Add ruma_serde::is_empty

This will be used for serde attributes that are copied to incoming types
and thus actually have to function for multiple types.

ruma-serde/src/can_be_empty.rs

@@ -0,0 +1,9 @@
+pub trait CanBeEmpty {
+    /// Check whether `self` is empty.
+    fn is_empty(&self) -> bool;
+}
+
+/// Check whether a value is empty.
+pub fn is_empty<T: CanBeEmpty>(val: &T) -> bool {
+    val.is_empty()
+}

ruma-serde/src/lib.rs

@@ -2,6 +2,7 @@
 
 use serde::de::{Deserialize, IntoDeserializer};
 
+pub mod can_be_empty;
 pub mod duration;
 pub mod empty;
 pub mod json_string;
@@ -9,6 +10,7 @@ pub mod test;
 pub mod time;
 pub mod urlencoded;
 
+pub use can_be_empty::{is_empty, CanBeEmpty};
 pub use empty::vec_as_map_of_empty;
 
 /// Check whether a value is equal to its default value.