codewide: appease Clippy

Clippy started to complain again:
- direct impl of ToString instead of taking advantage of blanked impl
  over Display,
- no Default impl for LegacySerializedValues,
- assigning result of clone() instead of using clone_from().

The suggestions were applied.

Author: wprzytula

scylla-cql/src/frame/mod.rs

@@ -15,6 +15,7 @@ use thiserror::Error;
 use tokio::io::{AsyncRead, AsyncReadExt};
 use uuid::Uuid;
 
+use std::fmt::Display;
 use std::{collections::HashMap, convert::TryFrom};
 
 use request::SerializableRequest;
@@ -47,11 +48,11 @@ pub enum Compression {
     Snappy,
 }
 
-impl ToString for Compression {
-    fn to_string(&self) -> String {
+impl Display for Compression {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         match self {
-            Compression::Lz4 => "lz4".to_owned(),
-            Compression::Snappy => "snappy".to_owned(),
+            Compression::Lz4 => f.write_str("lz4"),
+            Compression::Snappy => f.write_str("snappy"),
         }
     }
 }

scylla-cql/src/frame/value.rs

@@ -699,6 +699,12 @@ pub trait ValueList {
     }
 }
 
+impl Default for LegacySerializedValues {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
 impl LegacySerializedValues {
     /// Creates empty value list
     pub const fn new() -> Self {

scylla/src/transport/connection_pool.rs

@@ -973,7 +973,7 @@ impl PoolRefiller {
             new_sharder,
         );
 
-        self.sharder = new_sharder.clone();
+        self.sharder.clone_from(&new_sharder);
 
         // If the sharder has changed, we can throw away all previous connections.
         // All connections to the same live node will have the same sharder,

scylla/src/utils/parse.rs

@@ -1,3 +1,5 @@
+use std::fmt::Display;
+
 /// An error that can occur during parsing.
 #[derive(Copy, Clone)]
 pub(crate) struct ParseError {
@@ -27,11 +29,11 @@ pub(crate) enum ParseErrorCause {
     Other(&'static str),
 }
 
-impl ToString for ParseErrorCause {
-    fn to_string(&self) -> String {
+impl Display for ParseErrorCause {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         match self {
-            ParseErrorCause::Expected(e) => format!("expected {:?}", e),
-            ParseErrorCause::Other(e) => e.to_string(),
+            ParseErrorCause::Expected(e) => write!(f, "expected {:?}", e),
+            ParseErrorCause::Other(e) => f.write_str(e),
         }
     }
 }