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)

2 files changed, 51 insertions, 0 deletions

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