Attribute `btf_type_tag` makes `const`ness of pointers disappear

[ojeda]

From Rust-for-Linux/linux#835, reported by @YakoYakoYokuYoku.

I have not checked yet whether this is due to libclang, but nevertheless I wanted to have the issue opened here.

Input C/C++ Header

void f(const char __attribute__((btf_type_tag("a"))) *);

Bindgen Invocation

$ bindgen input.h

Actual Results

extern "C" {
    pub fn f(arg1: *mut ::std::os::raw::c_char);
}

Expected Results

extern "C" {
    pub fn f(arg1: *const ::std::os::raw::c_char);
}

[emilio]

It seems clang treats these a bit differently internally. For that function we get:

[2022-07-25T08:57:49Z DEBUG bindgen::ir::item] Item::from_ty_with_id: ItemId(11)
    	ty = Type(char const * __attribute__((btf_type_tag)), kind: Pointer, cconv: 100, decl: Cursor( kind: NoDeclFound, loc: builtin definitions, usr: None), canon: Cursor( kind: NoDeclFound, loc: builtin definitions, usr: None)),
    	location = Cursor( kind: ParmDecl, loc: t.h:1:55, usr: None)
[2022-07-25T08:57:49Z DEBUG bindgen::ir::context] builtin_or_resolved_ty: Type(char const * __attribute__((btf_type_tag)), kind: Pointer, cconv: 100, decl: Cursor( kind: NoDeclFound, loc: builtin definitions, usr: None), canon: Cursor( kind: NoDeclFound, loc: builtin definitions, usr: None)), Some(Cursor( kind: ParmDecl, loc: t.h:1:55, usr: None)), ItemId(11), None
[2022-07-25T08:57:49Z DEBUG bindgen::ir::context] Not resolved, maybe builtin?
[2022-07-25T08:57:49Z DEBUG bindgen::ir::context] builtin_or_resolved_ty: Type(char const * __attribute__((btf_type_tag)), kind: Pointer, cconv: 100, decl: Cursor( kind: NoDeclFound, loc: builtin definitions, usr: None), canon: Cursor( kind: NoDeclFound, loc: builtin definitions, usr: None)), Some(Cursor( kind: ParmDecl, loc: t.h:1:55, usr: None)), ItemId(11), None
[2022-07-25T08:57:49Z DEBUG bindgen::ir::context] Not resolved, maybe builtin?
[2022-07-25T08:57:49Z DEBUG bindgen::ir::ty] from_clang_ty: ItemId(11), ty: Type(char const * __attribute__((btf_type_tag)), kind: Pointer, cconv: 100, decl: Cursor( kind: NoDeclFound, loc: builtin definitions, usr: None), canon: Cursor( kind: NoDeclFound, loc: builtin definitions, usr: None)), loc: Cursor( kind: ParmDecl, loc: t.h:1:55, usr: None)
[2022-07-25T08:57:49Z DEBUG bindgen::ir::ty] currently_parsed_types: []
[2022-07-25T08:57:49Z DEBUG bindgen::ir::item] from_ty_or_ref_with_id: ItemId(12) Type(char, kind: Char_S, cconv: 100, decl: Cursor( kind: NoDeclFound, loc: builtin definitions, usr: None), canon: Cursor( kind: NoDeclFound, loc: builtin definitions, usr: None)), Cursor( kind: ParmDecl, loc: t.h:1:55, usr: None), None
[2022-07-25T08:57:49Z DEBUG bindgen::ir::item] refs already collected, resolving directly
[2022-07-25T08:57:49Z DEBUG bindgen::ir::item] Item::from_ty_with_id: ItemId(12)
    	ty = Type(char, kind: Char_S, cconv: 100, decl: Cursor( kind: NoDeclFound, loc: builtin definitions, usr: None), canon: Cursor( kind: NoDeclFound, loc: builtin definitions, usr: None)),
    	location = Cursor( kind: ParmDecl, loc: t.h:1:55, usr: None)
[2022-07-25T08:57:49Z DEBUG bindgen::ir::context] builtin_or_resolved_ty: Type(char, kind: Char_S, cconv: 100, decl: Cursor( kind: NoDeclFound, loc: builtin definitions, usr: None), canon: Cursor( kind: NoDeclFound, loc: builtin definitions, usr: None)), Some(Cursor( kind: ParmDecl, loc: t.h:1:55, usr: None)), ItemId(12), None
[2022-07-25T08:57:49Z DEBUG bindgen::ir::context] Not resolved, maybe builtin?
[2022-07-25T08:57:49Z DEBUG bindgen::ir::context] add_builtin_item: item = Item { id: ItemId(13), local_id: LazyCell { inner: UnsafeCell { .. } }, next_child_local_id: Cell { value: 1 }, canonical_name: LazyCell { inner: UnsafeCell { .. } }, path_for_allowlisting: LazyCell { inner: UnsafeCell { .. } }, comment: None, annotations: Annotations { opaque: false, hide: false, use_instead_of: None, disallow_copy: false, disallow_debug: false, disallow_default: false, must_use_type: false, private_fields: None, accessor_kind: None, constify_enum_variant: false, derives: [] }, parent_id: ItemId(0), kind: Type(Type { name: Some("char"), layout: Some(Layout { size: 1, align: 1, packed: false }), kind: Int(Char { is_signed: true }), is_const: false }), location: Some(builtin definitions) }
[2022-07-25T08:57:49Z DEBUG bindgen::ir::context] add_item_to_module: adding ItemId(13) as child of parent module ItemId(0)
[2022-07-25T08:57:49Z DEBUG bindgen::ir::context] BindgenContext::add_item(Item { id: ItemId(11), local_id: LazyCell { inner: UnsafeCell { .. } }, next_child_local_id: Cell { value: 1 }, canonical_name: LazyCell { inner: UnsafeCell { .. } }, path_for_allowlisting: LazyCell { inner: UnsafeCell { .. } }, comment: None, annotations: Annotations { opaque: false, hide: false, use_instead_of: None, disallow_copy: false, disallow_debug: false, disallow_default: false, must_use_type: false, private_fields: None, accessor_kind: None, constify_enum_variant: false, derives: [] }, parent_id: ItemId(0), kind: Type(Type { name: None, layout: Some(Layout { size: 8, align: 8, packed: false }), kind: Pointer(TypeId(ItemId(13))), is_const: false }), location: Some(t.h:1:55) }, declaration: Some(Cursor( kind: NoDeclFound, loc: builtin definitions, usr: None)), loc: Some(Cursor( kind: ParmDecl, loc: t.h:1:55, usr: None))
[2022-07-25T08:57:49Z DEBUG bindgen::ir::context] add_item_to_module: adding ItemId(11) as child of parent module ItemId(0)
[2022-07-25T08:57:49Z DEBUG bindgen::ir::context] Invalid declaration Cursor( kind: NoDeclFound, loc: builtin definitions, usr: None) found for type Type { name: None, layout: Some(Layout { size: 8, align: 8, packed: false }), kind: Pointer(TypeId(ItemId(13))), is_const: false }

