From 82aa4b902a77ce703cbb94a36c6265143741285b Mon Sep 17 00:00:00 2001
From: Ian Jackson <ijackson@chiark.greenend.org.uk>
Date: Wed, 16 Mar 2022 19:30:10 +0000
Subject: [PATCH] Provide define_accessor_trait and use it to generate
 CircMgrConfig

---
 Cargo.lock                        |  1 +
 crates/tor-basic-utils/Cargo.toml |  1 +
 crates/tor-basic-utils/src/lib.rs | 68 +++++++++++++++++++++++++++++++
 crates/tor-circmgr/src/config.rs  | 32 ++++++---------
 4 files changed, 82 insertions(+), 20 deletions(-)

diff --git a/Cargo.lock b/Cargo.lock
index 6e1b047c1..cdcae9108 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -3129,6 +3129,7 @@ dependencies = [
 name = "tor-basic-utils"
 version = "0.1.0"
 dependencies = [
+ "derive_more",
  "educe",
  "rand 0.8.5",
 ]
diff --git a/crates/tor-basic-utils/Cargo.toml b/crates/tor-basic-utils/Cargo.toml
index 9ae19c4c5..3ceea934c 100644
--- a/crates/tor-basic-utils/Cargo.toml
+++ b/crates/tor-basic-utils/Cargo.toml
@@ -15,4 +15,5 @@ repository = "https://gitlab.torproject.org/tpo/core/arti.git/"
 rand = "0.8"
 
 [dev-dependencies]
+derive_more = "0.99"
 educe = "0.4.6"
diff --git a/crates/tor-basic-utils/src/lib.rs b/crates/tor-basic-utils/src/lib.rs
index 2c5ec8cd7..fb7f423fa 100644
--- a/crates/tor-basic-utils/src/lib.rs
+++ b/crates/tor-basic-utils/src/lib.rs
@@ -72,3 +72,71 @@ pub fn skip_fmt<T>(_: &T, f: &mut fmt::Formatter) -> fmt::Result {
 }
 
 // ----------------------------------------------------------------------
+
+/// Define an "accessor trait", which describes structs that have fields of certain types
+///
+/// This can be useful if a large struct, living high up in the dependency graph,
+/// contains fields that lower-lever crates want to be able to use without having
+/// to copy the data about etc.
+///
+/// ```
+/// // imagine this in the lower-level module
+/// use tor_basic_utils::define_accessor_trait;
+/// define_accessor_trait! {
+///     pub trait View {
+///         lorem: String,
+///         ipsum: usize,
+///    }
+/// }
+///
+/// fn test_view<V: View>(v: &V) {
+///     assert_eq!(v.lorem(), "sit");
+///     assert_eq!(v.ipsum(), &42);
+/// }
+///
+/// // imagine this in the higher-level module
+/// use derive_more::AsRef;
+/// #[derive(AsRef)]
+/// struct Everything {
+///     #[as_ref] lorem: String,
+///     #[as_ref] ipsum: usize,
+///     dolor: Vec<()>,
+/// }
+/// impl View for Everything { }
+///
+/// let everything = Everything {
+///     lorem: "sit".into(),
+///     ipsum: 42,
+///     dolor: vec![()],
+/// };
+///
+/// test_view(&everything);
+/// ```
+///
+/// ### Generated code
+///
+/// ```
+/// pub trait View: AsRef<String> + AsRef<usize> {
+///     fn lorem(&self) -> &String { self.as_ref() }
+///     fn ipsum(&self) -> &usize { self.as_ref() }
+/// }
+/// ```
+#[macro_export]
+macro_rules! define_accessor_trait {
+    {
+        $( #[ $attr:meta ])*
+        $vis:vis trait $Trait:ident {
+            $( $accessor:ident: $type:ty, )*
+        }
+    } => {
+        $( #[ $attr ])*
+        $vis trait $Trait: $( core::convert::AsRef<$type> + )* {
+            $(
+                /// Access the field
+                fn $accessor(&self) -> &$type { core::convert::AsRef::as_ref(self) }
+            )*
+        }
+    }
+}
+
+// ----------------------------------------------------------------------
diff --git a/crates/tor-circmgr/src/config.rs b/crates/tor-circmgr/src/config.rs
index 6e0adecc8..dbce3df8a 100644
--- a/crates/tor-circmgr/src/config.rs
+++ b/crates/tor-circmgr/src/config.rs
@@ -4,6 +4,7 @@
 //!
 //! Most types in this module are re-exported by `arti-client`.
 
+use tor_basic_utils::define_accessor_trait;
 use tor_config::ConfigBuildError;
 
 use derive_builder::Builder;
@@ -248,26 +249,17 @@ impl PreemptiveCircuitConfig {
     }
 }
 
-/// Configuration for a circuit manager
-///
-/// If the circuit manager gains new configurabilities, this trait will gain additional
-/// supertraits, as an API break.
-///
-/// Prefer to use `TorClientConfig`, which will always implement this trait.
-pub trait CircMgrConfig:
-    AsRef<PathConfig> + AsRef<CircuitTiming> + AsRef<PreemptiveCircuitConfig>
-{
-    /// Get the PathConfig
-    fn path_rules(&self) -> &PathConfig {
-        self.as_ref()
-    }
-    /// Get the CircuitTiming
-    fn circuit_timing(&self) -> &CircuitTiming {
-        self.as_ref()
-    }
-    /// Get the PreemptiveCircuitConfig
-    fn preemptive_circuits(&self) -> &PreemptiveCircuitConfig {
-        self.as_ref()
+define_accessor_trait! {
+    /// Configuration for a circuit manager
+    ///
+    /// If the circuit manager gains new configurabilities, this trait will gain additional
+    /// supertraits, as an API break.
+    ///
+    /// Prefer to use [`TorClientConfig`], which will always implement this trait.
+    pub trait CircMgrConfig {
+        path_rules: PathConfig,
+        circuit_timing: CircuitTiming,
+        preemptive_circuits: PreemptiveCircuitConfig,
     }
 }