no_diff diagnostics improvement (#7655)

close #6286.

This PR is to improve the diagnostics for no_diff usage.

In a differentiable function, any calls to a non-diff function with constant arguments should not require no_diff attribute.
This PR adds this extra check at `checkAutoDiffUsages` where it checks the differentiability on IR.

In a differentiable method, we will force to use `[NoDiffThis]` attribute if there is access to non-differentiable `This` type. Once this access is detected we will report a warning to bring users attention that this access won't generate any derivative, they have to use `[NoDiffThis]` to suppress that warning.
This PR adds this check at type checking stage, because it's the easiest way to find out all the `This` accesses.

1 files changed, 42 insertions, 0 deletions

diff --git a/tests/diagnostics/force-no-diff-this.slang b/tests/diagnostics/force-no-diff-this.slang
new file mode 100644
index 000000000..ae1464ffb
--- /dev/null
+++ b/tests/diagnostics/force-no-diff-this.slang
@@ -0,0 +1,42 @@
+//DIAGNOSTIC_TEST:SIMPLE(filecheck=CHECK):
+
+struct MyStruct<T> where T: __BuiltinFloatingPointType
+{
+    float a;
+    __init(float a) { this.a = a;}
+
+    [Differentiable]
+    T eval(T x)
+    {
+        //CHECK: ([[# @LINE+1]]): warning 31159
+        return exp(x * T(a) * T(a));
+    }
+
+    [Differentiable]
+    [NoDiffThis]
+    T eval1(T x)
+    {
+        //CHECK-NOT: ([[# @LINE+1]]): warning 31159
+        return exp(x * T(a) * T(a));
+    }
+};
+
+[Differentiable]
+float evalFunc(float x)
+{
+    MyStruct<float> s = {x};
+    return s.eval(x) + s.eval1(x);
+}
+
+RWStructuredBuffer<float> output;
+
+[shader("compute")]
+[numthreads(1,1,1)]
+void computeMain(uint id : SV_DispatchThreadID)
+{
+    var x = diffPair(2.0f);
+    bwd_diff(evalFunc)(x, 1.0f);
+
+    output[0] = x.d;
+}
+