Make Schema::fields and Schema::metadata pub (#2239)

arrow/src/datatypes/schema.rs

@@ -33,11 +33,11 @@ use super::Field;
 /// memory layout.
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq)]
 pub struct Schema {
-    pub(crate) fields: Vec<Field>,
+    pub fields: Vec<Field>,
     /// A map of key-value pairs containing additional meta data.
     #[serde(skip_serializing_if = "HashMap::is_empty")]
     #[serde(default)]
-    pub(crate) metadata: HashMap<String, String>,
+    pub metadata: HashMap<String, String>,
 }
 
 impl Schema {

arrow/tests/schema.rs

@@ -0,0 +1,46 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+use arrow::datatypes::{DataType, Field, Schema};
+use std::collections::HashMap;
+/// The tests in this file ensure a `Schema` can be manipulated
+/// outside of the arrow crate
+
+#[test]
+fn schema_destructure() {
+    let meta = [("foo".to_string(), "baz".to_string())]
+        .into_iter()
+        .collect::<HashMap<String, String>>();
+
+    let field = Field::new("c1", DataType::Utf8, false);
+    let schema = Schema::new(vec![field]).with_metadata(meta);
+
+    // Destructuring a Schema allows rewriting fields and metadata
+    // without copying
+    //
+    // Model this usecase below:
+
+    let Schema {
+        mut fields,
+        metadata,
+    } = schema;
+    fields.push(Field::new("c2", DataType::Utf8, false));
+
+    let new_schema = Schema::new(fields).with_metadata(metadata);
+
+    assert_eq!(new_schema.fields().len(), 2);
+}