Support visibility control and default to `internal`. (#3380)

* Support visibility control and default to `internal`.

* Fix wip.

* Fixes.

* Fix.

* Fix test.

* Add legacy language detection and compatibility for existing code.

* Add doc.

---------

Co-authored-by: Yong He <yhe@nvidia.com>

3 files changed, 68 insertions, 0 deletions

diff --git a/tests/diagnostics/internal-visibility/that-module-impl.slang b/tests/diagnostics/internal-visibility/that-module-impl.slang
new file mode 100644
index 000000000..893510afe
--- /dev/null
+++ b/tests/diagnostics/internal-visibility/that-module-impl.slang
@@ -0,0 +1,17 @@
+implementing "that-module";
+
+
+public void publicFunc2() { }
+
+internal void internalFunc1() {}
+
+public struct PublicStruct
+{
+    int x;
+    public int y;
+}
+
+public namespace Namespace
+{
+    public static int publicVar2;
+}
diff --git a/tests/diagnostics/internal-visibility/that-module.slang b/tests/diagnostics/internal-visibility/that-module.slang
new file mode 100644
index 000000000..b072c7231
--- /dev/null
+++ b/tests/diagnostics/internal-visibility/that-module.slang
@@ -0,0 +1,25 @@
+module "that-module";
+
+__include "that-module-impl";
+
+void internalMethod();
+
+public void publicMethod();
+
+public namespace Namespace
+{
+    static int internalVar;
+
+    public static int publicVar;
+}
+
+enum InternalEnum
+{
+    A,B,C
+}
+public enum PublicEnum
+{
+    D,E,F
+}
+
+struct InternalStruct { int x; }
diff --git a/tests/diagnostics/internal-visibility/this-module.slang b/tests/diagnostics/internal-visibility/this-module.slang
new file mode 100644
index 000000000..98e0f3d08
--- /dev/null
+++ b/tests/diagnostics/internal-visibility/this-module.slang
@@ -0,0 +1,26 @@
+//DIAGNOSTIC_TEST:SIMPLE(filecheck=CHECK):
+
+module "this-module";
+
+import "that-module";
+
+void test()
+{
+    PublicStruct s;
+    // CHECK:{{.*}}(11): error 30600:
+    s.x = 1; // Error.
+    // CHECK-NOT:{{.*}}error
+    s.y = 2; // OK.
+    publicMethod(); // OK.
+    publicFunc2();  // OK.
+    Namespace.publicVar = 1; // OK.
+    Namespace.publicVar2 = 1; // OK.
+    // CHECK:{{.*}}(19): error 30600:
+    Namespace.internalVar = 1; // error.
+    // CHECK:{{.*}}(21): error 30600:
+    InternalEnum e; // Error.
+    // CHECK:{{.*}}(23): error 30600:
+    InternalStruct s1; // Error.
+    // CHECK:{{.*}}(25): error 30600:
+    internalMethod(); // Error.
+}