SIL: Introduce TypeConverter::getForwardingSubstitutionsForLowering()

Author: slavapestov

include/swift/SIL/TypeLowering.h

@@ -1076,6 +1076,13 @@ class TypeConverter {
   /// Get the generic environment for a constant.
   GenericEnvironment *getConstantGenericEnvironment(SILDeclRef constant);
 
+  /// Get the generic environment for SILGen to use. The substitution map
+  /// sends the generic parameters of the function's interface type into
+  /// archetypes, which will either be primary archetypes from this
+  /// environment, or local archetypes captured by this function.
+  std::pair<GenericEnvironment *, SubstitutionMap>
+  getForwardingSubstitutionsForLowering(SILDeclRef constant);
+
   /// Returns the SIL type of a constant reference.
   SILType getConstantType(TypeExpansionContext context, SILDeclRef constant) {
     return getConstantInfo(context, constant).getSILType();

lib/SIL/IR/TypeLowering.cpp

@@ -4086,6 +4086,28 @@ TypeConverter::getConstantGenericEnvironment(SILDeclRef c) {
       .genericSig.getGenericEnvironment();
 }
 
+std::pair<GenericEnvironment *, SubstitutionMap>
+TypeConverter::getForwardingSubstitutionsForLowering(SILDeclRef constant) {
+  auto sig = getGenericSignatureWithCapturedEnvironments(constant);
+
+  GenericEnvironment *genericEnv = nullptr;
+  SubstitutionMap forwardingSubs;
+
+  if (sig.baseGenericSig) {
+    genericEnv = sig.baseGenericSig.getGenericEnvironment();
+    forwardingSubs = genericEnv->getForwardingSubstitutionMap();
+  }
+
+  if (!sig.capturedEnvs.empty()) {
+    assert(sig.genericSig && !sig.genericSig->isEqual(sig.baseGenericSig));
+
+    forwardingSubs = buildSubstitutionMapWithCapturedEnvironments(
+        forwardingSubs, sig.genericSig, sig.capturedEnvs);
+  }
+
+  return std::make_pair(genericEnv, forwardingSubs);
+}
+
 SILType TypeConverter::getSubstitutedStorageType(TypeExpansionContext context,
                                                  AbstractStorageDecl *value,
                                                  Type lvalueType) {