From c228c6ff93fb89f9c003dfaf0f0d1342dc2c91cb Mon Sep 17 00:00:00 2001
From: "Adam H. Leventhal" <ahl@oxide.computer>
Date: Sat, 19 Sep 2026 14:41:42 -0700
Subject: [PATCH] emit shared default functions only where something calls them

The defaults set records a helper when a default value is considered,
not when a call site survives rendering, so vega emitted a default_i64
nothing called. Filter at output time, the way the error module now
works.
---
 typify-impl/src/lib.rs     | 28 ++++++++++++++++++++++++----
 typify-impl/tests/vega.out |  7 -------
 2 files changed, 24 insertions(+), 11 deletions(-)

diff --git a/typify-impl/src/lib.rs b/typify-impl/src/lib.rs
index 462e30e2..ee1a2615 100644
--- a/typify-impl/src/lib.rs
+++ b/typify-impl/src/lib.rs
@@ -235,6 +235,18 @@ pub(crate) enum DefaultImpl {
     NZU64,
 }
 
+impl DefaultImpl {
+    /// The name of the shared function this renders as.
+    fn fn_name(&self) -> &'static str {
+        match self {
+            DefaultImpl::Boolean => "default_bool",
+            DefaultImpl::I64 => "default_i64",
+            DefaultImpl::U64 => "default_u64",
+            DefaultImpl::NZU64 => "default_nzu64",
+        }
+    }
+}
+
 /// Type name to use in generated code.
 #[derive(Clone)]
 pub struct MapType(pub syn::Type);
@@ -907,14 +919,22 @@ impl TypeSpace {
             .values()
             .for_each(|type_entry| type_entry.output(self, &mut output));
 
-        // Add all shared default functions.
-        self.defaults
+        // Add the shared default functions that some emitted item actually
+        // calls. This is gross, and may have false-positives, but those should
+        // be basically benign.
+        let called = self
+            .defaults
             .iter()
+            .filter(|x| output.contains(x.fn_name()))
+            .collect::<Vec<_>>();
+        called
+            .into_iter()
             .for_each(|x| output.add_item(output::OutputSpaceMod::Defaults, "", x.into()));
 
         // Add the error type conversions use, but only when some emitted item
-        // references it. This is kind of gross that we're groveling around
-        // through output to decide, but it will--I hope--be short-lived.
+        // references it. This is also kind of gross that we're groveling
+        // around through output to decide, but it will--I hope--be
+        // short-lived.
         if output.contains("ConversionError") {
             self.add_error_item(&mut output);
         }
diff --git a/typify-impl/tests/vega.out b/typify-impl/tests/vega.out
index cdb59c94..6c3dea2c 100644
--- a/typify-impl/tests/vega.out
+++ b/typify-impl/tests/vega.out
@@ -45358,13 +45358,6 @@ pub mod defaults {
     pub(super) fn default_bool<const V: bool>() -> bool {
         V
     }
-    pub(super) fn default_i64<T, const V: i64>() -> T
-    where
-        T: ::std::convert::TryFrom<i64>,
-        <T as ::std::convert::TryFrom<i64>>::Error: ::std::fmt::Debug,
-    {
-        T::try_from(V).unwrap()
-    }
     pub(super) fn aggregate_transform_drop() -> super::AggregateTransformDrop {
         super::AggregateTransformDrop::Boolean(true)
     }