Slightly improve performance of builders

Builders would keep a `serde_json::Map<String, Value>`, which would
require re-creating owned strings for the same parameter multiple times
in some cases, depending on builder defaults and keying strategies.

This commit uses a `std::collections::HashMap<&'static str, Value>`
internally, and moves over values to a `serde_json::Map<String, Value>`
when it comes time to sending them to the appropriate `http` module
function.

This saves the number of heap-allocated string creations on most
builders, with specific performance increase on `builder::CreateMessage`
and `builder::CreateEmbed` & co.

5 files changed, 39 insertions, 35 deletions

diff --git a/src/model/channel/channel_category.rs b/src/model/channel/channel_category.rs
index f567cee..73c50a7 100644
--- a/src/model/channel/channel_category.rs
+++ b/src/model/channel/channel_category.rs
@@ -93,20 +93,14 @@ impl ChannelCategory {
             }
         }
 
-        let mut map = Map::new();
-        map.insert("name".to_string(), Value::String(self.name.clone()));
-        map.insert(
-            "position".to_string(),
-            Value::Number(Number::from(self.position)),
-        );
-        map.insert(
-            "type".to_string(),
-            Value::String(self.kind.name().to_string()),
-        );
+        let mut map = HashMap::new();
+        map.insert("name", Value::String(self.name.clone()));
+        map.insert("position", Value::Number(Number::from(self.position)));
+        map.insert("type", Value::String(self.kind.name().to_string()));
 
-        let edited = f(EditChannel(map)).0;
+        let map = serenity_utils::hashmap_to_json_map(f(EditChannel(map)).0);
 
-        http::edit_channel(self.id.0, &edited).map(|channel| {
+        http::edit_channel(self.id.0, &map).map(|channel| {
             let GuildChannel {
                 id,
                 category_id,
diff --git a/src/model/channel/channel_id.rs b/src/model/channel/channel_id.rs
index 4034fb9..3bed660 100644
--- a/src/model/channel/channel_id.rs
+++ b/src/model/channel/channel_id.rs
@@ -11,6 +11,8 @@ use builder::{CreateMessage, EditChannel, GetMessages};
 use CACHE;
 #[cfg(feature = "model")]
 use http::{self, AttachmentType};
+#[cfg(feature = "model")]
+use utils;
 
 #[cfg(feature = "model")]
 impl ChannelId {
@@ -185,7 +187,9 @@ impl ChannelId {
     /// [Manage Channel]: permissions/constant.MANAGE_CHANNELS.html
     #[inline]
     pub fn edit<F: FnOnce(EditChannel) -> EditChannel>(&self, f: F) -> Result<GuildChannel> {
-        http::edit_channel(self.0, &f(EditChannel::default()).0)
+        let map = utils::hashmap_to_json_map(f(EditChannel::default()).0);
+
+        http::edit_channel(self.0, &map)
     }
 
     /// Edits a [`Message`] in the channel given its Id.
@@ -209,9 +213,9 @@ impl ChannelId {
     /// [`the limit`]: ../builder/struct.CreateMessage.html#method.content
     pub fn edit_message<F, M>(&self, message_id: M, f: F) -> Result<Message>
         where F: FnOnce(CreateMessage) -> CreateMessage, M: Into<MessageId> {
-        let map = f(CreateMessage::default()).0;
+        let msg = f(CreateMessage::default());
 
-        if let Some(content) = map.get("content") {
+        if let Some(content) = msg.0.get("content") {
             if let Value::String(ref content) = *content {
                 if let Some(length_over) = Message::overflow_length(content) {
                     return Err(Error::Model(ModelError::MessageTooLong(length_over)));
@@ -219,6 +223,8 @@ impl ChannelId {
             }
         }
 
+        let map = utils::hashmap_to_json_map(msg.0);
+
         http::edit_message(self.0, message_id.into().0, &Value::Object(map))
     }
 
@@ -441,9 +447,9 @@ impl ChannelId {
     /// [Send Messages]: permissions/constant.SEND_MESSAGES.html
     pub fn send_files<'a, F, T, It: IntoIterator<Item=T>>(&self, files: It, f: F) -> Result<Message>
         where F: FnOnce(CreateMessage) -> CreateMessage, T: Into<AttachmentType<'a>> {
-        let mut map = f(CreateMessage::default()).0;
+        let mut msg = f(CreateMessage::default());
 
