Introduce a cargo clippy run (#2025)

This might help catch stylistic problems in our Rust code.

Related to #2587.

src/concurrency/async-exercises/chat-async/src/bin/client.rs

@@ -41,7 +41,7 @@ async fn main() -> Result<(), tokio_websockets::Error> {
                             println!("From server: {}", text);
                         }
                     },
-                    Some(Err(err)) => return Err(err.into()),
+                    Some(Err(err)) => return Err(err),
                     None => return Ok(()),
                 }
             }

src/lifetimes/exercise.rs

@@ -116,7 +116,7 @@ fn unpack_tag(tag: u64) -> (u64, WireType) {
 
 // ANCHOR: parse_field
 /// Parse a field, returning the remaining bytes
-fn parse_field(data: &[u8]) -> (Field, &[u8]) {
+fn parse_field(data: &[u8]) -> (Field<'_>, &[u8]) {
     let (tag, remainder) = parse_varint(data);
     let (field_num, wire_type) = unpack_tag(tag);
     let (fieldvalue, remainder) = match wire_type {

src/tuples-and-arrays/exercise.rs

@@ -12,6 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+// Iterators are covered later.
+#[allow(clippy::needless_range_loop)]
 // ANCHOR: solution
 // ANCHOR: transpose
 fn transpose(matrix: [[i32; 3]; 3]) -> [[i32; 3]; 3] {

src/types-and-values/exercise.rs

@@ -12,6 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+// Omitting `return` is covered later.
+#[allow(clippy::needless_return)]
 // ANCHOR: solution
 // ANCHOR: fib
 fn fib(n: u32) -> u32 {

src/unsafe-rust/exercise.rs

@@ -12,6 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+// `DIR` matches name of C struct.
+#[allow(clippy::upper_case_acronyms)]
 // ANCHOR: solution
 // ANCHOR: ffi
 mod ffi {
@@ -40,7 +42,7 @@ mod ffi {
     }
 
     // Layout according to the macOS man page for dir(5).
-    #[cfg(all(target_os = "macos"))]
+    #[cfg(target_os = "macos")]
     #[repr(C)]
     pub struct dirent {
         pub d_fileno: u64,