ARROW-4377: [Rust] Implement std::fmt::Debug for PrimitiveArrays

rust/arrow/src/array.rs

@@ -56,6 +56,7 @@
 
 use std::any::Any;
 use std::convert::From;
+use std::fmt;
 use std::io::Write;
 use std::mem;
 use std::sync::Arc;
@@ -248,6 +249,20 @@ impl<T: ArrowNumericType> PrimitiveArray<T> {
     }
 }
 
+impl<T: ArrowNumericType> fmt::Debug for PrimitiveArray<T> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "PrimitiveArray<{:?}>\n[\n", T::get_data_type())?;
+        for i in 0..self.len() {
+            if self.is_null(i) {
+                write!(f, "  null,\n")?;
+            } else {
+                write!(f, "  {:?},\n", self.value(i))?;
+            }
+        }
+        write!(f, "]")
+    }
+}
+
 /// Specific implementation for Boolean arrays due to bit-packing
 impl PrimitiveArray<BooleanType> {
     pub fn new(length: usize, values: Buffer, null_count: usize, offset: usize) -> Self {
@@ -280,6 +295,20 @@ impl PrimitiveArray<BooleanType> {
     }
 }
 
+impl fmt::Debug for PrimitiveArray<BooleanType> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "PrimitiveArray<{:?}>\n[\n", BooleanType::get_data_type())?;
+        for i in 0..self.len() {
+            if self.is_null(i) {
+                write!(f, "  null,\n")?
+            } else {
+                write!(f, "  {:?},\n", self.value(i))?
+            }
+        }
+        write!(f, "]")
+    }
+}
+
 // TODO: the macro is needed here because we'd get "conflicting implementations" error
 // otherwise with both `From<Vec<T::Native>>` and `From<Vec<Option<T::Native>>>`.
 // We should revisit this in future.
@@ -761,6 +790,52 @@ mod tests {
         let _slice = arr.value_slice(0, 4);
     }
 
+    #[test]
+    fn test_int32_fmt_debug() {
+        let buf = Buffer::from(&[0, 1, 2, 3, 4].to_byte_slice());
+        let arr = Int32Array::new(5, buf, 0, 0);
+        assert_eq!(
+            "PrimitiveArray<Int32>\n[\n  0,\n  1,\n  2,\n  3,\n  4,\n]",
+            format!("{:?}", arr)
+        );
+    }
+
+    #[test]
+    fn test_int32_with_null_fmt_debug() {
+        let mut builder = Int32Array::builder(3);
+        builder.append_slice(&[0, 1]).unwrap();
+        builder.append_null().unwrap();
+        builder.append_slice(&[3, 4]).unwrap();
+        let arr = builder.finish();
+        assert_eq!(
+            "PrimitiveArray<Int32>\n[\n  0,\n  1,\n  null,\n  3,\n  4,\n]",
+            format!("{:?}", arr)
+        );
+    }
+
+    #[test]
+    fn test_boolean_fmt_debug() {
+        let buf = Buffer::from(&[true, false, false].to_byte_slice());
+        let arr = BooleanArray::new(3, buf, 0, 0);
+        assert_eq!(
+            "PrimitiveArray<Boolean>\n[\n  true,\n  false,\n  false,\n]",
+            format!("{:?}", arr)
+        );
+    }
+
+    #[test]
+    fn test_boolean_with_null_fmt_debug() {
+        let mut builder = BooleanArray::builder(3);
+        builder.append_value(true).unwrap();
+        builder.append_null().unwrap();
+        builder.append_value(false).unwrap();
+        let arr = builder.finish();
+        assert_eq!(
+            "PrimitiveArray<Boolean>\n[\n  true,\n  null,\n  false,\n]",
+            format!("{:?}", arr)
+        );
+    }
+
     #[test]
     fn test_primitive_array_builder() {
         // Test building an primitive array with ArrayData builder and offset

rust/arrow/src/datatypes.rs

@@ -96,7 +96,10 @@ pub struct Field {
     nullable: bool,
 }
 
-pub trait ArrowNativeType: Send + Sync + Copy + PartialOrd + FromStr + 'static {}
+pub trait ArrowNativeType:
+    fmt::Debug + Send + Sync + Copy + PartialOrd + FromStr + 'static
+{
+}
 
 /// Trait indicating a primitive fixed-width type (bool, ints and floats).
 pub trait ArrowPrimitiveType: 'static {