Initial support for explicit default initializers (#1235)

This change makes it so that for a suitable type `MyType`, a variable declaration like:

    MyType v;

is treated as if it were written:

    MyType v = MyType();

The definition of "suitable" here is that `MyType` needs to have an available `__init` declaration that can be invoked with zero arguments. I've added a test to confirm that the new behavior works in this specific case.

There are a bunch of caveats to the feature as it stands today:

* Just because `MyType` has a zero-parameter `__init`, that doesn't mean an array type like `MyType[10]` does, so arrays currently remain uninitialized by default. Fixing this gap requires careful consideration because some, but not all, array types should be default-initializable.

* The change here should mean that a `struct` type with a field like `MyType f;` should count as having a default initial-value expression for that field, but I haven't confirmed that.

* Even if a `struct` provides initial values for all its fields (e.g., `struct S { float f = 0; }`), that doesn't mean it has a default `__init` right now, so those `struct` types will still be left uninitialized by default. Converging all this behavior is still TBD.

Just to be clear: there is no provision or plan in Slang to support destructors, RAII, copy constructors, move constructors, overloaded assignment operations, or any other features that buy heavily into the C++ model of how construction and destruction of values gets done.

In fact, I'm not even 100% sure I like having this change in place at all, and I think we should reserve the right to revert it and say that only specific stdlib types get to opt in to default initialization along these lines.

2 files changed, 39 insertions, 0 deletions

diff --git a/tests/compute/default-initializer.slang b/tests/compute/default-initializer.slang
new file mode 100644
index 000000000..1cda60084
--- /dev/null
+++ b/tests/compute/default-initializer.slang
@@ -0,0 +1,35 @@
+// default-initializer.slang
+
+// Confirm that a type with a default initializer gets
+// default-initialized where appropriate.
+
+//TEST(compute):COMPARE_COMPUTE:
+//TEST(compute):COMPARE_COMPUTE:-cpu
+
+struct Stuff
+{
+	int value;
+
+	__init()
+	{
+		value = 16;
+	}
+}
+
+int test(int value)
+{
+	Stuff s;
+	return value * s.value + value;
+}
+
+//TEST_INPUT:ubuffer(data=[0 1 2 3], stride=4):out,name=outputBuffer
+RWStructuredBuffer<int> outputBuffer : register(u0);
+
+[numthreads(4, 1, 1)]
+void computeMain(uint3 dispatchThreadID : SV_DispatchThreadID)
+{
+	uint tid = dispatchThreadID.x;
+	int inVal = outputBuffer[tid];
+	int outVal = test(inVal);
+	outputBuffer[tid] = outVal;
+}
\ No newline at end of file
diff --git a/tests/compute/default-initializer.slang.expected.txt b/tests/compute/default-initializer.slang.expected.txt
new file mode 100644
index 000000000..d4cb1cc00
--- /dev/null
+++ b/tests/compute/default-initializer.slang.expected.txt
@@ -0,0 +1,4 @@
+0
+11
+22
+33