FFI: Implement tts_new(backend)

This required giving the Backends enum and Features struct a C
representation.

src/ffi.rs

@@ -5,7 +5,7 @@ use std::{
     ptr,
 };
 
-use crate::TTS;
+use crate::{Backends, TTS};
 
 thread_local! {
     /// Stores the last reported error, so it can be retrieved at will from C
@@ -38,6 +38,20 @@ pub extern "C" fn tts_clear_error() {
     });
 }
 
+/// Creates a new TTS object with the specified backend and returns a pointer to it.
+/// If an error occured, a null pointer is returned,
+/// Call `tts_get_error()` for more information about the specific error.
+#[no_mangle]
+pub extern "C" fn tts_new(backend: Backends) -> *mut TTS {
+    match TTS::new(backend) {
+        Ok(tts) => Box::into_raw(Box::new(tts)),
+        Err(e) => {
+            set_last_error(e.to_string()).unwrap();
+            ptr::null_mut()
+        }
+    }
+}
+
 /// Creates a new TTS object with the default backend and returns a pointer to it.
 /// If an error occured, a null pointer is returned,
 /// Call `tts_get_error()` for more information about the specific error.

src/lib.rs

@@ -31,6 +31,7 @@ mod backends;
 #[cfg(feature = "ffi")]
 pub mod ffi;
 
+#[repr(C)]
 #[derive(Clone, Copy, Debug)]
 pub enum Backends {
     #[cfg(target_os = "linux")]
@@ -75,6 +76,7 @@ unsafe impl Send for UtteranceId {}
 
 unsafe impl Sync for UtteranceId {}
 
+#[repr(C)]
 pub struct Features {
     pub stop: bool,
     pub rate: bool,