Refactor substitution representation in DeclRefBase (#363)

This commit changes the type of `DeclRefBase::substitutions` from `RefPtr<Substitutions>` to `SubstitutionSet`, which is a new type defined as following:
```
struct SubstitutionSet
{
    RefPtr<GenericSubstitution> genericSubstitutions;
    RefPtr<ThisTypeSubstitution> thisTypeSubstitution;
    RefPtr<GlobalGenericParamSubstitution> globalGenParamSubstitutions;
}
```
This change get rid of most helper functions to retreive the substitution of a certain type, as well as surgery operations to insert a `ThisTypeSubstitution` or `GlobalGenericTypeSubstittuion` at top or bottom of the substitution chain. It also simplies type comparison when certain type of substitution should not be considered as part of type definition.

1 files changed, 3 insertions, 3 deletions

diff --git a/source/slang/parameter-binding.cpp b/source/slang/parameter-binding.cpp
index fcd3fa525..e5ea1d531 100644
--- a/source/slang/parameter-binding.cpp
+++ b/source/slang/parameter-binding.cpp
@@ -1529,7 +1529,7 @@ static RefPtr<TypeLayout> processEntryPointParameter(
 static void collectEntryPointParameters(
     ParameterBindingContext*        context,
     EntryPointRequest*              entryPoint,
-    Substitutions*                  typeSubst)
+    SubstitutionSet                 typeSubst)
 {
     FuncDecl* entryPointFuncDecl = entryPoint->decl;
     if (!entryPointFuncDecl)
@@ -1722,7 +1722,7 @@ static void collectParameters(
         for( auto& entryPoint : translationUnit->entryPoints )
         {
             context->stage = entryPoint->profile.GetStage();
-            collectEntryPointParameters(context, entryPoint.Ptr(), nullptr);
+            collectEntryPointParameters(context, entryPoint.Ptr(), SubstitutionSet());
         }
         context->entryPointLayout = nullptr;
     }
@@ -2059,7 +2059,7 @@ StructTypeLayout* getGlobalStructLayout(
 RefPtr<ProgramLayout> specializeProgramLayout(
     TargetRequest * targetReq,
     ProgramLayout* programLayout, 
-    Substitutions * typeSubst)
+    SubstitutionSet typeSubst)
 {
     RefPtr<ProgramLayout> newProgramLayout;
     newProgramLayout = new ProgramLayout();