If I remove the attribute:

[2022-07-25T08:57:49Z DEBUG bindgen::ir::item] Item::from_ty_with_id: ItemId(14)
    	ty = Type(const char *, kind: Pointer, cconv: 100, decl: Cursor( kind: NoDeclFound, loc: builtin definitions, usr: None), canon: Cursor( kind: NoDeclFound, loc: builtin definitions, usr: None)),
    	location = Cursor( kind: ParmDecl, loc: t.h:2:19, usr: None)
[2022-07-25T08:57:49Z DEBUG bindgen::ir::context] builtin_or_resolved_ty: Type(const char *, kind: Pointer, cconv: 100, decl: Cursor( kind: NoDeclFound, loc: builtin definitions, usr: None), canon: Cursor( kind: NoDeclFound, loc: builtin definitions, usr: None)), Some(Cursor( kind: ParmDecl, loc: t.h:2:19, usr: None)), ItemId(14), None
[2022-07-25T08:57:49Z DEBUG bindgen::ir::context] Not resolved, maybe builtin?
[2022-07-25T08:57:49Z DEBUG bindgen::ir::context] builtin_or_resolved_ty: Type(const char *, kind: Pointer, cconv: 100, decl: Cursor( kind: NoDeclFound, loc: builtin definitions, usr: None), canon: Cursor( kind: NoDeclFound, loc: builtin definitions, usr: None)), Some(Cursor( kind: ParmDecl, loc: t.h:2:19, usr: None)), ItemId(14), None
[2022-07-25T08:57:49Z DEBUG bindgen::ir::context] Not resolved, maybe builtin?
[2022-07-25T08:57:49Z DEBUG bindgen::ir::ty] from_clang_ty: ItemId(14), ty: Type(const char *, kind: Pointer, cconv: 100, decl: Cursor( kind: NoDeclFound, loc: builtin definitions, usr: None), canon: Cursor( kind: NoDeclFound, loc: builtin definitions, usr: None)), loc: Cursor( kind: ParmDecl, loc: t.h:2:19, usr: None)
[2022-07-25T08:57:49Z DEBUG bindgen::ir::ty] currently_parsed_types: []
[2022-07-25T08:57:49Z DEBUG bindgen::ir::item] from_ty_or_ref_with_id: ItemId(15) Type(const char, kind: Char_S, cconv: 100, decl: Cursor( kind: NoDeclFound, loc: builtin definitions, usr: None), canon: Cursor( kind: NoDeclFound, loc: builtin definitions, usr: None)), Cursor( kind: ParmDecl, loc: t.h:2:19, usr: None), None
[2022-07-25T08:57:49Z DEBUG bindgen::ir::item] refs already collected, resolving directly
[2022-07-25T08:57:49Z DEBUG bindgen::ir::item] Item::from_ty_with_id: ItemId(15)
    	ty = Type(const char, kind: Char_S, cconv: 100, decl: Cursor( kind: NoDeclFound, loc: builtin definitions, usr: None), canon: Cursor( kind: NoDeclFound, loc: builtin definitions, usr: None)),
    	location = Cursor( kind: ParmDecl, loc: t.h:2:19, usr: None)
[2022-07-25T08:57:49Z DEBUG bindgen::ir::context] builtin_or_resolved_ty: Type(const char, kind: Char_S, cconv: 100, decl: Cursor( kind: NoDeclFound, loc: builtin definitions, usr: None), canon: Cursor( kind: NoDeclFound, loc: builtin definitions, usr: None)), Some(Cursor( kind: ParmDecl, loc: t.h:2:19, usr: None)), ItemId(15), None
[2022-07-25T08:57:49Z DEBUG bindgen::ir::context] Not resolved, maybe builtin?
[2022-07-25T08:57:49Z DEBUG bindgen::ir::context] add_builtin_item: item = Item { id: ItemId(16), local_id: LazyCell { inner: UnsafeCell { .. } }, next_child_local_id: Cell { value: 1 }, canonical_name: LazyCell { inner: UnsafeCell { .. } }, path_for_allowlisting: LazyCell { inner: UnsafeCell { .. } }, comment: None, annotations: Annotations { opaque: false, hide: false, use_instead_of: None, disallow_copy: false, disallow_debug: false, disallow_default: false, must_use_type: false, private_fields: None, accessor_kind: None, constify_enum_variant: false, derives: [] }, parent_id: ItemId(0), kind: Type(Type { name: Some("const char"), layout: Some(Layout { size: 1, align: 1, packed: false }), kind: Int(Char { is_signed: true }), is_const: true }), location: Some(builtin definitions) }
[2022-07-25T08:57:49Z DEBUG bindgen::ir::context] add_item_to_module: adding ItemId(16) as child of parent module ItemId(0)

