sql: Use itertools to multiunzip instead of doing it manually

Summary:
I always wanted this feature in Rust and just found out `itertools` has it :)

So I'm using it here, where we were doing this manually because vanilla Rust only knows how to unzip 2-tuples.

Doesn't change any behaviour, purely a refactor.

Reviewed By: RajivTS

Differential Revision: D40943893

fbshipit-source-id: bd4f63c89e07d69469152b8db78621f49219818f

shed/sql/common/Cargo.toml

@@ -18,6 +18,7 @@ anyhow = "1.0.65"
 cloned = { version = "0.1.0", path = "../../cloned" }
 futures = { version = "0.3.22", features = ["async-await", "compat"] }
 futures_stats = { version = "0.1.0", path = "../../futures_stats" }
+itertools = "0.10.3"
 lazy_static = "1.4"
 mysql_async = "0.27.1"
 mysql_derive = { version = "0.1.0", path = "../derive" }

shed/sql/common/lib.rs

@@ -125,14 +125,14 @@ pub struct SqlShardedConnections {
 impl From<Vec1<SqlConnections>> for SqlShardedConnections {
     fn from(shard_connections: Vec1<SqlConnections>) -> Self {
         let (head, last) = shard_connections.split_off_last();
-        let mut write_connections = Vec::with_capacity(head.len());
-        let mut read_connections = Vec::with_capacity(head.len());
-        let mut read_master_connections = Vec::with_capacity(head.len());
-        for connections in head {
-            write_connections.push(connections.write_connection);
-            read_connections.push(connections.read_connection);
-            read_master_connections.push(connections.read_master_connection);
-        }
+        let (write_connections, read_connections, read_master_connections) =
+            itertools::multiunzip(head.into_iter().map(|conn| {
+                (
+                    conn.write_connection,
+                    conn.read_connection,
+                    conn.read_master_connection,
+                )
+            }));
 
         Self {
             read_connections: Vec1::from_vec_push(read_connections, last.read_connection),