optimize alias to var conversion

compiler/solve/src/solve.rs

@@ -837,25 +837,13 @@ fn type_to_variable<'a>(
             actual,
             lambda_set_variables,
         } => {
-            // the rank of these variables is NONE (encoded as 0 in practice)
+            if let Some(reserved) = Variable::get_reserved(*symbol) {
-            // using them for other ranks causes issues
                 if rank.is_none() {
-                // TODO replace by arithmetic?
+                    // reserved variables are stored with rank NONE
-                match *symbol {
+                    return reserved;
-                    Symbol::NUM_I128 => return Variable::I128,
+                } else {
-                    Symbol::NUM_I64 => return Variable::I64,
+                    // for any other rank, we need to copy; it takes care of adjusting the rank
-                    Symbol::NUM_I32 => return Variable::I32,
+                    return deep_copy_var(subs, rank, pools, reserved);
-                    Symbol::NUM_I16 => return Variable::I16,
-                    Symbol::NUM_I8 => return Variable::I8,
-
-                    Symbol::NUM_U128 => return Variable::U128,
-                    Symbol::NUM_U64 => return Variable::U64,
-                    Symbol::NUM_U32 => return Variable::U32,
-                    Symbol::NUM_U16 => return Variable::U16,
-                    Symbol::NUM_U8 => return Variable::U8,
-
-                    Symbol::NUM_NAT => return Variable::NAT,
-                    _ => {}
                 }
             }
 
@@ -868,7 +856,11 @@ fn type_to_variable<'a>(
                 lambda_set_variables,
             );
 
-            let alias_variable = type_to_variable(subs, rank, pools, arena, actual);
+            let alias_variable = if let Symbol::RESULT_RESULT = *symbol {
+                roc_result_to_var(subs, rank, pools, arena, actual)
+            } else {
+                type_to_variable(subs, rank, pools, arena, actual)
+            };
             let content = Content::Alias(*symbol, alias_variables, alias_variable);
 
             register(subs, rank, pools, content)
@@ -941,6 +933,52 @@ fn alias_to_var<'a>(
     }
 }
 
+fn roc_result_to_var<'a>(
+    subs: &mut Subs,
+    rank: Rank,
+    pools: &mut Pools,
+    arena: &'a bumpalo::Bump,
+    result_type: &Type,
+) -> Variable {
+    match result_type {
+        Type::TagUnion(tags, ext) => {
+            debug_assert!(ext.is_empty_tag_union());
+            debug_assert!(tags.len() == 2);
+
+            if let [(err, err_args), (ok, ok_args)] = &tags[..] {
+                debug_assert_eq!(err, &subs.tag_names[0]);
+                debug_assert_eq!(ok, &subs.tag_names[1]);
+
+                if let ([err_type], [ok_type]) = (err_args.as_slice(), ok_args.as_slice()) {
+                    let err_var = type_to_variable(subs, rank, pools, arena, err_type);
+                    let ok_var = type_to_variable(subs, rank, pools, arena, ok_type);
+
+                    let start = subs.variables.len() as u32;
+                    let err_slice = SubsSlice::new(start, 1);
+                    let ok_slice = SubsSlice::new(start + 1, 1);
+
+                    subs.variables.push(err_var);
+                    subs.variables.push(ok_var);
+
+                    let variables = SubsSlice::new(subs.variable_slices.len() as _, 2);
+                    subs.variable_slices.push(err_slice);
+                    subs.variable_slices.push(ok_slice);
+
+                    let union_tags = UnionTags::from_slices(Subs::RESULT_TAG_NAMES, variables);
+                    let ext = Variable::EMPTY_TAG_UNION;
+
+                    let content = Content::Structure(FlatType::TagUnion(union_tags, ext));
+
+                    return register(subs, rank, pools, content);
+                }
+            }
+
+            unreachable!("invalid arguments to Result.Result; canonicalization should catch this!")
+        }
+        _ => unreachable!("not a valid type inside a Result.Result alias"),
+    }
+}
+
 fn insertion_sort_by<T, F>(arr: &mut [T], mut compare: F)
 where
     F: FnMut(&T, &T) -> std::cmp::Ordering,

compiler/types/src/subs.rs

@@ -708,6 +708,29 @@ impl Variable {
     pub const fn index(&self) -> u32 {
         self.0
     }
+
+    pub const fn get_reserved(symbol: Symbol) -> Option<Variable> {
+        // Must be carefule here: the variables must in fact be in Subs
+        match symbol {
+            Symbol::NUM_I128 => Some(Variable::I128),
+            Symbol::NUM_I64 => Some(Variable::I64),
+            Symbol::NUM_I32 => Some(Variable::I32),
+            Symbol::NUM_I16 => Some(Variable::I16),
+            Symbol::NUM_I8 => Some(Variable::I8),
+
+            Symbol::NUM_U128 => Some(Variable::U128),
+            Symbol::NUM_U64 => Some(Variable::U64),
+            Symbol::NUM_U32 => Some(Variable::U32),
+            Symbol::NUM_U16 => Some(Variable::U16),
+            Symbol::NUM_U8 => Some(Variable::U8),
+
+            Symbol::NUM_NAT => Some(Variable::NAT),
+
+            Symbol::BOOL_BOOL => Some(Variable::BOOL),
+
+            _ => None,
+        }
+    }
 }
 
 impl From<Variable> for OptVariable {
@@ -1012,6 +1035,8 @@ fn define_integer_types(subs: &mut Subs) {
 }
 
 impl Subs {
+    pub const RESULT_TAG_NAMES: SubsSlice<TagName> = SubsSlice::new(0, 2);
+
     pub fn new() -> Self {
         Self::with_capacity(0)
     }
@@ -1019,10 +1044,15 @@ impl Subs {
     pub fn with_capacity(capacity: usize) -> Self {
         let capacity = capacity.max(Variable::NUM_RESERVED_VARS);
 
+        let mut tag_names = Vec::with_capacity(32);
+
+        tag_names.push(TagName::Global("Err".into()));
+        tag_names.push(TagName::Global("Ok".into()));
+
         let mut subs = Subs {
             utable: UnificationTable::default(),
             variables: Default::default(),
-            tag_names: Default::default(),
+            tag_names,
             field_names: Default::default(),
             record_fields: Default::default(),
             // store an empty slice at the first position