ARROW-11603: [Rust] Fix Clippy Lints for Rust 1.50

# Rationale:

CI uses "stable" rust. 1.50 stable was released today: https://blog.rust-lang.org/2021/02/11/Rust-1.50.0.html

The new clippy is pickier resulting in many clippy warnings such as https://github.com/apache/arrow/pull/9469/checks?check_run_id=1881854256

We need to get CI back green

# Changes
Based on based on #9475 from @nevi-me, this PR aims to get the CI green as soon as possible.

Ideally, we would fix the actual lint problems, However, when I tried to do so the lints propagated into a significant change -- as clippy says to remove the `Result` but then there are a bunch of call sites that then also need t be changed

I want to get CI back clean as soon as possible so I just hammered through and cleaned up as best I could as well as sprinkling in various `#[allow(clippy::unnecessary_wraps)]` as necessary to get a clean run.

My rationale is that the code is no worse than it was before. Though it could be better!

Closes #9476 from alamb/ARROW-11602-lints

Authored-by: Neville Dipale <[email protected]>
Signed-off-by: Jorge C. Leitao <[email protected]>

Author: nevi-me

rust/arrow/examples/read_csv.rs

@@ -22,11 +22,10 @@ use std::sync::Arc;
 
 use arrow::csv;
 use arrow::datatypes::{DataType, Field, Schema};
-use arrow::error::Result;
 #[cfg(feature = "prettyprint")]
 use arrow::util::pretty::print_batches;
 
-fn main() -> Result<()> {
+fn main() {
     let schema = Schema::new(vec![
         Field::new("city", DataType::Utf8, false),
         Field::new("lat", DataType::Float64, false),
@@ -41,5 +40,4 @@ fn main() -> Result<()> {
     {
         print_batches(&[_batch]).unwrap();
     }
-    Ok(())
 }

rust/arrow/examples/read_csv_infer_schema.rs

@@ -18,12 +18,11 @@
 extern crate arrow;
 
 use arrow::csv;
-use arrow::error::Result;
 #[cfg(feature = "prettyprint")]
 use arrow::util::pretty::print_batches;
 use std::fs::File;
 
-fn main() -> Result<()> {
+fn main() {
     let file = File::open("test/data/uk_cities_with_headers.csv").unwrap();
     let builder = csv::ReaderBuilder::new()
         .has_header(true)
@@ -34,5 +33,4 @@ fn main() -> Result<()> {
     {
         print_batches(&[_batch]).unwrap();
     }
-    Ok(())
 }

rust/arrow/src/array/builder.rs

@@ -1499,6 +1499,7 @@ impl FieldData {
     }
 
     /// Appends a single value to this `FieldData`'s `values_buffer`.
+    #[allow(clippy::unnecessary_wraps)]
     fn append_to_values_buffer<T: ArrowPrimitiveType>(
         &mut self,
         v: T::Native,
@@ -1521,6 +1522,7 @@ impl FieldData {
     }
 
     /// Appends a null to this `FieldData`.
+    #[allow(clippy::unnecessary_wraps)]
     fn append_null<T: ArrowPrimitiveType>(&mut self) -> Result<()> {
         if let Some(b) = &mut self.bitmap_builder {
             let values_buffer = self

rust/arrow/src/array/transform/mod.rs

@@ -1075,7 +1075,7 @@ mod tests {
     }
 
     #[test]
-    fn test_fixed_size_binary_append() -> Result<()> {
+    fn test_fixed_size_binary_append() {
         let a = vec![Some(vec![1, 2]), Some(vec![3, 4]), Some(vec![5, 6])];
         let a = FixedSizeBinaryArray::from(a).data();
 
@@ -1118,7 +1118,6 @@ mod tests {
         ];
         let expected = FixedSizeBinaryArray::from(expected).data();
         assert_eq!(&result, expected.as_ref());
-        Ok(())
     }
 
     /*

rust/arrow/src/buffer.rs

@@ -1619,7 +1619,7 @@ mod tests {
     }
 
     #[test]
-    fn test_mutable_equal() -> Result<()> {
+    fn test_mutable_equal() {
         let mut buf = MutableBuffer::new(1);
         let mut buf2 = MutableBuffer::new(1);
 
@@ -1632,8 +1632,6 @@ mod tests {
 
         buf2.reserve(65);
         assert!(buf != buf2);
-
-        Ok(())
     }
 
     #[test]

rust/arrow/src/compute/kernels/cast.rs

@@ -821,6 +821,7 @@ const EPOCH_DAYS_FROM_CE: i32 = 719_163;
 /// Arrays should have the same primitive data type, otherwise this should fail.
 /// We do not perform this check on primitive data types as we only use this
 /// function internally, where it is guaranteed to be infallible.
+#[allow(clippy::unnecessary_wraps)]
 fn cast_array_data<TO>(array: &ArrayRef, to_type: DataType) -> Result<ArrayRef>
 where
     TO: ArrowNumericType,
@@ -838,6 +839,7 @@ where
 }
 
