ffi: Simplify hex_encode helper

Ever so slightly less efficient in how it will check the arguments,
but also reuses existing conversion logic.

rust/bridge/ffi/src/lib.rs

@@ -8,7 +8,9 @@
 
 use std::ffi::{c_char, c_uchar, CString};
 
-use libsignal_bridge::ffi::*;
+use libsignal_bridge::ffi::{self, *};
+use libsignal_bridge::ffi_arg_type;
+use libsignal_bridge_macros::bridge_fn;
 #[cfg(feature = "libsignal-bridge-testing")]
 #[allow(unused_imports)]
 use libsignal_bridge_testing::*;
@@ -99,33 +101,8 @@ pub unsafe extern "C" fn signal_identitykeypair_deserialize(
     })
 }
 
-#[no_mangle]
-pub unsafe extern "C" fn signal_hex_encode(
-    output: *mut c_char,
-    output_len: usize,
-    input: *const u8,
-    input_len: usize,
-) -> *mut SignalFfiError {
-    run_ffi_safe(|| {
-        if input_len == 0 {
-            return Ok(());
-        }
-        if input_len > output_len / 2 {
-            // We check this early because an output buffer of {NULL, 0} is *valid*, just too small
-            // for anything but a zero-length input, while std::slice::from_raw_parts_mut requires a
-            // non-null base pointer.
-            return Err(SignalProtocolError::InvalidArgument(
-                "output buffer too small".to_string(),
-            )
-            .into());
-        }
-        if input.is_null() || output.is_null() {
-            return Err(NullPointerError.into());
-        }
-        let output = std::slice::from_raw_parts_mut(output, output_len);
-        let output = zerocopy::IntoBytes::as_mut_bytes(output);
-        let input = std::slice::from_raw_parts(input, input_len);
-        hex::encode_to_slice(input, output).expect("checked above");
-        Ok(())
-    })
+#[bridge_fn(jni = false, node = false)]
+fn hex_encode(output: &mut [u8], input: &[u8]) -> Result<(), SignalProtocolError> {
+    hex::encode_to_slice(input, output)
+        .map_err(|_| SignalProtocolError::InvalidArgument("output buffer too small".to_string()))
 }