serialize: add more tests and docstrings

scylla-cql/src/frame/value.rs

@@ -36,6 +36,7 @@ pub struct ValueTooBig;
 pub struct ValueOverflow;
 
 /// Represents an unset value
+#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
 pub struct Unset;
 
 /// Represents an counter value

scylla-cql/src/types/serialize/mod.rs

@@ -1,3 +1,7 @@
+#![warn(missing_docs)]
+
+//! Types and traits related to serialization of values to the CQL format.
+
 use std::{error::Error, fmt::Display, sync::Arc};
 
 use thiserror::Error;
@@ -7,6 +11,25 @@ pub mod value;
 pub mod writers;
 
 pub use writers::{CellValueBuilder, CellWriter, RowWriter};
+
+/// An error indicating that a failure happened during serialization.
+///
+/// The error is type-erased so that the crate users can define their own
+/// serialization impls and their errors. As for the impls defined or generated
+/// by the driver itself, the following errors can be returned:
+///
+/// - [`row::BuiltinSerializationError`] is returned when serialization of
+///   one of types with an impl built into the driver fails. It is also returned
+///   from impls generated by the `SerializeRow` macro.
+/// - [`value::BuiltinSerializationError`] is analogous to the above but is
+///   returned from [`SerializeCql::serialize`](value::SerializeCql::serialize)
+///   instead both in the case of builtin impls and impls generated by the
+///   `SerializeCql` macro. It won't be returned by the `Session` directly,
+///   but it might be nested in the [`row::BuiltinSerializationError`].
+/// - [`row::ValueListToSerializeRowAdapterError`] is returned in case when
+///   a list of named values encoded with the legacy `ValueList` trait is passed
+///   as an argument to the statement, and rewriting it using the new
+///   `SerializeRow` interface fails.
 #[derive(Debug, Clone, Error)]
 pub struct SerializationError(Arc<dyn Error + Send + Sync>);