lint ImproperCTypes: overhaul (take 2 of "better handling of indirections")

compiler/rustc_codegen_cranelift/example/std_example.rs

@@ -209,7 +209,7 @@ fn rust_call_abi() {
 struct I64X2([i64; 2]);
 
 #[cfg_attr(target_arch = "s390x", allow(dead_code))]
-#[allow(improper_ctypes_definitions)]
+#[allow(improper_c_fn_definitions)]
 extern "C" fn foo(_a: I64X2) {}
 
 #[cfg(target_arch = "x86_64")]

compiler/rustc_lint/messages.ftl

@@ -350,21 +350,22 @@ lint_implicit_unsafe_autorefs = implicit autoref creates a reference to the dere
     .method_def = method calls to `{$method_name}` require a reference
     .suggestion = try using a raw pointer method instead; or if this reference is intentional, make it explicit
 
-lint_improper_ctypes = `extern` {$desc} uses type `{$ty}`, which is not FFI-safe
+lint_improper_ctypes = {$desc} uses type `{$ty}`, which is not FFI-safe
     .label = not FFI-safe
     .note = the type is defined here
 
 lint_improper_ctypes_array_help = consider passing a pointer to the array
 
 lint_improper_ctypes_array_reason = passing raw arrays by value is not FFI-safe
-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 passing a `*const std::ffi::c_char` instead, and use `CStr::as_ptr()`
+lint_improper_ctypes_cstr_help_const =
+    consider passing a `*const std::ffi::c_char` instead, converting to/from `{$ty}` as needed
+lint_improper_ctypes_cstr_help_mut =
+    consider passing a `*mut std::ffi::c_char` instead, converting to/from `{$ty}` as needed
 lint_improper_ctypes_cstr_reason = `CStr`/`CString` do not have a guaranteed layout
 
 lint_improper_ctypes_dyn = trait objects have no C equivalent
@@ -375,40 +376,51 @@ lint_improper_ctypes_enum_repr_help =
 lint_improper_ctypes_enum_repr_reason = enum has no representation hint
 lint_improper_ctypes_fnptr_help = consider using an `extern fn(...) -> ...` function pointer instead
 
+lint_improper_ctypes_fnptr_indirect_reason = the function pointer to `{$ty}` is FFI-unsafe due to `{$inner_ty}`
 lint_improper_ctypes_fnptr_reason = this function pointer has Rust-specific calling convention
+
 lint_improper_ctypes_non_exhaustive = this enum is non-exhaustive
 lint_improper_ctypes_non_exhaustive_variant = this enum has non-exhaustive variants
 
 lint_improper_ctypes_only_phantomdata = composed only of `PhantomData`
 
 lint_improper_ctypes_opaque = opaque types have no C equivalent
 
-lint_improper_ctypes_slice_help = consider using a raw pointer instead
-
+lint_improper_ctypes_slice_help = consider using a raw pointer to the slice's first element (and a length) instead
 lint_improper_ctypes_slice_reason = slices have no C equivalent
-lint_improper_ctypes_str_help = consider using `*const u8` and a length instead
 
+lint_improper_ctypes_str_help = consider using `*const u8` and a length instead
 lint_improper_ctypes_str_reason = string slices have no C equivalent
-lint_improper_ctypes_struct_fieldless_help = consider adding a member to this struct
 
-lint_improper_ctypes_struct_fieldless_reason = this struct has no fields
-lint_improper_ctypes_struct_layout_help = consider adding a `#[repr(C)]` or `#[repr(transparent)]` attribute to this struct
+lint_improper_ctypes_struct_consider_transparent = `{$ty}` has exactly one non-zero-sized field, consider making it `#[repr(transparent)]` instead
+lint_improper_ctypes_struct_dueto = this struct/enum/union (`{$ty}`) is FFI-unsafe due to a `{$inner_ty}` field
+
+lint_improper_ctypes_struct_fieldless_help = consider adding a member to this struct
+lint_improper_ctypes_struct_fieldless_reason = `{$ty}` has no fields
 
-lint_improper_ctypes_struct_layout_reason = this struct has unspecified layout
-lint_improper_ctypes_struct_non_exhaustive = this struct is non-exhaustive
-lint_improper_ctypes_struct_zst = this struct contains only zero-sized fields
+lint_improper_ctypes_struct_layout_help = consider adding a `#[repr(C)]` (not `#[repr(C,packed)]`) or `#[repr(transparent)]` attribute to `{$ty}`
+lint_improper_ctypes_struct_layout_reason = `{$ty}` has unspecified layout
+lint_improper_ctypes_struct_non_exhaustive = `{$ty}` is non-exhaustive
+lint_improper_ctypes_struct_zst = `{$ty}` contains only zero-sized fields
 
 lint_improper_ctypes_tuple_help = consider using a struct instead
-
 lint_improper_ctypes_tuple_reason = tuples have unspecified layout
-lint_improper_ctypes_union_fieldless_help = consider adding a member to this union
 
