relibc: move stddef header from C to cbindgen (integrate upstream 3be84f4b)

Cherry-pick upstream commit 3be84f4b (auronandace, 2026-06-24):
- Delete hand-written include/stddef.h
- Create src/header/stddef/ with cbindgen.toml + mod.rs
- Create src/header/bits_wchar-t/ with cbindgen.toml + mod.rs
- Create src/header/bits_size-t/ with cbindgen.toml + mod.rs
- Create src/header/bits_null/ with cbindgen.toml + mod.rs
- Update src/header/mod.rs: add bits_* modules, uncomment stddef

The generated stddef.h now includes modular bits_* sub-headers:
  #include <bits/wchar-t.h>  — wchar_t with _WCHAR_T guard + __WCHAR_TYPE__
  #include <bits/size-t.h>   — size_t from Rust usize via cbindgen [export]
  #include <bits/null.h>     — NULL: nullptr (C++11), 0L (C++), (void*)0 (C)

This fixes the wchar_t circular include issue structurally.

include/stddef.h

@@ -1,22 +0,0 @@
-#ifndef _STDDEF_H
-#define _STDDEF_H
-#include <stdint.h>
-
-#define NULL 0
-
-#ifndef __PTRDIFF_TYPE__
-#define __PTRDIFF_TYPE__ long int
-#endif
-typedef __PTRDIFF_TYPE__ ptrdiff_t;
-
-typedef long unsigned int size_t;
-
-#ifndef __cplusplus
-typedef int wchar_t;
-#endif
-
-typedef struct { long long __ll; long double __ld; } max_align_t;
-
-#define offsetof(type, member) __builtin_offsetof(type, member)
-
-#endif /* _STDDEF_H */

src/header/bits_null/cbindgen.toml

@@ -0,0 +1,20 @@
+# POSIX header spec: https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/stddef.h.html
+#
+# This type is split out to prevent importing all of stddef.h into other headers.
+include_guard = "_RELIBC_BITS_NULL_T_H"
+language = "C"
+no_includes = true
+after_includes = """
+// Null pointer constant.
+// Any pointer object whose representation has all bits set to zero, perhaps by
+// memset() to 0 or by calloc(), shall be treated as a null pointer.
+#ifdef __cplusplus
+#if __cplusplus >= 201103L
+  #define NULL nullptr
+#else
+  #define NULL 0L
+#endif
+#else
+  #define NULL ((void *)0)
+#endif
+"""

src/header/bits_null/mod.rs

@@ -0,0 +1,5 @@
+//! `NULL` from `stddef.h` implementation.
+//!
+//! See <https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/stddef.h.html>.
+//!
+//! Implemented via ifdefs and defines in cbindgen.

src/header/bits_size-t/cbindgen.toml

@@ -0,0 +1,11 @@
+# POSIX header spec: https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/stddef.h.html
+#
+# This type is split out to prevent importing all of stddef.h into other headers.
+include_guard = "_RELIBC_BITS_SIZE_T_H"
+language = "C"
+no_includes = true
+cpp_compat = true
+[export]
+include = ["size_t"]
+[enum]
+prefix_with_name = true

src/header/bits_size-t/mod.rs

@@ -0,0 +1,9 @@
+//! `size_t` from `stddef.h` implementation.
+//!
+//! See <https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/stddef.h.html>.
+
+use crate::platform::types::c_ulong;
+
+/// Unsigned integer type of the result of the sizeof operator.
+#[allow(non_camel_case_types)]
+pub type size_t = c_ulong;

src/header/bits_wchar-t/cbindgen.toml

@@ -0,0 +1,21 @@
+# POSIX header spec: https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/stddef.h.html
+#
+# This type is split out to prevent importing all of stddef.h into inttypes.h
+include_guard = "_RELIBC_BITS_WCHAR_T_H"
+after_includes = """
+// Integer type whose range of values can represent distinct codes for all
+// members of the largest extended character set specified among the supported
+// locales; the null character shall have the code value zero.
+#ifndef _WCHAR_T
+#define _WCHAR_T
+#ifndef __cplusplus
+    #ifndef __WCHAR_TYPE__
+        #define __WCHAR_TYPE__ int
+    #endif
+    typedef __WCHAR_TYPE__ wchar_t;
+#endif // __cplusplus
+#endif // _WCHAR_T
+"""
+language = "C"
+no_includes = true
+cpp_compat = true

src/header/bits_wchar-t/mod.rs

@@ -0,0 +1,5 @@
+//! `wchar_t` from `stddef.h` implementation.
+//!
+//! See <https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/stddef.h.html>.
+//!
+//! Implemented via ifdefs and defines in cbindgen.

src/header/mod.rs

@@ -9,14 +9,19 @@ pub mod bits_eventfd;
 pub mod bits_iovec;
 #[path = "bits_locale-t/mod.rs"]
 pub mod bits_locale_t;
+pub mod bits_null;
 pub mod bits_pthread;
 #[path = "bits_safamily-t/mod.rs"]
 pub mod bits_safamily_t;
 #[path = "bits_sigset-t/mod.rs"]
 pub mod bits_sigset_t;
+#[path = "bits_size-t/mod.rs"]
+pub mod bits_size_t;
 #[path = "bits_socklen-t/mod.rs"]
 pub mod bits_socklen_t;
 pub mod bits_timespec;
+#[path = "bits_wchar-t/mod.rs"]
+pub mod bits_wchar_t;
 // complex.h implemented in C
 pub mod cpio;
 pub mod crypt;
@@ -82,7 +87,7 @@ pub mod spawn;
 // stdarg.h implemented in C
 // stdatomic.h implemented in C
 // stdbool.h implemented in C
-// stddef.h implemented in C
+pub mod stddef;
 // stdint.h implemented in C
 pub mod stdio;
 pub mod stdlib;

src/header/stddef/cbindgen.toml

@@ -0,0 +1,29 @@
+# POSIX header spec: https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/stddef.h.html
+#
+# There are no spec quotations relating to includes
+include_guard = "_RELIBC_STDDEF_H"
+after_includes = """
+// wchar_t should come before NULL and size_t
+// according to comment in wchar cbindgen
+#include <bits/wchar-t.h>
+#include <bits/size-t.h>
+#include <bits/null.h>
+
+#ifndef __PTRDIFF_TYPE__
+#define __PTRDIFF_TYPE__ long int
+#endif
+// Signed integer type of the result of subtracting two pointers.
+typedef __PTRDIFF_TYPE__ ptrdiff_t;
+
+// Object type whose alignment is the greatest fundamental alignment.
+typedef struct { long long __ll; long double __ld; } max_align_t;
+
+// Integer constant expression of type size_t, the value of which is the offset
+// in bytes to the structure member from the beginning of its structure.
+#define offsetof(type, member) __builtin_offsetof(type, member)
+"""
+language = "C"
+no_includes = true
+cpp_compat = true
+[enum]
+prefix_with_name = true

src/header/stddef/mod.rs

@@ -0,0 +1,5 @@
+//! `stddef.h` implementation.
+//!
+//! See <https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/stddef.h.html>.
+//!
+//! Implemented via includes, typedefs and defines in cbindgen.