Initial support for a using construct (#1506)

The basic idea is that if you have a namespace:

    namespace MyCoolNamespace { void f() { ... } ... }

then you can bring the declarations from that namespace into scope with:

    using MyCoolNamespace;
    f();

The `using` construct is allowed in any scope where declarations are allowed. As an additional feature, the construct allows and then ignores the keyword `namespace` if it occurs right after `using`:

    using namespace MyCoolNamespace;

Note that unlike in C++, `using` a namespace inside another namespace doesn't implicitly make the symbols available to clients of that namespace:

    namespace hidden { void secret() {...} ... }
    namespace api { using hidden; ... }

    api.secret(); // ERROR: `secret()` isn't a member of `api`

The implementation of this feature was relatively simple, although it does leave out more advanced features that might be desirable in the future:

* No support for `using MCN = MyCoolNamespace` sorts of tricks to define a short name

* No support for `using` anything that isn't a namespace (e.g., to make the members of a type available without qualification)

* No support for cases where multiple visible modules have a namespace of the same name (or dealing with overloaded namespaces in general)

7 files changed, 163 insertions, 6 deletions

diff --git a/source/slang/slang-ast-decl.h b/source/slang/slang-ast-decl.h
index 833acd681..e88260414 100644
--- a/source/slang/slang-ast-decl.h
+++ b/source/slang/slang-ast-decl.h
@@ -373,6 +373,18 @@ class ModuleDecl : public NamespaceDeclBase
     Dictionary<AggTypeDecl*, RefPtr<CandidateExtensionList>> mapTypeToCandidateExtensions;
 };
 
+    /// A declaration that brings members of another declaration or namespace into scope
+class UsingDecl : public Decl
+{
+    SLANG_CLASS(UsingDecl)
+
+        /// An expression that identifies the entity (e.g., a namespace) to be brought into `scope`
+    Expr* arg;
+
+        /// The scope that the entity named by `arg` will be brought into
+    RefPtr<Scope> scope;
+};
+
 class ImportDecl : public Decl
 {
     SLANG_CLASS(ImportDecl)
diff --git a/source/slang/slang-check-decl.cpp b/source/slang/slang-check-decl.cpp
index 10e3818c4..4ea8aa7f6 100644
--- a/source/slang/slang-check-decl.cpp
+++ b/source/slang/slang-check-decl.cpp
@@ -56,6 +56,8 @@ namespace Slang
 
         void visitImportDecl(ImportDecl* decl);
 
+        void visitUsingDecl(UsingDecl* decl);
+
         void visitGenericTypeParamDecl(GenericTypeParamDecl* decl);
 
         void visitGenericValueParamDecl(GenericValueParamDecl* decl);
@@ -4615,6 +4617,58 @@ namespace Slang
         }
     }
 
