Add a special case for CStr/CString in the improper_ctypes lint

Instead of saying to "consider adding a `#[repr(C)]` or `#[repr(transparent)]` attribute to this struct",
we now tell users to "Use `*const ffi::c_char` instead, and pass the value from `CStr::as_ptr()`" when the
type involved is a `CStr` or a `CString`.

compiler/rustc_lint/messages.ftl

@@ -258,6 +258,10 @@ lint_improper_ctypes_box = box cannot be represented as a single pointer
 lint_improper_ctypes_char_help = consider using `u32` or `libc::wchar_t` instead
 
 lint_improper_ctypes_char_reason = the `char` type has no C equivalent
+
+lint_improper_ctypes_cstr_help = consider using `*const std::ffi::c_char` instead, and pass the value from `CStr::as_ptr()`
+lint_improper_ctypes_cstr_reason = `CStr`/`CString` do not have a guaranteed layout
+
 lint_improper_ctypes_dyn = trait objects have no C equivalent
 
 lint_improper_ctypes_enum_repr_help =

compiler/rustc_lint/src/types.rs

@@ -1229,6 +1229,16 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
                 }
                 match def.adt_kind() {
                     AdtKind::Struct | AdtKind::Union => {
+                        if let Some(sym::cstring_type | sym::cstr_type) =
+                            tcx.get_diagnostic_name(def.did())
+                        {
+                            return FfiUnsafe {
+                                ty,
+                                reason: fluent::lint_improper_ctypes_cstr_reason,
+                                help: Some(fluent::lint_improper_ctypes_cstr_help),
+                            };
+                        }
+
                         if !def.repr().c() && !def.repr().transparent() {
                             return FfiUnsafe {
                                 ty,

compiler/rustc_span/src/symbol.rs

@@ -616,6 +616,7 @@ symbols! {
         crate_visibility_modifier,
         crt_dash_static: "crt-static",
         csky_target_feature,
+        cstr_type,
         cstring_type,
         ctlz,
         ctlz_nonzero,

library/core/src/ffi/c_str.rs

@@ -80,6 +80,7 @@ use crate::str;
 /// [str]: prim@str "str"
 #[derive(Hash)]
 #[stable(feature = "core_c_str", since = "1.64.0")]
+#[rustc_diagnostic_item = "cstr_type"]
 #[rustc_has_incoherent_inherent_impls]
 #[lang = "CStr"]
 // `fn from` in `impl From<&CStr> for Box<CStr>` current implementation relies

tests/ui/lint/lint-ctypes-cstr.rs

@@ -0,0 +1,16 @@
+#![crate_type = "lib"]
+#![deny(improper_ctypes)]
+
+use std::ffi::{CStr, CString};
+
+extern "C" {
+    fn take_cstr(s: CStr);
+    //~^ ERROR `extern` block uses type `CStr`, which is not FFI-safe
+    //~| HELP consider using `*const std::ffi::c_char` instead, and pass the value from `CStr::as_ptr()`
+    fn take_cstring(s: CString);
+    //~^ ERROR `extern` block uses type `CString`, which is not FFI-safe
+    //~| HELP consider using `*const std::ffi::c_char` instead, and pass the value from `CStr::as_ptr()`
+    fn take_cstring_ref(s: &CString);
+    //~^ ERROR `extern` block uses type `CString`, which is not FFI-safe
+    //~| HELP consider using `*const std::ffi::c_char` instead, and pass the value from `CStr::as_ptr()`
+}

tests/ui/lint/lint-ctypes-cstr.stderr

@@ -0,0 +1,34 @@
+error: `extern` block uses type `CStr`, which is not FFI-safe
+  --> $DIR/lint-ctypes-cstr.rs:7:21
+   |
+LL |     fn take_cstr(s: CStr);
+   |                     ^^^^ not FFI-safe
+   |
+   = help: consider using `*const std::ffi::c_char` instead, and pass the value from `CStr::as_ptr()`
+   = note: `CStr`/`CString` do not have a guaranteed layout
+note: the lint level is defined here
+  --> $DIR/lint-ctypes-cstr.rs:2:9
+   |
+LL | #![deny(improper_ctypes)]
+   |         ^^^^^^^^^^^^^^^
+
+error: `extern` block uses type `CString`, which is not FFI-safe
+  --> $DIR/lint-ctypes-cstr.rs:10:24
+   |
+LL |     fn take_cstring(s: CString);
+   |                        ^^^^^^^ not FFI-safe
+   |
+   = help: consider using `*const std::ffi::c_char` instead, and pass the value from `CStr::as_ptr()`
+   = note: `CStr`/`CString` do not have a guaranteed layout
+
+error: `extern` block uses type `CString`, which is not FFI-safe
+  --> $DIR/lint-ctypes-cstr.rs:13:28
+   |
+LL |     fn take_cstring_ref(s: &CString);
+   |                            ^^^^^^^^ not FFI-safe
+   |
+   = help: consider using `*const std::ffi::c_char` instead, and pass the value from `CStr::as_ptr()`
+   = note: `CStr`/`CString` do not have a guaranteed layout
+
+error: aborting due to 3 previous errors
+