fix: only use diagnostic attribute on versions after 1.78

server_fn/Cargo.toml

@@ -16,6 +16,7 @@ server_fn_macro_default = { workspace = true }
 const_format = "0.2.33"
 const-str = "0.5.7"
 xxhash-rust = { version = "0.8.12", features = ["const_xxh64"] }
+rustversion = { workspace = true}
 # used across multiple features
 serde = { version = "1.0", features = ["derive"] }
 send_wrapper = { version = "0.6.0", features = ["futures"], optional = true }

server_fn/src/error.rs

@@ -560,13 +560,13 @@ where
 }
 
 #[doc(hidden)]
-#[diagnostic::on_unimplemented(
+#[rustversion::attr(since(1.78), diagnostic::on_unimplemented(
     message = "{Self} is not a `Result` or aliased `Result`. Server functions \
                must return a `Result` or aliased `Result`.",
     label = "Must return a `Result` or aliased `Result`.",
     note = "If you are trying to return an alias of `Result`, you must also \
             implement `FromServerFnError` for the error type."
-)]
+))]
 /// A trait for extracting the error and ok types from a [`Result`]. This is used to allow alias types to be returned from server functions.
 pub trait ServerFnMustReturnResult {
     /// The error type of the [`Result`].