+    void SemanticsDeclHeaderVisitor::visitUsingDecl(UsingDecl* decl)
+    {
+        // First, we need to look up whatever the argument of the `using`
+        // declaration names.
+        //
+        decl->arg = CheckTerm(decl->arg);
+
+        // Next, we want to ensure that whatever is being named by `decl->arg`
+        // is a namespace (or a module, since modules are namespace-like).
+        //
+        // TODO: The logic here assumes that we can't have multiple `NamespaceDecl`s
+        // with the same name in scope, but that assumption is only valid in the
+        // context of a single module (where we deduplicate `namespace`s during
+        // parsing). If a user `import`s multiple modules that all have namespaces
+        // of the same name, it would be possible for `decl->arg` to be overloaded.
+        // In that case we should really iterate over all the entities that are
+        // named and import any that are namespace-like.
+        //
+        NamespaceDeclBase* namespaceDecl = nullptr;
+        if( auto declRefExpr = as<DeclRefExpr>(decl->arg) )
+        {
+            if( auto namespaceDeclRef = declRefExpr->declRef.as<NamespaceDeclBase>() )
+            {
+                SLANG_ASSERT(!namespaceDeclRef.substitutions.substitutions);
+                namespaceDecl = namespaceDeclRef.getDecl();
+            }
+        }
+        if( !namespaceDecl )
+        {
+            getSink()->diagnose(decl->arg, Diagnostics::expectedANamespace, decl->arg->type);
+            return;
+        }
+
+        // Once we have identified the namespace to bring into scope,
+        // we need to create a new sibling sub-scope to add to the
+        // lookup scope that was in place when the `using` was parsed.
+        //
+        // Subsequent lookup in that scope will walk through our new
+        // sub-scope and see the namespace.
+        //
+        // TODO: If we update the `containerDecl` in a scope to allow
+        // for a more general `DeclRef`, or even a full `DeclRefExpr`,
+        // then it would be possible for `using` to apply to more kinds
+        // of entities than just namespaces.
+        //
+        auto scope = decl->scope;
+        auto subScope = new Scope();
+        subScope->containerDecl = namespaceDecl;
+        subScope->nextSibling = scope->nextSibling;
+        scope->nextSibling = subScope;
+    }
+
         /// Get a reference to the candidate extension list for `typeDecl` in the given dictionary
         ///
         /// Note: this function creates an empty list of candidates for the given type if
diff --git a/source/slang/slang-diagnostic-defs.h b/source/slang/slang-diagnostic-defs.h
index 3e0d488f9..c268fd241 100644
--- a/source/slang/slang-diagnostic-defs.h
+++ b/source/slang/slang-diagnostic-defs.h
@@ -255,6 +255,7 @@ DIAGNOSTIC(30052, Error, invalidSwizzleExpr, "invalid swizzle pattern '$0' on ty
 DIAGNOSTIC(30043, Error, getStringHashRequiresStringLiteral, "getStringHash parameter can only accept a string literal")
 
 DIAGNOSTIC(30060, Error, expectedAType, "expected a type, got a '$0'")
+DIAGNOSTIC(30061, Error, expectedANamespace, "expected a namespace, got a '$0'")
 
 DIAGNOSTIC(30100, Error, staticRefToNonStaticMember, "type '$0' cannot be used to refer to non-static member '$1'")
 
diff --git a/source/slang/slang-lower-to-ir.cpp b/source/slang/slang-lower-to-ir.cpp
index 864491f7e..f87b6bd69 100644
--- a/source/slang/slang-lower-to-ir.cpp
+++ b/source/slang/slang-lower-to-ir.cpp
@@ -5032,6 +5032,7 @@ struct DeclLoweringVisitor : DeclVisitor<DeclLoweringVisitor, LoweredValInfo>
     LoweredValInfo visit##NAME(NAME*) { return LoweredValInfo(); }
 
     IGNORED_CASE(ImportDecl)
+    IGNORED_CASE(UsingDecl)
     IGNORED_CASE(EmptyDecl)
     IGNORED_CASE(SyntaxDecl)
     IGNORED_CASE(AttributeDecl)
diff --git a/source/slang/slang-parser.cpp b/source/slang/slang-parser.cpp
index f2bc2eb9a..e02ccf245 100644
--- a/source/slang/slang-parser.cpp
+++ b/source/slang/slang-parser.cpp
@@ -933,12 +933,16 @@ namespace Slang
         return parser->getNamePool()->getName(text);
     }
 
+    static bool expect(Parser* parser, TokenType tokenType)
+    {
+        return parser->ReadToken(tokenType).type == tokenType;
+    }
+
     static NameLoc expectIdentifier(Parser* parser)
     {
         return NameLoc(parser->ReadToken(TokenType::Identifier));
     }
 
-
     static NodeBase* parseImportDecl(
         Parser* parser, void* /*userData*/)
     {
@@ -2629,6 +2633,44 @@ namespace Slang
         return result;
     }
 
