Define ID types as specializations, rather than newtypes.

This is based heavily on the work done for normalized strings, in `crate::normalize`. The key realization in that module is that the logic distinguishing one kind of thing (normalized strings in that case, IDs, in this case) can be packaged up as a type token, and that doing so may reduce the overall complexity. This implementation for ID also borrows heavily from the implementation for normalized strings.

It's less flexible: an ID implemented this way can't expose _less_ of `crate::id::ID`'s interface, whereas newtype wrappers can, for example. However, our code doesn't use that flexiblity on purpose anywhere and we're relatively unlikely to change that. In return, the individual ID types require substantially less code - they do not, for example, need to re-implement `Display` for themselves.

I very nearly made the trait `Prefix`:

```rust
pub trait Prefix {
    const PREFIX: &str;
}
```

however, I think having an effectively-constant method is less surprising overall.

1 files changed, 137 insertions, 33 deletions

diff --git a/src/id.rs b/src/id.rs
index d79f600..8d71c5b 100644
--- a/src/id.rs
+++ b/src/id.rs
@@ -1,4 +1,7 @@
 use rand::{seq::SliceRandom, thread_rng};
+use serde::{Deserializer, Serializer};
+use sqlx::encode::IsNull;
+use sqlx::{Database, Decode, Encode, Type};
 use std::fmt;
 
 // Make IDs that:
@@ -23,48 +26,149 @@ const ALPHABET: [char; 23] = [
 // 68 bits per ID.
 const ID_SIZE: usize = 15;
 
-// Intended to be wrapped in a newtype that provides both type-based separation
-// from other identifier types, and a unique prefix to allow the intended type
-// of an ID to be determined by eyeball when debugging.
+// Intended to be wrapped in a type aliases that specalizes Id on a type that
+// implements Prefix, both so that IDs are type-wise distinct within the server
+// and so that IDs are readily distinguishable from one another outside of it.
 //
 // By convention, the prefix should be UPPERCASE - note that the alphabet for
 // this is entirely lowercase.
-#[derive(
-    Clone,
-    Debug,
-    Hash,
-    Eq,
-    Ord,
-    PartialEq,
-    PartialOrd,
-    sqlx::Type,
-    serde::Deserialize,
-    serde::Serialize,
-)]
-#[sqlx(transparent)]
-#[serde(transparent)]
-pub struct Id(String);
-
-impl fmt::Display for Id {
+//
+// To build a new ID type, create a "marker" type that implements this trait, plus the following list of derives:
+//
+// ```
+// #[derive(Clone, Debug, Default, Eq, Hash, Ord, PartialEq, PartialOrd)]
+// pub struct Person;
+//
+// impl crate::id::Prefix for Person {
+//     fn prefix(&self) -> &str {
+//         "P" // try not to conflict with other prefixes
+//     }
+// }
+// ```
+//
+// Then provide a type alias of the form
+//
+// ```
+// pub type Id = crate::id::Id<Person>;
+// ```
+//
+// The `Id` type will provide a `generate()` method that can generate new random IDs, and the
+// resulting type alias can be serialized to and deserialized from strings, and stored in string
+// compatible database columns.
+#[derive(Clone, Debug, Hash, Eq, Ord, PartialEq, PartialOrd)]
+pub struct Id<T>(String, T);
+
+impl<T> fmt::Display for Id<T> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         self.0.fmt(f)
     }
 }
 
-impl Id {
-    pub fn generate<T>(prefix: &str) -> T
+pub trait Prefix {
+    fn prefix(&self) -> &str;
+}
+
+impl<T> Id<T>
+where
+    T: Prefix + Default,
+{
+    pub fn generate() -> Self {
+        let instance = T::default();
+        let prefix = instance.prefix();
+
+        let random_part = (0..ID_SIZE)
+            .filter_map(|_| ALPHABET.choose(&mut thread_rng()))
+            .copied();
+
+        let id = prefix.chars().chain(random_part).collect();
+
+        Self(id, instance)
+    }
+}
+
+impl<T> serde::ser::Serialize for Id<T> {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        let Self(id, _) = self;
+        id.serialize(serializer)
+    }
+}
+
+impl<'de, T> serde::de::Deserialize<'de> for Id<T>
+where
+    T: Default,
+{
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
     where
-        T: From<Self>,
+        D: Deserializer<'de>,
     {
-        let mut rng = thread_rng();
-        let id = prefix
-            .chars()
-            .chain(
-                (0..ID_SIZE)
-                    .filter_map(|_| ALPHABET.choose(&mut rng)) /* usize -> &char */
-                    .copied(), /* &char -> char */
-            )
-            .collect();
-        T::from(Self(id))
+        let instance = T::default();
+        let id = String::deserialize(deserializer)?;
+
+        Ok(Self(id, instance))
+    }
+}
+
+// Type is manually implemented so that we can implement Decode to do
+// recover the type token on read. Implementation is otherwise based on
+// `#[derive(sqlx::Type)]` with the `#[sqlx(transparent)]` attribute.
+impl<DB, T> Type<DB> for Id<T>
+where
+    DB: Database,
+    String: Type<DB>,
+{
+    fn type_info() -> <DB as Database>::TypeInfo {
+        <String as Type<DB>>::type_info()
+    }
+
+    fn compatible(ty: &<DB as Database>::TypeInfo) -> bool {
+        <String as Type<DB>>::compatible(ty)
+    }
+}
+
+impl<'r, DB, T> Decode<'r, DB> for Id<T>
+where
+    DB: Database,
+    String: Decode<'r, DB>,
+    T: Default,
+{
+    fn decode(value: <DB as Database>::ValueRef<'r>) -> Result<Self, sqlx::error::BoxDynError> {
+        let instance = T::default();
+        let id = String::decode(value)?;
+        Ok(Self(id, instance))
+    }
+}
+
+impl<'q, DB, T> Encode<'q, DB> for Id<T>
+where
+    DB: Database,
+    String: Encode<'q, DB>,
+{
+    fn encode(
+        self,
+        buf: &mut <DB as Database>::ArgumentBuffer<'q>,
+    ) -> Result<IsNull, sqlx::error::BoxDynError> {
+        let Self(id, _) = self;
+        id.encode(buf)
+    }
+
+    fn encode_by_ref(
+        &self,
+        buf: &mut <DB as Database>::ArgumentBuffer<'q>,
+    ) -> Result<IsNull, sqlx::error::BoxDynError> {
+        let Self(id, _) = self;
+        id.encode_by_ref(buf)
+    }
+
+    fn produces(&self) -> Option<<DB as Database>::TypeInfo> {
+        let Self(id, _) = self;
+        id.produces()
+    }
+
+    fn size_hint(&self) -> usize {
+        let Self(id, _) = self;
+        id.size_hint()
     }
 }