Avoid allocating OrderedSet in UnionBuilder::simplify (#13206)

Co-authored-by: Alex Waygood <[email protected]>

crates/red_knot_python_semantic/src/types/builder.rs

@@ -25,8 +25,10 @@
 //!   * No type in an intersection can be a supertype of any other type in the intersection (just
 //!     eliminate the supertype from the intersection).
 //!   * An intersection containing two non-overlapping types should simplify to [`Type::Never`].
+
 use crate::types::{IntersectionType, Type, UnionType};
 use crate::{Db, FxOrderSet};
+use ordermap::set::MutableValues;
 
 use super::builtins_symbol_ty_by_name;
 
@@ -63,23 +65,31 @@ impl<'db> UnionBuilder<'db> {
     ///     - TODO For enums `E` with members `X1`,...,`Xn`, replaces
     ///     `Literal[E.X1,...,E.Xn]` with `E`.
     fn simplify(&mut self) {
-        if self
-            .elements
-            .is_superset(&[Type::BooleanLiteral(true), Type::BooleanLiteral(false)].into())
-        {
-            let bool_ty = builtins_symbol_ty_by_name(self.db, "bool");
-            self.elements.remove(&Type::BooleanLiteral(true));
-            self.elements.remove(&Type::BooleanLiteral(false));
-            self.elements.insert(bool_ty);
+        if let Some(true_index) = self.elements.get_index_of(&Type::BooleanLiteral(true)) {
+            if self.elements.contains(&Type::BooleanLiteral(false)) {
+                *self.elements.get_index_mut2(true_index).unwrap() =
+                    builtins_symbol_ty_by_name(self.db, "bool");
+                self.elements.remove(&Type::BooleanLiteral(false));
+            }
         }
     }
 
     pub(crate) fn build(mut self) -> Type<'db> {
-        self.simplify();
         match self.elements.len() {
             0 => Type::Never,
             1 => self.elements[0],
-            _ => Type::Union(UnionType::new(self.db, self.elements)),
+            _ => {
+                self.simplify();
+
+                match self.elements.len() {
+                    0 => Type::Never,
+                    1 => self.elements[0],
+                    _ => {
+                        self.elements.shrink_to_fit();
+                        Type::Union(UnionType::new(self.db, self.elements))
+                    }
+                }
+            }
         }
     }
 }
@@ -360,7 +370,7 @@ mod tests {
             panic!("expected a union");
         };
 
-        assert_eq!(union.elements_vec(&db), &[t3, bool_ty]);
+        assert_eq!(union.elements_vec(&db), &[bool_ty, t3]);
     }
 
     #[test]