Sort fields in one pass, and drop 0-sized fields

compiler/mono/src/expr.rs

@@ -640,16 +640,16 @@ fn from_can<'a>(
         } => {
             let arena = env.arena;
 
-            let btree = crate::layout::record_fields_btree(
+            let sorted_fields = crate::layout::sort_record_fields(
                 env.arena,
                 record_var,
                 env.subs,
                 env.pointer_size,
             );
 
-            let mut field_tuples = Vec::with_capacity_in(btree.len(), arena);
+            let mut field_tuples = Vec::with_capacity_in(sorted_fields.len(), arena);
 
-            for (label, layout) in btree {
+            for (label, layout) in sorted_fields {
                 let field = fields.remove(&label).unwrap();
                 let expr = from_can(env, field.loc_expr.value, procs, layout_cache);
 
@@ -749,7 +749,7 @@ fn from_can<'a>(
         } => {
             let arena = env.arena;
 
-            let btree = crate::layout::record_fields_btree(
+            let sorted_fields = crate::layout::sort_record_fields(
                 env.arena,
                 record_var,
                 env.subs,
@@ -757,9 +757,9 @@ fn from_can<'a>(
             );
 
             let mut index = None;
-            let mut field_layouts = Vec::with_capacity_in(btree.len(), env.arena);
+            let mut field_layouts = Vec::with_capacity_in(sorted_fields.len(), env.arena);
 
-            for (current, (label, field_layout)) in btree.into_iter().enumerate() {
+            for (current, (label, field_layout)) in sorted_fields.into_iter().enumerate() {
                 field_layouts.push(field_layout);
 
                 if label == field {
@@ -1675,16 +1675,16 @@ fn from_can_pattern<'a>(
             let mut it = destructs.iter();
             let mut opt_destruct = it.next();
 
-            let btree = crate::layout::record_fields_btree(
+            let sorted_fields = crate::layout::sort_record_fields(
                 env.arena,
                 *whole_var,
                 env.subs,
                 env.pointer_size,
             );
 
-            let mut field_layouts = Vec::with_capacity_in(btree.len(), env.arena);
+            let mut field_layouts = Vec::with_capacity_in(sorted_fields.len(), env.arena);
 
-            for (label, field_layout) in btree.into_iter() {
+            for (label, field_layout) in sorted_fields.into_iter() {
                 if let Some(destruct) = opt_destruct {
                     if destruct.value.label == label {
                         opt_destruct = it.next();

compiler/mono/src/layout.rs

@@ -4,7 +4,6 @@ use roc_collections::all::MutMap;
 use roc_module::ident::{Lowercase, TagName};
 use roc_module::symbol::Symbol;
 use roc_types::subs::{Content, FlatType, Subs, Variable};
-use std::collections::BTreeMap;
 
 pub const MAX_ENUM_SIZE: usize = (std::mem::size_of::<u8>() * 8) as usize;
 
@@ -304,13 +303,19 @@ fn layout_from_flat_type<'a>(
         Record(fields, ext_var) => {
             debug_assert!(ext_var_is_empty_record(subs, ext_var));
 
-            let btree = fields
-                .into_iter()
-                .collect::<BTreeMap<Lowercase, Variable>>();
+            // Sort the fields by label
+            let mut sorted_fields = Vec::with_capacity_in(fields.len(), arena);
 
-            let mut layouts = Vec::with_capacity_in(btree.len(), arena);
+            for tuple in fields {
+                sorted_fields.push(tuple);
+            }
 
-            for (_, field_var) in btree {
+            sorted_fields.sort_by(|(label1, _), (label2, _)| label1.cmp(label2));
+
+            // Determine the layouts of the fields, maintaining sort order
+            let mut layouts = Vec::with_capacity_in(sorted_fields.len(), arena);
+
+            for (_, field_var) in sorted_fields {
                 let field_content = subs.get_without_compacting(field_var).content;
 
                 match Layout::new(arena, field_content, subs, pointer_size) {
@@ -327,8 +332,14 @@ fn layout_from_flat_type<'a>(
                 }
             }
 
+            if layouts.len() == 1 {
+                // If the record has only one field that isn't zero-sized,
+                // unwrap it.
+                Ok(layouts.pop().unwrap())
+            } else {
                 Ok(Layout::Struct(layouts.into_bump_slice()))
             }
+        }
         TagUnion(tags, ext_var) => {
             debug_assert!(ext_var_is_empty_tag_union(subs, ext_var));
 
@@ -348,26 +359,32 @@ fn layout_from_flat_type<'a>(
     }
 }
 
-pub fn record_fields_btree<'a>(
+pub fn sort_record_fields<'a>(
     arena: &'a Bump,
     var: Variable,
     subs: &Subs,
     pointer_size: u32,
-) -> BTreeMap<Lowercase, Layout<'a>> {
+) -> Vec<'a, (Lowercase, Layout<'a>)> {
     let mut fields_map = MutMap::default();
+
     match roc_types::pretty_print::chase_ext_record(subs, var, &mut fields_map) {
         Ok(()) | Err((_, Content::FlexVar(_))) => {
-            // collect into btreemap to sort
-            fields_map
-                .into_iter()
-                .map(|(label, var)| {
-                    (
-                        label,
-                        Layout::from_var(arena, var, subs, pointer_size)
-                            .expect("invalid layout from var"),
-                    )
-                })
-                .collect::<BTreeMap<Lowercase, Layout<'a>>>()
+            // Sort the fields by label
+            let mut sorted_fields = Vec::with_capacity_in(fields_map.len(), arena);
+
+            for (label, var) in fields_map {
+                let layout = Layout::from_var(arena, var, subs, pointer_size)
+                    .expect("invalid layout from var");
+
+                // Drop any zero-sized fields like {}
+                if layout.stack_size(pointer_size) != 0 {
+                    sorted_fields.push((label, layout));
+                }
+            }
+
+            sorted_fields.sort_by(|(label1, _), (label2, _)| label1.cmp(label2));
+
+            sorted_fields
         }
         Err(other) => panic!("invalid content in record variable: {:?}", other),
     }