fix(model): Unavailable guild must always have unavailable as true (#…

…2361)

This adds a new construct that can be used to only deserialize a
specific boolean value. We then use that value to ensure that only
guilds which are actually unavailable get deserialized as such.

---------

Signed-off-by: Erk <[email protected]>
Co-authored-by: Jens Reidel <[email protected]>

twilight-model/src/gateway/payload/incoming/guild_create.rs

@@ -4,11 +4,13 @@ use crate::{
 };
 use serde::{Deserialize, Serialize};
 
+// Developer note: Do not change order as we want unavailable to fail
+// first.
 #[derive(Clone, Debug, Deserialize, Eq, PartialEq, Serialize)]
 #[serde(untagged)]
 pub enum GuildCreate {
-    Available(Guild),
     Unavailable(UnavailableGuild),
+    Available(Guild),
 }
 
 impl GuildCreate {

twilight-model/src/guild/unavailable_guild.rs

@@ -1,12 +1,37 @@
-use crate::id::{marker::GuildMarker, Id};
+use crate::{
+    id::{marker::GuildMarker, Id},
+    util::mustbe::MustBeBool,
+};
 use serde::{Deserialize, Serialize};
 
-#[derive(Clone, Debug, Deserialize, Eq, Hash, PartialEq, Serialize)]
+#[derive(Clone, Debug, Eq, Hash, PartialEq, Serialize)]
 pub struct UnavailableGuild {
     pub id: Id<GuildMarker>,
     pub unavailable: bool,
 }
 
+impl<'de> Deserialize<'de> for UnavailableGuild {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: serde::Deserializer<'de>,
+    {
+        #[derive(Deserialize)]
+        #[serde(rename = "UnavailableGuild")] // Tests expect this struct name
+        struct UnavailableGuildIntermediate {
+            id: Id<GuildMarker>,
+            #[allow(unused)] // Only used in the derived impl
+            unavailable: MustBeBool<true>,
+        }
+
+        let intermediate = UnavailableGuildIntermediate::deserialize(deserializer)?;
+
+        Ok(Self {
+            id: intermediate.id,
+            unavailable: true,
+        })
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::UnavailableGuild;

twilight-model/src/util/mod.rs

@@ -3,6 +3,7 @@
 pub mod datetime;
 pub mod hex_color;
 pub mod image_hash;
+pub(crate) mod mustbe;
 
 pub use self::{datetime::Timestamp, hex_color::HexColor, image_hash::ImageHash};
 

twilight-model/src/util/mustbe.rs

@@ -0,0 +1,84 @@
+//! A struct that only deserializes from one specific boolean value.
+//!
+//! This module is heavily based upon
+//! <https://github.com/dtolnay/monostate>.
+
+use std::fmt;
+
+use serde::{
+    de::{Error, Unexpected, Visitor},
+    Deserialize,
+};
+
+/// Struct that will only serialize from the bool specified as `T`.
+pub struct MustBeBool<const T: bool>;
+
+impl<'de, const T: bool> Deserialize<'de> for MustBeBool<T> {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: serde::Deserializer<'de>,
+    {
+        struct MustBeBoolVisitor(bool);
+
+        impl<'de> Visitor<'de> for MustBeBoolVisitor {
+            type Value = ();
+
+            fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+                write!(formatter, "boolean `{}`", self.0)
+            }
+
+            fn visit_bool<E>(self, v: bool) -> Result<Self::Value, E>
+            where
+                E: Error,
+            {
+                if v == self.0 {
+                    Ok(())
+                } else {
+                    Err(E::invalid_value(Unexpected::Bool(v), &self))
+                }
+            }
+        }
+
+        deserializer
+            .deserialize_any(MustBeBoolVisitor(T))
+            .map(|()| MustBeBool)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::MustBeBool;
+
+    use serde::Deserialize;
+
+    #[derive(Deserialize)]
+    struct MTrue {
+        #[allow(unused)]
+        m: MustBeBool<true>,
+    }
+
+    #[derive(Deserialize)]
+    struct MFalse {
+        #[allow(unused)]
+        m: MustBeBool<false>,
+    }
+
+    #[derive(Deserialize)]
+    #[serde(untagged)]
+    enum TestEnum {
+        VariantTrue(MTrue),
+        VariantFalse(MFalse),
+    }
+
+    #[test]
+    #[allow(unused)]
+    fn true_false_enum() {
+        let json_1 = r#"{ "m": false }"#;
+        let result_1 = serde_json::from_str::<TestEnum>(json_1).unwrap();
+        assert!(matches!(result_1, TestEnum::VariantFalse(_)));
+
+        let json_2 = r#"{ "m": true }"#;
+        let result_2 = serde_json::from_str::<TestEnum>(json_2).unwrap();
+        assert!(matches!(result_2, TestEnum::VariantTrue(_)));
+    }
+}