Explicitly detach derivative when forming a non-differentiable struct out of differentiable args (#4901)

* Explicitly detach derivative when forming a non-differentiable struct out of differentiable args

This fixes an issue where initializer lists get optimized out and lose information about non-differentiability.

There are 2 places where this could have been fixed:
1. When coercing initializer-list exprs, we can check for non-differentiable aggregate types and use a detach derivative on all the args.
2. Add an extra case in the peephole optimization step that adds detach-derivative when simplifying a make-struct of a non-differentiable type.

Even though solution 2 is more elegant, this PR goes with solution 1 simply to avoid having to use a differentiable-type-conformance-context that is used in the auto-diff IR passes to check for differentiability.

* Change test name + add expected vals

2 files changed, 49 insertions, 0 deletions

diff --git a/tests/autodiff/field-extract-of-make-struct.slang b/tests/autodiff/field-extract-of-make-struct.slang
new file mode 100644
index 000000000..1cf2b40c4
--- /dev/null
+++ b/tests/autodiff/field-extract-of-make-struct.slang
@@ -0,0 +1,43 @@
+//TEST(compute):COMPARE_COMPUTE_EX:-slang -compute -shaderobj -output-using-type
+//TEST(compute, vulkan):COMPARE_COMPUTE_EX:-vk -compute -shaderobj -output-using-type
+//TEST(compute):COMPARE_COMPUTE_EX:-cpu -compute -output-using-type -shaderobj
+
+//TEST_INPUT:ubuffer(data=[0 0 0 0 0], stride=4):out,name=outputBuffer
+RWStructuredBuffer<float> outputBuffer;
+
+// This test checks to ensure that expressions of the form FieldExtract(MakeStruct(...))
+// are handled correctly in the presence of differentiation. 
+// Such expressions are often optimized out, and the non-differentiability of the struct 
+// type can be accidentally overlooked.
+//
+
+typedef DifferentialPair<float> dpfloat;
+typedef float.Differential dfloat;
+
+struct Data
+{
+    [PreferRecompute]
+    __init(float tin)
+    {
+        this.t = tin;
+    }
+    float t;
+};
+
+[BackwardDifferentiable]
+float test_make_struct(float y)
+{
+    Data d = { y };
+    return d.t;
+}
+
+[numthreads(1, 1, 1)]
+void computeMain(uint3 dispatchThreadID : SV_DispatchThreadID)
+{
+    {
+        dpfloat dpa = dpfloat(1.0, 0.0);
+
+        bwd_diff(test_make_struct)(dpa, 1.0f);
+        outputBuffer[0] = dpa.d; // Expect: 0.0
+    }
+}
\ No newline at end of file
diff --git a/tests/autodiff/field-extract-of-make-struct.slang.expected.txt b/tests/autodiff/field-extract-of-make-struct.slang.expected.txt
new file mode 100644
index 000000000..e070cf84d
--- /dev/null
+++ b/tests/autodiff/field-extract-of-make-struct.slang.expected.txt
@@ -0,0 +1,6 @@
+type: float
+0.000000
+0.000000
+0.000000
+0.000000
+0.000000