From dd1e70aa3b51be4e14753142bc46cc78a892eaf8 Mon Sep 17 00:00:00 2001
From: Ian Jackson <ijackson@chiark.greenend.org.uk>
Date: Thu, 12 May 2022 00:01:47 +0100
Subject: [PATCH] ambassador: impl_via_ambassador: Introduce and use it once

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>
---
 src/fastsplit.rs |  7 ++++---
 src/prelude.rs   |  1 +
 src/utils.rs     | 18 ++++++++++++++++++
 3 files changed, 23 insertions(+), 3 deletions(-)

diff --git a/src/fastsplit.rs b/src/fastsplit.rs
index f6c23f69..23622621 100644
--- a/src/fastsplit.rs
+++ b/src/fastsplit.rs
@@ -56,10 +56,11 @@ impl Piece {
   }
 }
 
-#[dyn_upcast]
-impl OutlineTrait for Piece {
-  ambassador_impl_OutlineTrait! {body_struct(<>, _, (), (inner()), ())}
+impl_via_ambassador!{
+  #[dyn_upcast]
+  impl OutlineTrait for Piece { inner() } 
 }
+
 #[dyn_upcast]
 impl PieceBaseTrait for Piece {
   ambassador_impl_PieceBaseTrait! {body_struct(<>, _, (), (inner()), ())}
diff --git a/src/prelude.rs b/src/prelude.rs
index a8c1610a..e502c85d 100644
--- a/src/prelude.rs
+++ b/src/prelude.rs
@@ -136,6 +136,7 @@ pub use base_misc::*;
 pub use crate::{deref_to_field, deref_to_field_mut};
 pub use crate::ensure_eq;
 pub use crate::format_by_fmt_hex;
+pub use crate::impl_via_ambassador;
 pub use crate::matches_doesnot;
 pub use crate::trace_dbg;
 pub use crate::{want, wantok, wants, want_let, want_failed_internal};
diff --git a/src/utils.rs b/src/utils.rs
index bb0953f9..40d3031b 100644
--- a/src/utils.rs
+++ b/src/utils.rs
@@ -773,6 +773,24 @@ paste!{
   pub fn [<x x x>]<T>() -> T { panic!("todo item triggered") }
 }
 
+#[macro_export]
+macro_rules! impl_via_ambassador{
+  { 
+    $( #[ $attr:meta ] )*
+    impl $Trait:ident for $Type:ty
+    { $($how_immut:tt)* }
+  } => { paste!{
+    $( #[ $attr ] )*
+    impl $Trait for $Type {
+      [< ambassador_impl_ $Trait >]!{ body_struct( <>, _,
+          (),
+          ($($how_immut)*),
+          ()
+      ) }
+    }
+  } }
+}
+
 #[macro_export]
 macro_rules! trace_dbg {
   ($msg:expr $(,$val:expr)*) => {
-- 
2.30.2