`nullptr_t` bindings name a type that is never defined

Input C/C++ Header

#include <stddef.h>

nullptr_t ident(nullptr_t p);
int is_null(nullptr_t p);

Bindgen Invocation

$ bindgen input.h --no-rustfmt-bindings --output bindings.rs -- -std=c23

Actual Results

bindgen exits 0 and emits uses of nullptr_t without defining it:

pub type wchar_t = ::std::os::raw::c_int;
pub type max_align_t = f64;

unsafe extern "C" {
    pub fn ident(p: nullptr_t) -> nullptr_t;
}
unsafe extern "C" {
    pub fn is_null(p: nullptr_t) -> ::std::os::raw::c_int;
}

rustc then fails:

error[E0425]: cannot find type `nullptr_t` in this scope

clang's AST does have the type (BuiltinType 'nullptr_t', plus the stddef.h typedef). Layout is pointer-sized.

Expected Results

Either:

• emit a Rust alias for nullptr_t (for example *const c_void, which matches the pointer-sized builtin), or
• skip these declarations instead of generating code that does not compile.

Environment

bindgen release: 0.72.1
clang:           Apple clang 21.0.0 (also reproduced with clang 18 on x86_64-linux)
rustc:           1.97.1
target:          aarch64-apple-darwin

Reproduce the bindgen invocation with input.h and inspect bindings.rs, then trace how clang's BuiltinType nullptr_t and the stddef.h typedef are represented. Done means the generated bindings compile, either by defining a suitable Rust alias or by skipping the affected declarations; add coverage for this C23 input.