lint ImproperCTypes: split the two lints into four total [...]

- now the lint scans repr(C) struct/enum/union definitions
- it now also scans method declarations in traits
- many other changes in the underlying logic
- some extra tests

Author: niacdoial

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

@@ -368,7 +368,7 @@ 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
 
@@ -416,35 +416,34 @@ lint_improper_ctypes_pat_intrange_reason = integers constrained to a given range
 
 lint_improper_ctypes_ptr_validity_help = consider using a raw pointer, or wrapping `{$ty}` in an `Option<_>`
 lint_improper_ctypes_ptr_validity_reason =
-    boxes and references are assumed to be valid (non-null, non-dangling, aligned) pointers,
+    boxes, references, and function pointers are assumed to be valid (non-null, non-dangling, aligned) pointers,
     which cannot be garanteed if their values are produced by non-rust code
 
-lint_improper_ctypes_sized_ptr_to_unsafe_type =
-    this reference (`{$ty}`) is ABI-compatible with a C pointer, but `{$inner_ty}` itself does not have a C layout
-
 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_reason = string slices have no C equivalent
 
+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 = 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_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_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

compiler/rustc_lint/src/lib.rs

@@ -195,8 +195,7 @@ late_lint_methods!(
             DefaultCouldBeDerived: DefaultCouldBeDerived::default(),
             DerefIntoDynSupertrait: DerefIntoDynSupertrait,
             DropForgetUseless: DropForgetUseless,
-            ImproperCTypesDeclarations: ImproperCTypesDeclarations,
-            ImproperCTypesDefinitions: ImproperCTypesDefinitions,
+            ImproperCTypesLint: ImproperCTypesLint,
             InvalidFromUtf8: InvalidFromUtf8,
             VariantSizeDifferences: VariantSizeDifferences,
             PathStatements: PathStatements,
@@ -337,6 +336,14 @@ fn register_builtins(store: &mut LintStore) {
         REFINING_IMPL_TRAIT_INTERNAL
     );
 
+    add_lint_group!(
+        "improper_c_boundaries",
+        IMPROPER_C_CALLBACKS,
+        IMPROPER_C_FN_DEFINITIONS,
+        IMPROPER_CTYPE_DEFINITIONS,
+        IMPROPER_CTYPES
+    );
+
     add_lint_group!("deprecated_safe", DEPRECATED_SAFE_2024);
 
     add_lint_group!(
@@ -364,6 +371,7 @@ fn register_builtins(store: &mut LintStore) {
     store.register_renamed("static_mut_ref", "static_mut_refs");
     store.register_renamed("temporary_cstring_as_ptr", "dangling_pointers_from_temporaries");
     store.register_renamed("elided_named_lifetimes", "mismatched_lifetime_syntaxes");
+    store.register_renamed("improper_ctypes_definitions", "improper_c_fn_definitions");
 
     // These were moved to tool lints, but rustc still sees them when compiling normally, before
     // tool lints are registered, so `check_tool_name_for_backwards_compat` doesn't work. Use

compiler/rustc_lint/src/types.rs

@@ -11,7 +11,10 @@ use tracing::debug;
 use {rustc_ast as ast, rustc_hir as hir};
 
 mod improper_ctypes; // these filed do the implementation for ImproperCTypesDefinitions,ImproperCTypesDeclarations
-pub(crate) use improper_ctypes::{ImproperCTypesDeclarations, ImproperCTypesDefinitions};
+pub(crate) use improper_ctypes::{
+    IMPROPER_C_CALLBACKS, IMPROPER_C_FN_DEFINITIONS, IMPROPER_CTYPE_DEFINITIONS, IMPROPER_CTYPES,
+    ImproperCTypesLint,
+};
 
 use crate::lints::{
     AmbiguousWidePointerComparisons, AmbiguousWidePointerComparisonsAddrMetadataSuggestion,
@@ -705,6 +708,26 @@ pub(crate) fn transparent_newtype_field<'a, 'tcx>(
     })
 }
 
+/// for a given ADT variant, list which fields are non-1ZST
+/// (`repr(transparent)` guarantees that there is at most one)
+pub(crate) fn map_non_1zst_fields<'a, 'tcx>(
+    tcx: TyCtxt<'tcx>,
+    variant: &'a ty::VariantDef,
+) -> Vec<bool> {
+    let typing_env = ty::TypingEnv::non_body_analysis(tcx, variant.def_id);
+    variant
+        .fields
+        .iter()
+        .map(|field| {
+            let field_ty = tcx.type_of(field.did).instantiate_identity();
+            let is_1zst = tcx
+                .layout_of(typing_env.as_query_input(field_ty))
+                .is_ok_and(|layout| layout.is_1zst());
+            !is_1zst
+        })
+        .collect()
+}
+
 /// Is type known to be non-null?
 fn ty_is_known_nonnull<'tcx>(
     tcx: TyCtxt<'tcx>,