Support marking C functions as deprecated

Author: ctz

librustls/cbindgen.toml

@@ -1,8 +1,20 @@
 include_guard = "RUSTLS_H"
 language = "C"
+after_includes = """
+#if defined(__clang__) || defined(__GNUC__)
+# define DEPRECATED_FUNC(why) __attribute__((deprecated(why)))
+#elif defined(_MSC_VER)
+# define DEPRECATED_FUNC(why) __declspec(deprecated(why))
+#else
+# define DEPRECATED_FUNC(why)
+#endif
+"""
 
 usize_is_size_t = true
 
+[fn]
+deprecated_with_note = "DEPRECATED_FUNC({})"
+
 [enum]
 prefix_with_name = true
 rename_variants = "ScreamingSnakeCase"

librustls/src/rustls.h

@@ -6,6 +6,14 @@
 #include <stddef.h>
 #include <stdint.h>
 #include <stdlib.h>
+#if defined(__clang__) || defined(__GNUC__)
+# define DEPRECATED_FUNC(why) __attribute__((deprecated(why)))
+#elif defined(_MSC_VER)
+# define DEPRECATED_FUNC(why) __declspec(deprecated(why))
+#else
+# define DEPRECATED_FUNC(why)
+#endif
+
 
 /**
  * Describes which sort of handshake happened.