+    static NodeBase* parseUsingDecl(Parser* parser, void* /*userData*/)
+    {
+        UsingDecl* decl = parser->astBuilder->create<UsingDecl>();
+        parser->FillPosition(decl);
+
+        // A `using` declaration will need to know about the current
+        // scope at the point where it appears, so that it can know
+        // the scope it is attempting to extend.
+        //
+        decl->scope = parser->currentScope;
+
+        // TODO: We may eventually want to support declarations
+        // of the form `using <id> = <expr>;` which introduce
+        // a shorthand alias for a namespace/type/whatever.
+        //
+        // For now we are just sticking to the most basic form.
+
+        // As a compatibility feature for programmers used to C++,
+        // we allow the `namespace` keyword to come after `using`,
+        // where it has no effect.
+        //
+        if(parser->LookAheadToken("namespace"))
+        {
+            advanceToken(parser);
+        }
+
+        // The entity that is going to be used is identified
+        // using an arbitrary expression (although we expect
+        // that valid code will not typically use the full
+        // freedom of what the expression grammar supports.
+        //
+        decl->arg = parser->ParseExpression();
+
+        expect(parser, TokenType::Semicolon);
+
+        return decl;
+    }
+
     static NodeBase* parseConstructorDecl(Parser* parser, void* /*userData*/)
     {
         ConstructorDecl* decl = parser->astBuilder->create<ConstructorDecl>();
@@ -2750,11 +2792,6 @@ namespace Slang
         return decl;
     }
 
-    static bool expect(Parser* parser, TokenType tokenType)
-    {
-        return parser->ReadToken(tokenType).type == tokenType;
-    }
-
         /// Peek in the token stream and return `true` if it looks like a modern-style variable declaration is coming up.
     static bool _peekModernStyleVarDecl(Parser* parser)
     {
@@ -5659,6 +5696,7 @@ namespace Slang
         DECL(typealias,       parseTypeAliasDecl);
         DECL(__generic_value_param, parseGlobalGenericValueParamDecl);
         DECL(namespace,       parseNamespaceDecl);
+        DECL(using,           parseUsingDecl);
 
     #undef DECL
 
diff --git a/tests/language-feature/namespaces/using-namespace.slang b/tests/language-feature/namespaces/using-namespace.slang
new file mode 100644
index 000000000..b0b301929
--- /dev/null
+++ b/tests/language-feature/namespaces/using-namespace.slang
@@ -0,0 +1,47 @@
+// using-namespace.slang
+
+// Test that `using` can bring declarations from a namespace into scope
+
+//TEST(compute):COMPARE_COMPUTE:
+
+namespace X
+{
+	struct A { int a; }
+}
+
+namespace Y
+{
+	// A `using` that brings one namespace into scope of declarations in another
+	//
+    using X;
+
+	int b(A a) { return a.a; }
+}
+
+// A `using` that brings declarations in a namespace into the global scope
+//
+using Y;
+
+int test(int value)
+{
+	// A `using` that brings declarations into a local scope
+	// (and also uses the optional `namespace` placeholder keyword)
+	//
+	using namespace X;
+
+	A a = { value };
+	return b(a);
+}
+
+//TEST_INPUT:ubuffer(data=[0 0 0 0], stride=4):out,name=outputBuffer
+RWStructuredBuffer<int> outputBuffer;
+
+[numthreads(4, 1, 1)]
+void computeMain(uint3 dispatchThreadID : SV_DispatchThreadID)
+{
+    uint tid = dispatchThreadID.x;
+    int inVal = tid;
+    int outVal = test(inVal);
+    outputBuffer[tid] = outVal;
+}
+
diff --git a/tests/language-feature/namespaces/using-namespace.slang.expected.txt b/tests/language-feature/namespaces/using-namespace.slang.expected.txt
new file mode 100644
index 000000000..bc856dafa
--- /dev/null
+++ b/tests/language-feature/namespaces/using-namespace.slang.expected.txt
@@ -0,0 +1,4 @@
+0
+1
+2
+3