From 29a882f9537e497319934e697d0668eb4dbfa7da Mon Sep 17 00:00:00 2001
From: Tony Garnock-Jones <tonyg@leastfixedpoint.com>
Date: Thu, 30 Sep 2021 11:15:26 +0200
Subject: [PATCH] Shrink commonest annotationless representation

---
 .../rust/preserves/src/value/repr.rs          | 30 ++++++++++++++-----
 1 file changed, 22 insertions(+), 8 deletions(-)

diff --git a/implementations/rust/preserves/src/value/repr.rs b/implementations/rust/preserves/src/value/repr.rs
index d41e66b..07cdfb7 100644
--- a/implementations/rust/preserves/src/value/repr.rs
+++ b/implementations/rust/preserves/src/value/repr.rs
@@ -75,7 +75,7 @@ pub trait NestedValue: Sized + Debug + Clone + Eq + Hash + Ord {
     {
         match &self.annotations().0 {
             None => (),
-            Some(vs) => for v in vs { v.foreach_embedded(f)? },
+            Some(vs) => for v in vs.iter() { v.foreach_embedded(f)? },
         }
         self.value().foreach_embedded(f)
     }
@@ -1007,7 +1007,7 @@ impl<'de> serde::Deserialize<'de> for UnwrappedIOValue {
 //---------------------------------------------------------------------------
 
 #[derive(Clone)]
-pub struct Annotations<N: NestedValue>(Option<Vec<N>>);
+pub struct Annotations<N: NestedValue>(Option<Box<Vec<N>>>);
 
 impl<N: NestedValue> Annotations<N> {
     pub fn empty() -> Self {
@@ -1015,7 +1015,7 @@ impl<N: NestedValue> Annotations<N> {
     }
 
     pub fn new(anns: Option<Vec<N>>) -> Self {
-        Annotations(anns)
+        Annotations(anns.map(Box::new))
     }
 
     pub fn maybe_slice(&self) -> Option<&[N]> {
@@ -1030,16 +1030,30 @@ impl<N: NestedValue> Annotations<N> {
     }
 
     pub fn to_vec(self) -> Vec<N> {
-        self.0.unwrap_or_default()
+        use std::ops::DerefMut;
+        self.0.map(|mut b| std::mem::take(b.deref_mut())).unwrap_or_default()
     }
 
     pub fn modify<F>(&mut self, f: F) -> &mut Self
     where
         F: FnOnce(&mut Vec<N>)
     {
-        let mut v: Vec<N> = self.0.take().unwrap_or_else(Vec::new);
-        f(&mut v);
-        self.0 = if v.is_empty() { None } else { Some(v) };
+        match &mut self.0 {
+            None => {
+                let mut v = Vec::new();
+                f(&mut v);
+                if !v.is_empty() {
+                    self.0 = Some(Box::new(v));
+                }
+            }
+            Some(b) => {
+                use std::ops::DerefMut;
+                f(b.deref_mut());
+                if b.is_empty() {
+                    self.0 = None;
+                }
+            }
+        }
         self
     }
 
@@ -1051,7 +1065,7 @@ impl<N: NestedValue> Annotations<N> {
             None =>
                 Annotations(None),
             Some(b) =>
-                Annotations(Some(b.iter().map(|a| a.copy_via(f)).collect::<Result<Vec<_>, _>>()?)),
+                Annotations(Some(Box::new(b.iter().map(|a| a.copy_via(f)).collect::<Result<Vec<_>, _>>()?))),
         })
     }
 }