Conditionally compiled extern "C" { blocks do not get #if defined( guards

[NoraCodes]

While conditional compilation works fine within extern "C" blocks, annotating an entire extern "C" block with #[cfg()] is valid in Rust, but doesn't work in cbindgen.

For example, for the following Rust project:

# Cargo.toml
[package]
name = "extern_block_conditional"
version = "0.1.0"
authors = ["cbindgen"]

[lib]
name = "extern_block_conditional"
crate-type = ["lib", "dylib"]
features = [ "myfeature" ]

# cbindgen.toml
[defines]
"feature = myfeature" = "DEFINED"

// src/lib.rs
#[repr(C)]
struct Normal {
    x: i32,
    y: f32,
}

#[cfg(feature = "myfeature")]
extern "C" {
    fn foo() -> i32;

    fn bar(a: Normal);
}

I would expect the following C output:

#include <stdarg.h>
#include <stdbool.h>
#include <stdint.h>
#include <stdlib.h>

typedef struct {
  int32_t x;
  float y;
} Normal;

#if defined(DEFINED)
extern int32_t foo(void);

extern void bar(Normal a);
#endif

But in fact, writing this as a test case in this project, the output I see is:

#include <stdarg.h>
#include <stdbool.h>
#include <stdint.h>
#include <stdlib.h>

typedef struct {
  int32_t x;
  float y;
} Normal;

extern int32_t foo(void);

extern void bar(Normal a);

cc @katef