chore: Clarify that the datafusion-comet crate does not have a public API

.github/actions/rust-test/action.yaml

@@ -43,6 +43,12 @@ runs:
         cd native
         cargo install cargo-machete && cargo machete
 
+    - name: Check no public items in datafusion-comet crate
+      shell: bash
+      run: |
+        cd native/core
+        cargo rustc -- -D unreachable_pub
+
     - name: Cache Maven dependencies
       uses: actions/cache@v4
       with:

native/core/README.md

@@ -0,0 +1,26 @@
+<!--
+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.
+-->
+
+# Apache DataFusion Comet: Plugin Native Code 
+
+This crate contains the native code that is invoked from the Comet's Spark plugin and is not intended to be 
+used as a dependency from other projects. Therefore, it does not provide a public API and is not published to crates.io.
+
+However, the `datafusion-comet-proto` and `datafusion-comet-spark-expr` crates do provide public APIs and are 
+published to crates.io.

native/core/benches/common.rs

@@ -28,11 +28,11 @@ use rand::{
 use std::sync::Arc;
 
 /// Returns fixed seedable RNG
-pub fn seedable_rng() -> StdRng {
+pub(crate) fn seedable_rng() -> StdRng {
     StdRng::seed_from_u64(42)
 }
 
-pub fn create_int64_array(size: usize, null_density: f32, min: i64, max: i64) -> Int64Array {
+pub(crate) fn create_int64_array(size: usize, null_density: f32, min: i64, max: i64) -> Int64Array {
     let mut rng = seedable_rng();
     (0..size)
         .map(|_| {
@@ -46,7 +46,7 @@ pub fn create_int64_array(size: usize, null_density: f32, min: i64, max: i64) ->
 }
 
 #[allow(dead_code)]
-pub fn create_primitive_array<T>(size: usize, null_density: f32) -> PrimitiveArray<T>
+pub(crate) fn create_primitive_array<T>(size: usize, null_density: f32) -> PrimitiveArray<T>
 where
     T: ArrowPrimitiveType,
     Standard: Distribution<T::Native>,
@@ -66,7 +66,7 @@ where
 /// Creates a dictionary with random keys and values, with value type `T`.
 /// Note here the keys are the dictionary indices.
 #[allow(dead_code)]
-pub fn create_dictionary_array<T>(
+pub(crate) fn create_dictionary_array<T>(
     size: usize,
     value_size: usize,
     null_density: f32,

native/core/benches/parquet_read.rs

@@ -156,7 +156,7 @@ struct TestColumnReader {
 }
 
 impl TestColumnReader {
-    pub fn new(
+    pub(crate) fn new(
         cd: ColumnDescriptor,
         promotion_info: TypePromotionInfo,
         batch_size: usize,

native/core/benches/perf.rs

@@ -25,13 +25,13 @@ use pprof::ProfilerGuard;
 /// Mostly followed this blog post: https://www.jibbow.com/posts/criterion-flamegraphs/
 /// After `cargo bench --bench <bench-name> -- --profile-time=<time>`
 /// You can find flamegraph.svg under `target/criterion/<bench-name>/<bench-method-name>/profile`
-pub struct FlamegraphProfiler<'a> {
+pub(crate) struct FlamegraphProfiler<'a> {
     frequency: c_int,
     active_profiler: Option<ProfilerGuard<'a>>,
 }
 
 impl<'a> FlamegraphProfiler<'a> {
-    pub fn new(frequency: c_int) -> Self {
+    pub(crate) fn new(frequency: c_int) -> Self {
         FlamegraphProfiler {
             frequency,
             active_profiler: None,

native/core/benches/row_columnar.rs

@@ -88,13 +88,13 @@ struct Row {
 }
 
 impl Row {
-    pub fn new() -> Self {
+    pub(crate) fn new() -> Self {
         Row {
             data: Box::new([0u8; ROW_SIZE]),
         }
     }
 
-    pub fn to_spark_row(&self, spark_row: &mut SparkUnsafeRow) {
+    pub(crate) fn to_spark_row(&self, spark_row: &mut SparkUnsafeRow) {
         spark_row.point_to_slice(self.data.as_ref());
     }
 }