Enable clippy::return_self_not_must_use lint (#24)

Cargo.toml

@@ -69,6 +69,7 @@ clippy.missing_errors_doc = "warn"
 clippy.missing_fields_in_debug = "warn"
 clippy.missing_panics_doc = "warn"
 clippy.partial_pub_fields = "warn"
+clippy.return_self_not_must_use = "warn"
 clippy.same_functions_in_if_condition = "warn"
 clippy.semicolon_if_nothing_returned = "warn"
 clippy.shadow_unrelated = "warn"
@@ -78,7 +79,6 @@ clippy.unseparated_literal_suffix = "warn"
 clippy.wildcard_imports = "warn"
 
 # TODO: Enable these and move them back above.
-# clippy.return_self_not_must_use = "warn"
 # clippy.use_self = "warn"
 # rust.elided_lifetimes_in_paths = "warn"
 # rust.missing_docs = "warn"

src/value.rs

@@ -156,6 +156,7 @@ impl<T: Tween> Animated<T> {
 
 /// Something that can be interpolated with an easing function.
 pub trait Tween: Clone + Default {
+    #[must_use]
     fn tween(&self, other: &Self, t: f64, easing: &Easing) -> Self;
 }