In particular, it seems clang_getPointeeType returns a different type for this case (and loses the constness). Not sure if working around it in bindgen is doable / easy yet.

[emilio]

Seems constness is completely lost at the libclang level :'(

From a quick printf like this:

diff --git a/src/ir/ty.rs b/src/ir/ty.rs
index d573408c..13701e28 100644
--- a/src/ir/ty.rs
+++ b/src/ir/ty.rs
@@ -1032,6 +1032,7 @@ impl Type {
                 CXType_MemberPointer |
                 CXType_Pointer => {
                     let pointee = ty.pointee_type().unwrap();
+                    println!("{:?} -> {:?} is_const {:?} -> {:?}", ty, pointee, ty.is_const(), pointee.is_const());
                     let inner =
                         Item::from_ty_or_ref(pointee, location, None, ctx);
                     TypeKind::Pointer(inner)

Type(char const * __attribute__((btf_type_tag)), kind: Pointer, cconv: 100, decl: Cursor( kind: NoDeclFound, loc: builtin definitions, usr: None), canon: Cursor( kind: NoDeclFound, loc: builtin definitions, usr: None)) -> Type(char, kind: Char_S, cconv: 100, decl: Cursor( kind: NoDeclFound, loc: builtin definitions, usr: None), canon: Cursor( kind: NoDeclFound, loc: builtin definitions, usr: None)) is_const false -> false
Type(const char *, kind: Pointer, cconv: 100, decl: Cursor( kind: NoDeclFound, loc: builtin definitions, usr: None), canon: Cursor( kind: NoDeclFound, loc: builtin definitions, usr: None)) -> Type(const char, kind: Char_S, cconv: 100, decl: Cursor( kind: NoDeclFound, loc: builtin definitions, usr: None), canon: Cursor( kind: NoDeclFound, loc: builtin definitions, usr: None)) is_const false -> true

However from a clang ast dump with --emit-clang-ast, it seems we could use the canonical type here:

     type.kind = Pointer
     type.cconv = 100
     type.spelling = "char const * __attribute__((btf_type_tag))"
     type.is-variadic? false

     type.canonical.kind = Pointer
     type.canonical.cconv = 100
     type.canonical.spelling = "const char *"
     type.canonical.is-variadic? false

     type.canonical.pointee.kind = Char_S
     type.canonical.pointee.cconv = 100
     type.canonical.pointee.spelling = "const char"
     type.canonical.pointee.is-variadic? false

     type.pointee.kind = Char_S
     type.pointee.cconv = 100
     type.pointee.spelling = "char"
     type.pointee.is-variadic? false

[nickdesaulniers]

@ojeda I think you can drop the workaround now from kbuild if you upgrade to a newer release of bindgen that has this fix?

[nickdesaulniers]

@ojeda rust/bindgen_parameters also contains a comment about 2aed6b0 in 0.59.2. Should the minimum version of bindgen be raised for these 2 issues?