add embed validation

Add embed validation for creating and updating messages. This checks the
the author name, description, field names, field values, footer text,
and title lengths to ensure they aren't too long. The total number of
fields is checked as well as the total combined length of all of the
above.

This addresses comment
<#167 (comment)>.

Limits are referenced in the documentation to point to
<https://discord.com/developers/docs/resources/channel#embed-limits>.

Signed-off-by: Vivian Hellyer <vivian@hellyer.dev>

http/Cargo.toml

@@ -24,4 +24,4 @@ percent-encoding = "2.1"
 url = "2"
 
 [dev-dependencies]
-tokio = "0.2"
+tokio = { features = ["macros"], version = "0.2" }

http/src/request/channel/message/create_message.rs

@@ -17,17 +17,30 @@ use twilight_model::{
 #[derive(Clone, Debug)]
 pub enum CreateMessageError {
     ContentInvalid,
+    EmbedTooLarge { source: EmbedValidationError },
 }
 
 impl Display for CreateMessageError {
     fn fmt(&self, f: &mut Formatter<'_>) -> FmtResult {
         match self {
             Self::ContentInvalid => f.write_str("the message content is invalid"),
+            Self::EmbedTooLarge {
+                ..
+            } => f.write_str("the embed's contents are too long"),
         }
     }
 }
 
-impl Error for CreateMessageError {}
+impl Error for CreateMessageError {
+    fn source(&self) -> Option<&(dyn Error + 'static)> {
+        match self {
+            Self::ContentInvalid => None,
+            Self::EmbedTooLarge {
+                source,
+            } => Some(source),
+        }
+    }
+}
 
 #[derive(Default, Serialize)]
 pub(crate) struct CreateMessageFields {
@@ -85,10 +98,14 @@ impl<'a> CreateMessage<'a> {
         Ok(self)
     }
 
-    pub fn embed(mut self, embed: Embed) -> Self {
+    pub fn embed(mut self, embed: Embed) -> Result<Self, CreateMessageError> {
+        validate::embed(&embed).map_err(|source| CreateMessageError::EmbedTooLarge {
+            source,
+        })?;
+
         self.fields.embed.replace(embed);
 
-        self
+        Ok(self)
     }
 
     pub fn allowed_mentions(

http/src/request/channel/message/mod.rs

@@ -17,3 +17,4 @@ pub use self::{
     get_message::GetMessage,
     update_message::UpdateMessage,
 };
+pub use super::super::validate::EmbedValidationError;

http/src/request/channel/message/update_message.rs

@@ -11,17 +11,30 @@ use twilight_model::{
 #[derive(Clone, Debug)]
 pub enum UpdateMessageError {
     ContentInvalid,
+    EmbedTooLarge { source: EmbedValidationError },
 }
 
 impl Display for UpdateMessageError {
     fn fmt(&self, f: &mut Formatter<'_>) -> FmtResult {
         match self {
             Self::ContentInvalid => f.write_str("the message content is invalid"),
+            Self::EmbedTooLarge {
+                ..
+            } => f.write_str("the embed's contents are too long"),
         }
     }
 }
 
-impl Error for UpdateMessageError {}
+impl Error for UpdateMessageError {
+    fn source(&self) -> Option<&(dyn Error + 'static)> {
+        match self {
+            Self::ContentInvalid => None,
+            Self::EmbedTooLarge {
+                source,
+            } => Some(source),
+        }
+    }
+}
 
 #[derive(Default, Serialize)]
 struct UpdateMessageFields {
@@ -129,10 +142,20 @@ impl<'a> UpdateMessage<'a> {
     /// Set the embed of the message.
     ///
     /// Pass `None` if you want to remove the message embed.
-    pub fn embed(mut self, embed: impl Into<Option<Embed>>) -> Self {
-        self.fields.embed.replace(embed.into());
+    pub fn embed(self, embed: impl Into<Option<Embed>>) -> Result<Self, UpdateMessageError> {
+        self._embed(embed.into())
+    }
 
-        self
+    fn _embed(mut self, embed: Option<Embed>) -> Result<Self, UpdateMessageError> {
+        if let Some(embed) = embed.as_ref() {
+            validate::embed(&embed).map_err(|source| UpdateMessageError::EmbedTooLarge {
+                source,
+            })?;
+        }
+
+        self.fields.embed.replace(embed);
+
+        Ok(self)
     }
 
     fn start(&mut self) -> Result<()> {