Defer start of feature-dependent transformations to a later point.

We previously would evaluate whether a unit field or method was required
by an optional feature before we had finished collecting all feature
requirements. This behavior was fine when we were visiting individual
modules' AST on by one, but breaks with #1462 were we changed using a
single AST to hold all modules.

This patch defers transformations until all feature requirements have
been collected.

hilti/toolchain/src/compiler/optimizer.cc

@@ -2,7 +2,6 @@
 
 #include "hilti/compiler/detail/optimizer.h"
 
-#include <algorithm>
 #include <optional>
 #include <string>
 #include <tuple>
@@ -200,7 +199,7 @@ struct FunctionVisitor : OptimizerVisitor {
 
                 if ( auto type = type_ ) {
                     for ( const auto& requirement : n->attributes()->findAll("&needed-by-feature") ) {
-                        auto feature = *requirement->valueAsString();
+                        const auto& feature = *requirement->valueAsString();
 
                         // If no feature constants were collected yet, reschedule us for the next collection pass.
                         //
@@ -212,9 +211,11 @@ struct FunctionVisitor : OptimizerVisitor {
                         }
 
                         auto it = _features.find(type->type()->type()->typeID());
-                        if ( it == _features.end() )
-                            // No feature requirements known for type.
+                        if ( it == _features.end() ) {
+                            // No feature requirements known for type yet.
+                            _collect_again = true;
                             continue;
+                        }
 
                         function.referenced = function.referenced || it->second.at(feature);
                     }
@@ -289,9 +290,11 @@ struct FunctionVisitor : OptimizerVisitor {
                         }
 
                         auto it = _features.find(decl->fullyQualifiedID());
-                        if ( it == _features.end() )
-                            // No feature requirements known for type.
+                        if ( it == _features.end() ) {
+                            // No feature requirements known for type yet.
+                            _collect_again = true;
                             continue;
+                        }
 
                         // Mark the function as referenced if it is needed by an active feature.
                         function.referenced = function.referenced || it->second.at(feature);

tests/spicy/optimization/file-order.spicy

@@ -0,0 +1,20 @@
+# @TEST-DOC: Checks that the order in which modules are compiled is irrelevant for optimizations, regression test for #1789.
+#
+# @TEST-EXEC: spicyc -dj y.spicy x.spicy
+# @TEST-EXEC: spicyc -dj x.spicy y.spicy
+
+# @TEST-START-FILE x.spicy
+module x;
+
+public type X = unit {
+    on %synced {
+        confirm;
+    }
+};
+# @TEST-END-FILE
+
+# @TEST-START-FILE y.spicy
+module y;
+
+type Y = unit {};
+# @TEST-END-FILE