Add flatten attribute to derive SerializeRow

Currently only the `match_by_name` flavor is supported. All the needed
structs/traits to make this work are marked as `#[doc(hidden)]` to not
increase the public API surface. Effort was done to not change any of
the existing API.

scylla-cql/src/lib.rs

@@ -76,4 +76,120 @@ pub mod _macro_internal {
     pub use crate::types::serialize::{
         CellValueBuilder, CellWriter, RowWriter, SerializationError,
     };
+
+    pub mod ser {
+        pub mod row {
+            pub use crate::{
+                frame::response::result::ColumnSpec,
+                types::serialize::{
+                    row::{
+                        mk_ser_err, mk_typck_err, BuiltinSerializationErrorKind,
+                        BuiltinTypeCheckError, BuiltinTypeCheckErrorKind, RowSerializationContext,
+                    },
+                    value::SerializeValue,
+                    writers::WrittenCellProof,
+                    RowWriter, SerializationError,
+                },
+            };
+
+            /// Whether a field used a column to finish its serialization or not
+            ///
+            /// Used when serializing by name as a single column may not have finished a rust
+            /// field in the case of a flattened struct
+            ///
+            /// For now this enum is an implementation detail of `#[derive(SerializeRow)]` when
+            /// serializing by name
+            #[derive(Debug)]
+            #[doc(hidden)]
+            pub enum FieldStatus {
+                /// The column finished the serialization for this field
+                Done,
+                /// The column was used but there are other fields not yet serialized
+                NotDone,
+                /// The column did not belong to this field
+                NotUsed,
+            }
+
+            /// Represents a set of values that can be sent along a CQL statement when serializing by name
+            ///
+            /// For now this trait is an implementation detail of `#[derive(SerializeRow)]` when
+            /// serializing by name
+            #[doc(hidden)]
+            pub trait SerializeRowByName {
+                /// A type that can handle serialization of this struct column-by-column
+                type Partial<'d>: PartialSerializeRowByName
+                where
+                    Self: 'd;
+
+                /// Returns a type that can serialize this row "column-by-column"
+                fn partial(&self) -> Self::Partial<'_>;
+            }
+
+            /// How to serialize a row column-by-column
+            ///
+            /// For now this trait is an implementation detail of `#[derive(SerializeRow)]` when
+            /// serializing by name
+            #[doc(hidden)]
+            pub trait PartialSerializeRowByName {
+                /// Serializes a single column in the row according to the information in the
+                /// given context
+                ///
+                /// It returns whether the column finished the serialization of the struct, did
+                /// it partially, none of at all, or errored
+                fn serialize_field(
+                    &mut self,
+                    spec: &ColumnSpec,
+                    writer: &mut RowWriter<'_>,
+                ) -> Result<FieldStatus, SerializationError>;
+
+                /// Checks if there are any missing columns to finish the serialization
+                fn check_missing(self) -> Result<(), SerializationError>;
+            }
+
+            pub struct ByName<'t, T: SerializeRowByName>(pub &'t T);
+
+            impl<T: SerializeRowByName> ByName<'_, T> {
+                /// Serializes all the fields/columns by name
+                pub fn serialize(
+                    self,
+                    ctx: &RowSerializationContext,
+                    writer: &mut RowWriter<'_>,
+                ) -> Result<(), SerializationError> {
+                    let mut partial = self.0.partial();
+
+                    for spec in ctx.columns() {
+                        let serialized = partial.serialize_field(spec, writer)?;
+
+                        if matches!(serialized, FieldStatus::NotUsed) {
+                            return Err(mk_typck_err::<Self>(
+                                BuiltinTypeCheckErrorKind::NoColumnWithName {
+                                    name: spec.name().to_owned(),
+                                },
+                            ));
+                        }
+                    }
+
+                    partial.check_missing()?;
+
+                    Ok(())
+                }
+            }
+
+            pub fn serialize_column<'b, T>(
+                value: &impl SerializeValue,
+                spec: &ColumnSpec,
+                writer: &'b mut RowWriter<'_>,
+            ) -> Result<WrittenCellProof<'b>, SerializationError> {
+                let sub_writer = writer.make_cell_writer();
+                value.serialize(spec.typ(), sub_writer).map_err(|err| {
+                    super::row::mk_ser_err::<T>(
+                        BuiltinSerializationErrorKind::ColumnSerializationFailed {
+                            name: spec.name().to_owned(),
+                            err,
+                        },
+                    )
+                })
+            }
+        }
+    }
 }

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

@@ -556,7 +556,8 @@ pub struct BuiltinTypeCheckError {
     pub kind: BuiltinTypeCheckErrorKind,
 }
 
-fn mk_typck_err<T>(kind: impl Into<BuiltinTypeCheckErrorKind>) -> SerializationError {
+#[doc(hidden)]
+pub fn mk_typck_err<T>(kind: impl Into<BuiltinTypeCheckErrorKind>) -> SerializationError {
     mk_typck_err_named(std::any::type_name::<T>(), kind)
 }
 
@@ -582,7 +583,8 @@ pub struct BuiltinSerializationError {
     pub kind: BuiltinSerializationErrorKind,
 }
 
-fn mk_ser_err<T>(kind: impl Into<BuiltinSerializationErrorKind>) -> SerializationError {
+#[doc(hidden)]
+pub fn mk_ser_err<T>(kind: impl Into<BuiltinSerializationErrorKind>) -> SerializationError {
     mk_ser_err_named(std::any::type_name::<T>(), kind)
 }
 
@@ -1634,4 +1636,56 @@ pub(crate) mod tests {
 
         assert_eq!(reference, row);
     }
+
+    #[test]
+    fn test_row_serialization_nested_structs() {
+        #[derive(SerializeRow, Debug)]
+        #[scylla(crate = crate)]
+        struct InnerColumnsOne {
+            x: i32,
+            y: f64,
+        }
+
+        #[derive(SerializeRow, Debug)]
+        #[scylla(crate = crate)]
+        struct InnerColumnsTwo {
+            z: bool,
+        }
+
+        #[derive(SerializeRow, Debug)]
+        #[scylla(crate = crate)]
+        struct OuterColumns {
+            #[scylla(flatten)]
+            inner_one: InnerColumnsOne,
+            a: String,
+            #[scylla(flatten)]
+            inner_two: InnerColumnsTwo,
+        }
+
+        let spec = [
+            col("a", ColumnType::Text),
+            col("x", ColumnType::Int),
+            col("z", ColumnType::Boolean),
+            col("y", ColumnType::Double),
+        ];
+
+        let value = OuterColumns {
+            inner_one: InnerColumnsOne { x: 5, y: 1.0 },
+            a: "something".to_owned(),
+            inner_two: InnerColumnsTwo { z: true },
+        };
+
+        let reference = do_serialize(
+            (
+                &value.a,
+                &value.inner_one.x,
+                &value.inner_two.z,
+                &value.inner_one.y,
+            ),
+            &spec,
+        );
+        let row = do_serialize(value, &spec);
+
+        assert_eq!(reference, row);
+    }
 }