Add _or_default accessors to generated messages

Update map_type_to_rust_accessor to provide default values for each
type, which are used to generate helper methods that return a default
value when a field is missing.

codegen/src/generator.rs

@@ -33,7 +33,7 @@ pub fn to_snake_case(s: &str) -> String {
     result
 }
 
-fn map_type_to_rust_accessor(field_type: i32, label: i32, is_map: bool) -> (String, String) {
+fn map_type_to_rust_accessor(field_type: i32, label: i32, is_map: bool) -> (String, String, String) {
     if label == 3 {
         // LABEL_REPEATED
         let iterator_type = if is_map {
@@ -44,6 +44,7 @@ fn map_type_to_rust_accessor(field_type: i32, label: i32, is_map: bool) -> (Stri
         return (
             iterator_type.to_string(),
             "".to_string(), // Not used for repeated fields in the same way
+            "".to_string(), // Not used for repeated fields
         );
     }
 
@@ -51,37 +52,53 @@ fn map_type_to_rust_accessor(field_type: i32, label: i32, is_map: bool) -> (Stri
         9 => (
             "&'a str".to_string(),
             "str::from_utf8(bytes).map_err(|_| crate::RotoError::WireFormatViolation)".to_string(),
+            "\"\"".to_string(),
         ), // TYPE_STRING
         1 => (
             "f64".to_string(),
             "Ok(f64::from_le_bytes(bytes.try_into().map_err(|_| crate::RotoError::WireFormatViolation)?))".to_string(),
+            "0.0".to_string(),
         ), // TYPE_DOUBLE
         2 => (
             "f32".to_string(),
             "Ok(f32::from_le_bytes(bytes.try_into().map_err(|_| crate::RotoError::WireFormatViolation)?))".to_string(),
+            "0.0".to_string(),
         ), // TYPE_FLOAT
         3 | 5 | 15 | 17 => (
             "i32".to_string(),
             "roto_runtime::read_varint(bytes).map(|(v, _)| v as i32).map_err(|_| roto_runtime::RotoError::WireFormatViolation)".to_string(),
+            "0".to_string(),
         ), // INT/SINT/SFIXED 32
         4 | 6 | 13 => (
             "u32".to_string(),
             "roto_runtime::read_varint(bytes).map(|(v, _)| v as u32).map_err(|_| roto_runtime::RotoError::WireFormatViolation)".to_string(),
+            "0".to_string(),
         ), // UINT/FIXED 32
         16 | 18 => (
             "i64".to_string(),
             "roto_runtime::read_varint(bytes).map(|(v, _)| v as i64).map_err(|_| roto_runtime::RotoError::WireFormatViolation)".to_string(),
+            "0".to_string(),
         ), // SINT/SFIXED 64
         7 | 14 => (
             "u64".to_string(),
             "roto_runtime::read_varint(bytes).map(|(v, _)| v as u64).map_err(|_| roto_runtime::RotoError::WireFormatViolation)".to_string(),
+            "0".to_string(),
         ), // UINT/FIXED 64
         8 => (
             "bool".to_string(),
             "roto_runtime::read_varint(bytes).map(|(v, _)| v != 0).map_err(|_| roto_runtime::RotoError::WireFormatViolation)".to_string(),
+            "false".to_string(),
         ), // TYPE_BOOL
-        11 | 12 => ("&'a [u8]".to_string(), "Ok(bytes)".to_string()), // MESSAGE/BYTES
+        11 | 12 => (
-        _ => ("&'a [u8]".to_string(), "Ok(bytes)".to_string()),
+            "&'a [u8]".to_string(),
+            "Ok(bytes)".to_string(),
+            "&[]".to_string(),
+        ), // MESSAGE/BYTES
+        _ => (
+            "&'a [u8]".to_string(),
+            "Ok(bytes)".to_string(),
+            "&[]".to_string(),
+        ),
     }
 }
 
@@ -248,7 +265,7 @@ fn write_message(msg_proto: &DescriptorProto, output: &mut String) {
     output.push_str("        })\n    }\n\n");
 
     for (field_name, tag, f_type, f_label, _oneof_index, is_map) in &fields_info {
-        let (rust_type, logic) = map_type_to_rust_accessor(*f_type, *f_label, *is_map);
+        let (rust_type, logic, default_val) = map_type_to_rust_accessor(*f_type, *f_label, *is_map);
         let safe_name = if field_name == "type" {
             format!("r#{}", field_name)
         } else {
@@ -290,6 +307,17 @@ fn write_message(msg_proto: &DescriptorProto, output: &mut String) {
             output.push_str("        let (bytes, _) = self.accessor.get_value_at(offset)?;\n");
             output.push_str(&format!("        {}\n", logic));
             output.push_str("    }\n\n");
+
+            output.push_str(&format!(
+                "    pub fn {}_or_default(&self) -> roto_runtime::Result<{}> {{\n",
+                 safe_name, rust_type
+            ));
+            output.push_str(&format!(
+                "        self.{}().or(Ok({}))\n",
+                safe_name, default_val
+            ));
+            output.push_str("    }\n\n");
+
             output.push_str(&format!(
                 "    pub fn has_{}(&self) -> bool {{ self.{}_offset.is_some() }}\n\n",
                 field_name, field_name
@@ -444,7 +472,7 @@ fn write_message(msg_proto: &DescriptorProto, output: &mut String) {
             output.push_str(&format!("pub enum {}<'a> {{\n", pascal_oneof_name));
             for (field_name, _tag, f_type, f_label, f_oneof_index, _is_map) in &fields_info {
                 if *f_oneof_index == Some(oneof_index as i32) {
-                    let (rust_type, _) = map_type_to_rust_accessor(*f_type, *f_label, *_is_map);
+                    let (rust_type, _, _) = map_type_to_rust_accessor(*f_type, *f_label, *_is_map);
                     let safe_field_name = if field_name == "type" {
                         format!("r#{}", field_name)
                     } else {