 /// Convert Array into a PrimitiveArray of type, and apply numeric cast
+#[allow(clippy::unnecessary_wraps)]
 fn cast_numeric_arrays<FROM, TO>(from: &ArrayRef) -> Result<ArrayRef>
 where
     FROM: ArrowNumericType,
@@ -869,6 +871,7 @@ where
 }
 
 /// Cast numeric types to Utf8
+#[allow(clippy::unnecessary_wraps)]
 fn cast_numeric_to_string<FROM>(array: &ArrayRef) -> Result<ArrayRef>
 where
     FROM: ArrowNumericType,
@@ -893,6 +896,7 @@ where
 }
 
 /// Cast numeric types to Utf8
+#[allow(clippy::unnecessary_wraps)]
 fn cast_string_to_numeric<T>(from: &ArrayRef) -> Result<ArrayRef>
 where
     T: ArrowNumericType,
@@ -959,6 +963,7 @@ where
 /// Cast Boolean types to numeric
 ///
 /// `false` returns 0 while `true` returns 1
+#[allow(clippy::unnecessary_wraps)]
 fn cast_bool_to_numeric<TO>(from: &ArrayRef) -> Result<ArrayRef>
 where
     TO: ArrowNumericType,

rust/arrow/src/compute/kernels/concat.rs

@@ -77,20 +77,18 @@ mod tests {
     use std::sync::Arc;
 
     #[test]
-    fn test_concat_empty_vec() -> Result<()> {
+    fn test_concat_empty_vec() {
         let re = concat(&[]);
         assert!(re.is_err());
-        Ok(())
     }
 
     #[test]
-    fn test_concat_incompatible_datatypes() -> Result<()> {
+    fn test_concat_incompatible_datatypes() {
         let re = concat(&[
             &PrimitiveArray::<Int64Type>::from(vec![Some(-1), Some(2), None]),
             &StringArray::from(vec![Some("hello"), Some("bar"), Some("world")]),
         ]);
         assert!(re.is_err());
-        Ok(())
     }
 
     #[test]

rust/arrow/src/compute/kernels/length.rs

@@ -24,6 +24,7 @@ use crate::{
 };
 use std::sync::Arc;
 
+#[allow(clippy::unnecessary_wraps)]
 fn length_string<OffsetSize>(array: &Array, data_type: DataType) -> Result<ArrayRef>
 where
     OffsetSize: OffsetSizeTrait,
@@ -178,11 +179,10 @@ mod tests {
 
     /// Tests that length is not valid for u64.
     #[test]
-    fn wrong_type() -> Result<()> {
+    fn wrong_type() {
         let array: UInt64Array = vec![1u64].into();
 
         assert!(length(&array).is_err());
-        Ok(())
     }
 
     /// Tests with an offset

rust/arrow/src/compute/kernels/sort.rs

@@ -254,6 +254,7 @@ impl Default for SortOptions {
 }
 
 /// Sort primitive values
+#[allow(clippy::unnecessary_wraps)]
 fn sort_boolean(
     values: &ArrayRef,
     value_indices: Vec<u32>,
@@ -316,6 +317,7 @@ fn sort_boolean(
 }
 
 /// Sort primitive values
+#[allow(clippy::unnecessary_wraps)]
 fn sort_primitive<T, F>(
     values: &ArrayRef,
     value_indices: Vec<u32>,
@@ -446,6 +448,7 @@ fn sort_string_dictionary<T: ArrowDictionaryKeyType>(
 
 /// shared implementation between dictionary encoded and plain string arrays
 #[inline]
+#[allow(clippy::unnecessary_wraps)]
 fn sort_string_helper<'a, A: Array, F>(
     values: &'a A,
     value_indices: Vec<u32>,
@@ -481,6 +484,7 @@ where
     Ok(UInt32Array::from(valid_indices))
 }
 
+#[allow(clippy::unnecessary_wraps)]
 fn sort_list<S, T>(
     values: &ArrayRef,
     value_indices: Vec<u32>,

rust/arrow/src/compute/kernels/substring.rs

@@ -24,6 +24,7 @@ use crate::{
 };
 use std::sync::Arc;
 
+#[allow(clippy::unnecessary_wraps)]
 fn generic_substring<OffsetSize: StringOffsetSizeTrait>(
     array: &GenericStringArray<OffsetSize>,
     start: OffsetSize,