Support explicit `this` expressions

This is the first step towards supporting traditional object-oriented method definitions; the second step will be to allow `this` expressions to be implicit.

- Add a test case using explicit `this`, and expected output

- Update parsing logic for expressions so that it handled identifiers similarly to the declaration and statement logic: first try to parse using a syntax declaration looked up in the curent scope, and otherwise fall back to the ordinary `VarExpr` case.

  * As long as I'm making that change: switch `true` and `false` to be parsed via the callback mechanism rather than be special-cased.

  * This change will also help out if we ever wanted to add `super`/`base` expressions, `new`, `sizeof`/`alignof` or any other expression keywords.

- Add a `ThisExpr` node and register a parser callback for it.

- Add semantic checks for `ThisExpr`: basically just look upwards through scopes until we find either an aggregate type declaration or an `extension` declaration, and then use that as the type of the expression.

  - TODO: eventually we need to guard against a `this` expression inside of a `static` member.

- The IR generation logic already handled creation of `this` parameters in function signatures; the missing piece was to register the appropriate parameter in the context, so that we can use it as the lowering of a `this` expression.

2 files changed, 38 insertions, 0 deletions

diff --git a/tests/compute/explicit-this-expr.slang b/tests/compute/explicit-this-expr.slang
new file mode 100644
index 000000000..7bd8dff99
--- /dev/null
+++ b/tests/compute/explicit-this-expr.slang
@@ -0,0 +1,34 @@
+//TEST(smoke,compute):COMPARE_COMPUTE:-xslang -use-ir
+//TEST_INPUT:ubuffer(data=[0 0 0 0], stride=4):dxbinding(0),glbinding(0),out
+
+// Access fields of a `struct` type from within a "method" by
+// using an explicit `this` expression.
+
+struct A
+{
+	float x;
+
+	float addWith(float y)
+	{
+		return this.x + y;
+	}
+};
+
+RWStructuredBuffer<float> outputBuffer : register(u0);
+
+
+float test(float inVal)
+{
+	A a;
+	a.x = inVal;
+	return a.addWith(inVal*inVal);
+}
+
+[numthreads(4, 1, 1)]
+void computeMain(uint3 dispatchThreadID : SV_DispatchThreadID)
+{
+	uint tid = dispatchThreadID.x;
+	float inVal = float(tid);
+	float outVal = test(inVal);
+	outputBuffer[tid] = outVal;
+}
\ No newline at end of file
diff --git a/tests/compute/explicit-this-expr.slang.expected.txt b/tests/compute/explicit-this-expr.slang.expected.txt
new file mode 100644
index 000000000..f73cfe6c3
--- /dev/null
+++ b/tests/compute/explicit-this-expr.slang.expected.txt
@@ -0,0 +1,4 @@
+0
+40000000
+40C00000
+41400000