define MBEDTLS_CHECK_RETURN_WARNING

Although, bindgen needs .enable_function_attribute_detection()
to process __attribute__((__warn_unused_result__)) because parsing
attrs can be really slow in certain cases. Benches were performed
to confirm our case doesn't face that issue.

References:
rust-lang/rust-bindgen#2149
rust-lang/rust-bindgen#1465
rust-lang/rust-bindgen#1466
rust-lang/rust-bindgen#1467

Author: MihirLuthra

mbedtls-sys/build/bindgen.rs

@@ -117,6 +117,7 @@ impl super::BuildConfig {
         }
 
         let bindings = bindgen::builder()
+            .enable_function_attribute_detection()
             .clang_args(cc.get_compiler().args().iter().map(|arg| arg.to_str().unwrap()))
             .header_contents("bindgen-input.h", &input)
             .allowlist_function("^(?i)mbedtls_.*")

mbedtls-sys/build/config.rs

@@ -146,7 +146,7 @@ const DEFAULT_DEFINES: &'static [CDefine] = &[
     ("MBEDTLS_AES_ROM_TABLES",                            Undefined),
     ("MBEDTLS_AES_FEWER_TABLES",                          Undefined),
     ("MBEDTLS_CAMELLIA_SMALL_MEMORY",                     Undefined),
-    ("MBEDTLS_CHECK_RETURN_WARNING",                      Undefined),
+    ("MBEDTLS_CHECK_RETURN_WARNING",                      Defined),
     ("MBEDTLS_CIPHER_MODE_CBC",                           Defined),
     ("MBEDTLS_CIPHER_MODE_CFB",                           Defined),
     ("MBEDTLS_CIPHER_MODE_CTR",                           Defined),