WIP: config: Type safe API prototype

If we represent settings with types, we can provide an api to
get/set/monitor settings that guarantees we can only access settings
that exist with the right types, without seperate getter/setter/callback
functions like GTK would use.

cosmic-config/examples/app.rs

@@ -0,0 +1,27 @@
+use cosmic_config::setting::{App, Setting, AppConfig};
+
+struct ExampleApp;
+
+impl App for ExampleApp {
+    const ID: &'static str = "com.Example.App";
+    const VERSION: u64 = 1;
+}
+
+struct DoFoo;
+
+impl Setting<ExampleApp> for DoFoo {
+    const NAME: &'static str = "do-foo";
+    type Type = bool;
+}
+
+struct WhatBar;
+
+impl Setting<ExampleApp> for WhatBar {
+    const NAME: &'static str = "what-bar";
+    type Type = String;
+}
+
+fn main() {
+    let config = AppConfig::<ExampleApp>::new().unwrap();
+    config.set::<DoFoo>(true).unwrap();
+}

cosmic-config/src/lib.rs

@@ -22,6 +22,8 @@ pub use cosmic_config_derive;
 #[cfg(feature = "calloop")]
 pub mod calloop;
 
+pub mod setting;
+
 #[derive(Debug)]
 pub enum Error {
     AtomicWrites(atomicwrites::Error<std::io::Error>),

cosmic-config/src/setting.rs

@@ -0,0 +1,36 @@
+use crate::{Config, ConfigGet, ConfigSet, Error};
+
+pub trait App {
+    const ID: &'static str;
+    // XXX how to handle versioning?
+    const VERSION: u64;
+}
+
+pub trait Setting<A: App> {
+    const NAME: &'static str;
+    // TODO can't use &str to set? Need to serialize owned value.
+    type Type: serde::Serialize + serde::de::DeserializeOwned;
+}
+
+pub struct AppConfig<A: App> {
+    config: Config,
+    _app: std::marker::PhantomData<A>,
+}
+
+impl<A: App> AppConfig<A> {
+    pub fn new() -> Result<Self, Error> {
+        Ok(Self {
+            config: Config::new(A::ID, A::VERSION)?,
+            _app: std::marker::PhantomData,
+        })
+    }
+
+    // XXX default value, if none set?
+    pub fn get<S: Setting<A>>(&self) -> Result<S::Type, Error> {
+        self.config.get(S::NAME)
+    }
+
+    pub fn set<S: Setting<A>>(&self, value: S::Type) -> Result<(), Error> {
+        self.config.set(S::NAME, value)
+    }
+}