Support scalarization of varying input/output for GLSL

GLSL technically supports varying (`in`, `out`) parameters of `struct` type, but there are some annoying constraints (not allowed for VS input), and it doesn't work with how an HLSL user would usually put "system-value" inputs/outputs into a `struct` together with ordinary inputs/outputs.

To work around this, this change adds support for using an imported Slang `struct` type for an `in` or `out` parameter, in which case it will (1) be scalarized and (2) will have system-value semantics mapped appropriately, just as for an entry-point parameter when cross-compiling an HLSL-style `main()`.

Changes:

- Add a notion of a `VaryingTupleExpr` and `VaryingTupleVarDecl`, similar to those for the resources-in-structs case

- Trigger use of these when we have a global-scope varying in/out using an imported `struct` type

- Also use these in the cross-compilation case for ordinary varying input/output (since this approach seems like it should be more general, and can hopefully handle stuff like GS input/output some day)

- When generating parameter binding information, special case global-scope input/output, and treat it the same as entry-point-parameter input/output

- Revamp how used resource ranges are computed so that we can eventually make this specific to an entry point

- Actually implement first signs of life for `maybeMoveTemp` so that assignments to the tuple-ified outputs will work better

- Add first test case that actually seems to work

- Add diagnostics for conflicting explicit bindings on a parameter

- Add diagnostic for different parameters with overlapping bindings

- Make global-scope varying input/output use a tracking data structure specific to the translation unit for computing locations (so that they are independent of other TUs)

2 files changed, 75 insertions, 0 deletions

diff --git a/tests/rewriter/varying-struct.slang b/tests/rewriter/varying-struct.slang
new file mode 100644
index 000000000..92e9dda2e
--- /dev/null
+++ b/tests/rewriter/varying-struct.slang
@@ -0,0 +1,21 @@
+//TEST_IGNORE_FILE:
+
+struct VS_IN
+{
+	float4 x : X;
+	float4 y : Y;	
+};
+
+struct VS_OUT
+{
+	float4 color : COLOR;
+	float4 posH : SV_Position;
+};
+
+VS_OUT doIt(VS_IN i)
+{
+	VS_OUT o;
+	o.color = i.x;
+	o.posH = i.y;
+	return o;
+}
diff --git a/tests/rewriter/varying-struct.vert b/tests/rewriter/varying-struct.vert
new file mode 100644
index 000000000..74ca8be37
--- /dev/null
+++ b/tests/rewriter/varying-struct.vert
@@ -0,0 +1,54 @@
+#version 450 core
+//TEST:COMPARE_GLSL:
+
+#if defined(__SLANG__)
+
+__import varying_struct;
+
+in VS_IN foo;
+out VS_OUT bar;
+
+void main()
+{
+    bar = doIt(foo);
+}
+
+#else
+
+struct VS_IN
+{
+	vec4 x;
+	vec4 y;	
+};
+
+struct VS_OUT
+{
+	vec4 color;
+	vec4 posH;
+};
+
+VS_OUT doIt(VS_IN i)
+{
+	VS_OUT o;
+	o.color = i.x;
+	o.posH = i.y;
+	return o;
+}
+
+layout(location = 0)
+out vec4 SLANG_out_bar_color;
+
+layout(location = 0)
+in vec4 SLANG_in_foo_x;
+
+layout(location = 1)
+in vec4 SLANG_in_foo_y;
+
+void main()
+{
+	VS_OUT SLANG_tmp_0 = doIt(VS_IN(SLANG_in_foo_x, SLANG_in_foo_y));
+	SLANG_out_bar_color = SLANG_tmp_0.color;
+	gl_Position = SLANG_tmp_0.posH;
+}
+
+#endif