From b1ed29657e9e934d2a08c820d521044ca3c5444a Mon Sep 17 00:00:00 2001
From: Tony Garnock-Jones <tonyg@leastfixedpoint.com>
Date: Mon, 20 Sep 2021 15:38:21 +0200
Subject: [PATCH] Make it so that any defined language can be used as
 placeholder to identity-(un)parse `NestedValue`s

---
 implementations/rust/preserves-schema/src/lib.rs      |  3 +++
 .../rust/preserves-schema/src/support/mod.rs          | 11 +++++++----
 2 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/implementations/rust/preserves-schema/src/lib.rs b/implementations/rust/preserves-schema/src/lib.rs
index ec34461..39be38c 100644
--- a/implementations/rust/preserves-schema/src/lib.rs
+++ b/implementations/rust/preserves-schema/src/lib.rs
@@ -64,6 +64,9 @@ macro_rules! define_language {
             }
         })*
 
+        impl<N: $crate::support::preserves::value::NestedValue> $crate::support::NestedValueCodec
+            for $lang<N> {}
+
         mod $fname {
             use super::*;
             lazy_static::lazy_static! {
diff --git a/implementations/rust/preserves-schema/src/support/mod.rs b/implementations/rust/preserves-schema/src/support/mod.rs
index e159574..e656671 100644
--- a/implementations/rust/preserves-schema/src/support/mod.rs
+++ b/implementations/rust/preserves-schema/src/support/mod.rs
@@ -19,12 +19,15 @@ use std::sync::Arc;
 
 use thiserror::Error;
 
+pub trait NestedValueCodec {} // marker trait
+impl NestedValueCodec for () {}
+
 pub trait Parse<L, Value: NestedValue>: Sized {
     fn parse(language: L, value: &Value) -> Result<Self, ParseError>;
 }
 
-impl<'a, Value: NestedValue> Parse<&'a (), Value> for Value {
-    fn parse(_language: &'a (), value: &Value) -> Result<Self, ParseError> {
+impl<'a, T: NestedValueCodec, Value: NestedValue> Parse<&'a T, Value> for Value {
+    fn parse(_language: &'a T, value: &Value) -> Result<Self, ParseError> {
         Ok(value.clone())
     }
 }
@@ -33,8 +36,8 @@ pub trait Unparse<L, Value: NestedValue> {
     fn unparse(&self, language: L) -> Value;
 }
 
-impl<'a, Value: NestedValue> Unparse<&'a (), Value> for Value {
-    fn unparse(&self, _language: &'a ()) -> Value {
+impl<'a, T: NestedValueCodec, Value: NestedValue> Unparse<&'a T, Value> for Value {
+    fn unparse(&self, _language: &'a T) -> Value {
         self.clone()
     }
 }