+lint_improper_ctypes_uninhabited_enum = zero-variant enums and other uninhabited types are not allowed in function arguments and static variables
+lint_improper_ctypes_uninhabited_never = the never type (`!`) and other uninhabited types are not allowed in function arguments and static variables
+
+lint_improper_ctypes_union_consider_transparent = `{$ty}` has exactly one non-zero-sized field, consider making it `#[repr(transparent)]` instead
+lint_improper_ctypes_union_fieldless_help = consider adding a member to this union
 lint_improper_ctypes_union_fieldless_reason = this union has no fields
-lint_improper_ctypes_union_layout_help = consider adding a `#[repr(C)]` or `#[repr(transparent)]` attribute to this union
+lint_improper_ctypes_union_layout_help = consider adding a `#[repr(C)]` attribute to this union
 
 lint_improper_ctypes_union_layout_reason = this union has unspecified layout
 lint_improper_ctypes_union_non_exhaustive = this union is non-exhaustive
 
+lint_improper_ctypes_unsized_box = this box for an unsized type contains metadata, which makes it incompatible with a C pointer
+lint_improper_ctypes_unsized_ptr = this pointer to an unsized type contains metadata, which makes it incompatible with a C pointer
+lint_improper_ctypes_unsized_ref = this reference to an unsized type contains metadata, which makes it incompatible with a C pointer
+
 lint_incomplete_include =
     include macro expected single expression in source
 

compiler/rustc_lint/src/lib.rs

@@ -335,6 +335,18 @@ fn register_builtins(store: &mut LintStore) {
         REFINING_IMPL_TRAIT_INTERNAL
     );
 
+    add_lint_group!(
+        "improper_c_boundaries",
+        IMPROPER_C_CALLBACKS,
+        IMPROPER_C_FN_DEFINITIONS,
+        IMPROPER_C_VAR_DEFINITIONS,
+        IMPROPER_CTYPES
+    );
+
+    // FIXME(ctypes, migration): when should this retrocompatibility-borne
+    // lint group disappear, if at all? Should it turn into a register_renamed?
+    add_lint_group!("improper_ctypes_definitions", IMPROPER_C_CALLBACKS, IMPROPER_C_FN_DEFINITIONS);
+
     add_lint_group!("deprecated_safe", DEPRECATED_SAFE_2024);
 
     add_lint_group!(

compiler/rustc_lint/src/lints.rs

@@ -1943,29 +1943,55 @@ pub(crate) enum UnpredictableFunctionPointerComparisonsSuggestion<'a, 'tcx> {
     },
 }
 
+pub(crate) struct ImproperCTypesLayer<'a> {
+    pub ty: Ty<'a>,
+    pub inner_ty: Option<Ty<'a>>,
+    pub note: DiagMessage,
+    pub span_note: Option<Span>,
+    pub help: Option<DiagMessage>,
+}
+
+impl<'a> Subdiagnostic for ImproperCTypesLayer<'a> {
+    fn add_to_diag<G: EmissionGuarantee>(self, diag: &mut Diag<'_, G>) {
+        diag.arg("ty", self.ty);
+        if let Some(ty) = self.inner_ty {
+            diag.arg("inner_ty", ty);
+        }
+
+        if let Some(help) = self.help {
+            diag.help(diag.eagerly_translate(help));
+        }
+
+        diag.note(diag.eagerly_translate(self.note));
+        if let Some(note) = self.span_note {
+            diag.span_note(note, fluent::lint_note);
+        };
+
+        diag.remove_arg("ty");
+        if self.inner_ty.is_some() {
+            diag.remove_arg("inner_ty");
+        }
+    }
+}
+
 pub(crate) struct ImproperCTypes<'a> {
     pub ty: Ty<'a>,
     pub desc: &'a str,
     pub label: Span,
-    pub help: Option<DiagMessage>,
-    pub note: DiagMessage,
-    pub span_note: Option<Span>,
+    pub reasons: Vec<ImproperCTypesLayer<'a>>,
 }
 
 // Used because of the complexity of Option<DiagMessage>, DiagMessage, and Option<Span>
 impl<'a> LintDiagnostic<'a, ()> for ImproperCTypes<'_> {
     fn decorate_lint<'b>(self, diag: &'b mut Diag<'a, ()>) {
         diag.primary_message(fluent::lint_improper_ctypes);
-        diag.arg("ty", self.ty);
-        diag.arg("desc", self.desc);
         diag.span_label(self.label, fluent::lint_label);
-        if let Some(help) = self.help {
-            diag.help(help);
-        }
-        diag.note(self.note);
-        if let Some(note) = self.span_note {
-            diag.span_note(note, fluent::lint_note);
+        for reason in self.reasons.into_iter() {
+            diag.subdiagnostic(reason);
         }
+        // declare the arguments at the end to avoid them being clobbered in the subdiagnostics
+        diag.arg("ty", self.ty);
+        diag.arg("desc", self.desc);
     }
 }