Reduce some allocations in glue

crates/glue/src/types.rs

@@ -1222,19 +1222,16 @@ fn add_tag_union<'a>(
                     }
                 }
                 NonNullableUnwrapped(_) => {
-                    let mut tags =
-                        union_tags_to_types(&name, union_tags, subs, env, types, layout, true);
-
-                    debug_assert_eq!(tags.len(), 1);
-
-                    let (tag_name, opt_payload) = tags.pop().unwrap();
+                    let (tag_name, payload_vars) = single_tag_payload(union_tags, subs);
+                    let (tag_name, opt_payload) =
+                        tag_to_type(&name, env, tag_name, payload_vars, types, layout, true);
 
                     // A recursive tag union with just one constructor
                     // Optimization: No need to store a tag ID (the payload is "unwrapped")
                     // e.g. `RoseTree a : [Tree a (List (RoseTree a))]`
                     RocTagUnion::NonNullableUnwrapped {
                         name: name.clone(),
-                        tag_name,
+                        tag_name: tag_name.to_string(),
                         payload: opt_payload.unwrap(),
                     }
                 }
@@ -1318,7 +1315,7 @@ fn add_tag_union<'a>(
             // e.g. `RoseTree a : [Tree a (List (RoseTree a))]`
             RocTagUnion::SingleTagStruct {
                 name: name.clone(),
-                tag_name,
+                tag_name: tag_name.to_string(),
                 payload_fields,
             }
         }
@@ -1334,7 +1331,7 @@ fn add_tag_union<'a>(
 
             RocTagUnion::SingleTagStruct {
                 name: name.clone(),
-                tag_name,
+                tag_name: tag_name.to_string(),
                 payload_fields: vec![type_id],
             }
         }
@@ -1344,7 +1341,7 @@ fn add_tag_union<'a>(
 
             RocTagUnion::SingleTagStruct {
                 name: name.clone(),
-                tag_name,
+                tag_name: tag_name.to_string(),
                 payload_fields,
             }
         }
@@ -1402,31 +1399,44 @@ fn union_tags_to_types<'a>(
             (name_str, payload_vars.to_vec())
         })
         .collect();
+
     // Sort tags alphabetically by tag name
     tags.sort_by(|(name1, _), (name2, _)| name1.cmp(name2));
 
-    tags_to_types(name, tags, env, types, layout, is_recursive)
+    tags.into_iter()
+        .map(|(tag_name, payload_vars)| {
+            tag_to_type(
+                name,
+                env,
+                tag_name,
+                &payload_vars,
+                types,
+                layout,
+                is_recursive,
+            )
+        })
+        .collect()
 }
 
 fn single_tag_payload<'a>(
     union_tags: &'a UnionLabels<TagName>,
     subs: &'a Subs,
-) -> (String, &'a [Variable]) {
+) -> (&'a str, &'a [Variable]) {
     let mut iter = union_tags.iter_from_subs(subs);
     let (tag_name, payload_vars) = iter.next().unwrap();
     // This should be a single-tag union.
     debug_assert_eq!(iter.next(), None);
 
-    (tag_name.0.as_str().to_string(), payload_vars)
+    (tag_name.0.as_str(), payload_vars)
 }
 
-fn single_tag_payload_fields<'a>(
-    union_tags: &UnionLabels<TagName>,
-    subs: &Subs,
+fn single_tag_payload_fields<'a, 'b>(
+    union_tags: &'b UnionLabels<TagName>,
+    subs: &'b Subs,
     field_layouts: &[Layout<'a>],
     env: &mut Env<'a>,
     types: &mut Types,
-) -> (String, Vec<TypeId>) {
+) -> (&'b str, Vec<TypeId>) {
     let (tag_name, payload_vars) = single_tag_payload(union_tags, subs);
 
     let payload_fields: Vec<TypeId> = payload_vars
@@ -1438,16 +1448,15 @@ fn single_tag_payload_fields<'a>(
     (tag_name, payload_fields)
 }
 
-fn tags_to_types<'a>(
+fn tag_to_type<'a, D: Display>(
     name: &str,
-    tags: Vec<(String, Vec<Variable>)>,
     env: &mut Env<'a>,
+    tag_name: D,
+    payload_vars: &[Variable],
     types: &mut Types,
     layout: Layout<'a>,
     is_recursive: bool,
-) -> Vec<(String, Option<TypeId>)> {
-    tags.into_iter()
-        .map(|(tag_name, payload_vars)| {
+) -> (D, Option<TypeId>) {
     match struct_fields_needed(env, payload_vars.iter().copied()) {
         0 => {
             // no payload
@@ -1470,16 +1479,13 @@ fn tags_to_types<'a>(
             // create a RocType for the payload and save it
             let struct_name = format!("{}_{}", &name, tag_name); // e.g. "MyUnion_MyVariant"
             let fields = payload_vars.iter().copied().enumerate();
-                    let struct_id =
-                        add_struct(env, struct_name, fields, types, layout, |name, fields| {
+            let struct_id = add_struct(env, struct_name, fields, types, layout, |name, fields| {
                 RocType::TagUnionPayload { name, fields }
             });
 
             (tag_name, Some(struct_id))
         }
     }
-        })
-        .collect()
 }
 
 fn struct_fields_needed<I: IntoIterator<Item = Variable>>(env: &mut Env<'_>, vars: I) -> usize {