-        if let Some(content) = map.get("content") {
+        if let Some(content) = msg.0.get("content") {
             if let Value::String(ref content) = *content {
                 if let Some(length_over) = Message::overflow_length(content) {
                     return Err(Error::Model(ModelError::MessageTooLong(length_over)));
@@ -451,7 +457,8 @@ impl ChannelId {
             }
         }
 
-        let _ = map.remove("embed");
+        let _ = msg.0.remove("embed");
+        let map = utils::hashmap_to_json_map(msg.0);
 
         http::send_files(self.0, files, map)
     }
@@ -477,14 +484,15 @@ impl ChannelId {
     /// [Send Messages]: permissions/constant.SEND_MESSAGES.html
     pub fn send_message<F>(&self, f: F) -> Result<Message>
         where F: FnOnce(CreateMessage) -> CreateMessage {
-        let CreateMessage(map, reactions) = f(CreateMessage::default());
+        let msg = f(CreateMessage::default());
+        let map = utils::hashmap_to_json_map(msg.0);
 
         Message::check_content_length(&map)?;
         Message::check_embed_length(&map)?;
 
         let message = http::send_message(self.0, &Value::Object(map))?;
 
-        if let Some(reactions) = reactions {
+        if let Some(reactions) = msg.1 {
             for reaction in reactions {
                 self.create_reaction(message.id, reaction)?;
             }
diff --git a/src/model/channel/embed.rs b/src/model/channel/embed.rs
index 435f706..74c503d 100644
--- a/src/model/channel/embed.rs
+++ b/src/model/channel/embed.rs
@@ -4,6 +4,8 @@ use utils::Colour;
 use internal::prelude::*;
 #[cfg(feature = "model")]
 use builder::CreateEmbed;
+#[cfg(feature = "model")]
+use utils;
 
 /// Represents a rich embed which allows using richer markdown, multiple fields
 /// and more. This was heavily inspired by [slack's attachments].
@@ -89,7 +91,9 @@ impl Embed {
     #[inline]
     pub fn fake<F>(f: F) -> Value
         where F: FnOnce(CreateEmbed) -> CreateEmbed {
-        Value::Object(f(CreateEmbed::default()).0)
+        let map = utils::hashmap_to_json_map(f(CreateEmbed::default()).0);
+
+        Value::Object(map)
     }
 }
 
diff --git a/src/model/channel/guild_channel.rs b/src/model/channel/guild_channel.rs
index cc74b9d..90bb926 100644
--- a/src/model/channel/guild_channel.rs
+++ b/src/model/channel/guild_channel.rs
@@ -116,7 +116,9 @@ impl GuildChannel {
             }
         }
 
-        http::create_invite(self.id.0, &f(CreateInvite::default()).0)
+        let map = serenity_utils::hashmap_to_json_map(f(CreateInvite::default()).0);
+
+        http::create_invite(self.id.0, &map)
     }
 
     /// Creates a [permission overwrite][`PermissionOverwrite`] for either a
@@ -312,18 +314,12 @@ impl GuildChannel {
             }
         }
 
-        let mut map = Map::new();
-        map.insert("name".to_string(), Value::String(self.name.clone()));
-        map.insert(
-            "position".to_string(),
-            Value::Number(Number::from(self.position)),
-        );
-        map.insert(
-            "type".to_string(),
-            Value::String(self.kind.name().to_string()),
-        );
-
-        let edited = f(EditChannel(map)).0;
+        let mut map = HashMap::new();
+        map.insert("name", Value::String(self.name.clone()));
+        map.insert("position", Value::Number(Number::from(self.position)));
+        map.insert("type", Value::String(self.kind.name().to_string()));
+
+        let edited = serenity_utils::hashmap_to_json_map(f(EditChannel(map)).0);
 
         match http::edit_channel(self.id.0, &edited) {
             Ok(channel) => {
diff --git a/src/model/channel/message.rs b/src/model/channel/message.rs
index 0e939b2..b5864fc 100644
--- a/src/model/channel/message.rs
+++ b/src/model/channel/message.rs
@@ -14,6 +14,8 @@ use constants;
 use CACHE;
 #[cfg(feature = "model")]
 use http;
+#[cfg(feature = "model")]
+use utils as serenity_utils;
 
 /// A representation of a message over a guild's text channel, a group, or a
 /// private channel.
@@ -230,7 +232,7 @@ impl Message {
             builder = builder.tts(true);
         }
 
-        let map = f(builder).0;
+        let map = serenity_utils::hashmap_to_json_map(f(builder).0);
 
         match http::edit_message(self.channel_id.0, self.id.0, &Value::Object(map)) {
             Ok(edited) => {