if ty.is_diag_item(cx, sym::cstring_type) {
-                ("CString", "as_bytes")
+            let ty = typeck.expr_ty(self_arg).peel_refs();
+            let ty_kind = if ty.is_diag_item(cx, sym::cstring_type) {
+                "`CString` value"
             } else if ty.is_lang_item(cx, LangItem::CStr) {
-                ("CStr", "to_bytes")
+                "`CStr` value"
             } else {
-                return;
+                "type that dereferences to `CStr`"
             };
 
             let ctxt = expr.span.ctxt();
@@ -71,11 +76,11 @@ impl<'tcx> LateLintPass<'tcx> for StrlenOnCStrings {
                 cx,
                 STRLEN_ON_C_STRINGS,
                 span,
-                format!("using `libc::strlen` on a `{ty_name}` value"),
+                format!("using `libc::strlen` on a {ty_kind}"),
                 |diag| {
                     let mut app = Applicability::MachineApplicable;
                     let val_name = snippet_with_context(cx, self_arg.span, ctxt, "_", &mut app).0;
-                    diag.span_suggestion(span, "try", format!("{val_name}.{method_name}().len()"), app);
+                    diag.span_suggestion(span, "try", format!("{val_name}.to_bytes().len()"), app);
                 },
             );
         }

tests/ui/strlen_on_c_strings.fixed

@@ -1,13 +1,13 @@
 #![warn(clippy::strlen_on_c_strings)]
-#![allow(clippy::manual_c_str_literals)]
+#![allow(clippy::manual_c_str_literals, clippy::boxed_local)]
 
 use libc::strlen;
 use std::ffi::{CStr, CString};
 
 fn main() {
     // CString
     let cstring = CString::new("foo").expect("CString::new failed");
-    let _ = cstring.as_bytes().len();
+    let _ = cstring.to_bytes().len();
     //~^ ERROR: using `libc::strlen` on a `CString` value
 
     // CStr
@@ -34,3 +34,13 @@ fn main() {
     let _ = unsafe { f(cstr).to_bytes().len() };
     //~^ 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();
+    //~^ ERROR: using `libc::strlen` on a type that dereferences to `CStr`
+    let _ = box_cstr.to_bytes().len();
+    //~^ ERROR: using `libc::strlen` on a type that dereferences to `CStr`
+    let _ = arc_cstring.to_bytes().len();
+    //~^ ERROR: using `libc::strlen` on a type that dereferences to `CStr`
+}

tests/ui/strlen_on_c_strings.rs

@@ -1,5 +1,5 @@
 #![warn(clippy::strlen_on_c_strings)]
-#![allow(clippy::manual_c_str_literals)]
+#![allow(clippy::manual_c_str_literals, clippy::boxed_local)]
 
 use libc::strlen;
 use std::ffi::{CStr, CString};
@@ -34,3 +34,13 @@ fn main() {
     let _ = unsafe { strlen(f(cstr).as_ptr()) };
     //~^ 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 _ = unsafe { libc::strlen(box_cstring.as_ptr()) };
+    //~^ ERROR: using `libc::strlen` on a type that dereferences to `CStr`
+    let _ = unsafe { libc::strlen(box_cstr.as_ptr()) };
+    //~^ ERROR: using `libc::strlen` on a type that dereferences to `CStr`
+    let _ = unsafe { libc::strlen(arc_cstring.as_ptr()) };
+    //~^ ERROR: using `libc::strlen` on a type that dereferences to `CStr`
+}

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: try: `cstring.as_bytes().len()`
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `cstring.to_bytes().len()`
    |
    = note: `-D clippy::strlen-on-c-strings` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::strlen_on_c_strings)]`
@@ -43,5 +43,23 @@ error: using `libc::strlen` on a `CStr` value
 LL |     let _ = unsafe { strlen(f(cstr).as_ptr()) };
    |                      ^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `f(cstr).to_bytes().len()`
 
-error: aborting due to 7 previous errors
+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: try: `box_cstring.to_bytes().len()`
+
+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: try: `box_cstr.to_bytes().len()`
+
+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: try: `arc_cstring.to_bytes().len()`
+
+error: aborting due to 10 previous errors