Add support for the HLSL "cast from zero" idiom (#1008)

If the user writes code like this:

    MyStruct s = (MyStruct) 0;

then we will interpret it as if they had written:

    MyStruct s = {};

That is, the "cast from zero" idiom will be taken as a legacy syntax for default construction (using an empty initializer list). This will be semantically equivalent to zero-initialization for all existing HLSL code (where `struct` fields can't have default initialization expressions defined), and is the easiest option for us to support in Slang (since we already support default-initialization using empty initializer lists).

The implementation of this feature is narrowly scoped:

* It only targets explicit cast expressions like `(MyStruct) 0` and not "constructor" syntax like `MyStruct(0)`
* It only applies when there is a single argument that is exactly an integer literal with a zero value (not a reference to a `static const int` that happens to be zero).

This change adds a test case to make sure that the feature works as expected. Because it relies on our existing initializer-list handling, the "cast from zero" idiom should work for any user-defined type where an initializer list would work.

2 files changed, 40 insertions, 0 deletions

diff --git a/tests/compute/cast-zero-to-struct.slang b/tests/compute/cast-zero-to-struct.slang
new file mode 100644
index 000000000..c2ed8fb6d
--- /dev/null
+++ b/tests/compute/cast-zero-to-struct.slang
@@ -0,0 +1,36 @@
+// cast-zero-to-struct.slang
+
+// Test that HLSL legacy syntax for casting from literal zero
+// to a `struct` type works.
+
+//TEST(compute):COMPARE_COMPUTE:
+
+struct S
+{
+	int2 a;
+	int2 b;
+	int2 c;
+}
+
+int test(int val)
+{
+	S s = (S) 0;
+
+	s.a.x = val;
+	s.b.y = val;
+	s.c.x = val;
+
+	int2 t = s.a + s.b*256 + s.c*65536;
+	return t.x + t.y*16;
+}
+
+//TEST_INPUT: ubuffer(data=[0 0 0 0], stride=4):dxbinding(0),glbinding(0),out
+RWStructuredBuffer<int> gOutputBuffer;
+
+[numthreads(4, 1, 1)]
+void computeMain(uint3 tid : SV_DispatchThreadID)
+{
+    int inVal = tid.x;
+    int outVal = test(inVal);
+    gOutputBuffer[tid.x] = outVal;
+}
diff --git a/tests/compute/cast-zero-to-struct.slang.expected.txt b/tests/compute/cast-zero-to-struct.slang.expected.txt
new file mode 100644
index 000000000..7e1d6b7b7
--- /dev/null
+++ b/tests/compute/cast-zero-to-struct.slang.expected.txt
@@ -0,0 +1,4 @@
+0
+11001
+22002
+33003
\ No newline at end of file