Remove dc_context_unref from Rust API

This removes the dc_context_unref function from the Rust API which was
just an alias for dc_close.  It still exists on the C API where it
makes sure to free the memory.

It also implements Drop for the context which just calls dc_close to
make sure all the memory is freed.  Since you can call dc_close as
many times as you like this ensures that at the Rust level you can't
Drop the struct without releasing the memory.

Finally since memory is now freed by dropping the struct this removes
the #[repr(C)] for the struct.  This struct is fully opaque to the C
API.

src/context.rs

@@ -20,7 +20,6 @@ use crate::sql::Sql;
 use crate::types::*;
 use crate::x::*;
 
-#[repr(C)]
 pub struct Context {
     pub userdata: *mut libc::c_void,
     pub dbfile: Arc<RwLock<*mut libc::c_char>>,
@@ -77,6 +76,14 @@ impl Context {
     }
 }
 
+impl Drop for Context {
+    fn drop(&mut self) {
+        unsafe {
+            dc_close(&self);
+        }
+    }
+}
+
 impl Default for RunningState {
     fn default() -> Self {
         RunningState {
@@ -165,16 +172,6 @@ pub fn dc_context_new(
     }
 }
 
-#[cfg(test)]
-mod tests {
-    use super::*;
-    #[test]
-    fn no_crashes_on_context_deref() {
-        let mut ctx = dc_context_new(None, std::ptr::null_mut(), Some("Test OS".into()));
-        unsafe { dc_context_unref(&mut ctx) };
-    }
-}
-
 unsafe fn cb_receive_imf(
     context: &Context,
     imf_raw_not_terminated: *const libc::c_char,
@@ -258,12 +255,6 @@ fn cb_get_config(context: &Context, key: &str) -> Option<String> {
     context.sql.get_config(context, key)
 }
 
-pub unsafe fn dc_context_unref(context: &mut Context) {
-    if 0 != dc_is_open(context) {
-        dc_close(context);
-    }
-}
-
 pub unsafe fn dc_close(context: &Context) {
     info!(context, 0, "disconnecting INBOX-watch",);
     context.inbox.read().unwrap().disconnect(context);
@@ -591,3 +582,24 @@ pub fn dc_is_mvbox(context: &Context, folder_name: impl AsRef<str>) -> bool {
         false
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn no_crashes_on_context_deref() {
+        let ctx = dc_context_new(None, std::ptr::null_mut(), Some("Test OS".into()));
+        std::mem::drop(ctx);
+    }
+
+    #[test]
+    fn test_context_double_close() {
+        let ctx = dc_context_new(None, std::ptr::null_mut(), None);
+        unsafe {
+            dc_close(&ctx);
+            dc_close(&ctx);
+        }
+        std::mem::drop(ctx);
+    }
+}