Add basic support for namespaces (#1304)

This change adds logic for parsing `namespace` declarations, referencing them, and looking up their members.

* The parser changes are a bit subtle, because that is where we deal with the issue of "re-opening" a namespace. We kludge things a bit by re-using an existing `NamespaceDecl` in the same parent if one is available, and thereby ensure that all the members in the same namespace can see on another.

* In order to allow namespaces to be referenced by name they need to have a type so that a `DeclRefExpr` to them can be formed. For this purpose we introduce `NamespaceType` which is the (singleton) type of a reference to a given namespace.

* The new `NamespaceType` case is detected in the `MemberExpr` checking logic and routed to the same logic that `StaticMemberExpr` uses, and the static lookup logic was extended with support for looking up in a namespace (a thin wrapper around one of the existing worker routines in `slang-lookup.cpp`.

* I made `NamespaceDecl` have a shared base class with `ModuleDecl` in the hopes that this would allow us to allow references to modules by name in the future. That hasn't been tested as part of this change.

* I cleaned up a bunch of logic around `ModuleDecl` holding a `Scope` pointer that was being used for some of the more ad hoc lookup routines in the public API. Those have been switched over to something that is a bit more sensible given the language rules and that doesn't rely on keeping state sititng around on the `ModuleDecl`.

* I added a test case to make sure the new funcitonality works, which includes re-opening a namespace, and it also tests both `.` and `::` operations for lookup in a namespace.

* The main missing feature here is the ability to do something like C++ `using`. It would probably be cleanest if we used `import` for this, since we already have that syntax (and having both `import` and `using` seems like a recipe for confusion). Most of the infrastructure is present to support `import`ing one namespace into another (in a way that wouldn't automatically pollute the namespace for clients), but some careful thought needs to be put into how import of namespaces vs. modules should work.

1 files changed, 29 insertions, 0 deletions

diff --git a/source/slang/slang-lookup.cpp b/source/slang/slang-lookup.cpp
index 02f74579c..6a2d31c32 100644
--- a/source/slang/slang-lookup.cpp
+++ b/source/slang/slang-lookup.cpp
@@ -234,6 +234,29 @@ static void _lookUpDirectAndTransparentMembers(
     }
 }
 
+    /// Perform "direct" lookup in a container declaration
+LookupResult lookUpDirectAndTransparentMembers(
+    Session*                session,
+    SemanticsVisitor*       semantics,
+    Name*                   name,
+    DeclRef<ContainerDecl>  containerDeclRef,
+    LookupMask              mask)
+{
+    LookupRequest request;
+    request.semantics = semantics;
+    request.mask = mask;
+    LookupResult result;
+    _lookUpDirectAndTransparentMembers(
+        session,
+        name,
+        containerDeclRef,
+        request,
+        result,
+        nullptr);
+    return result;
+}
+
+
 static RefPtr<SubtypeWitness> _makeSubtypeWitness(
     Type*                       subType,
     SubtypeWitness*             subToMidWitness,
@@ -607,6 +630,12 @@ static void _lookUpInScopes(
         {
             auto containerDecl = link->containerDecl;
 
+            // It is possible for the first scope in a list of
+            // siblings to be a "dummy" scope that only exists
+            // to combine the siblings; in that case it will
+            // have a null `containerDecl` and needs to be
+            // skipped over.
+            //
             if(!containerDecl)
                 continue;