Warn when inout parameter is never written (#4777)

Addresses #4698 as one approach to diagnose the potential problem.

Emit warnings when a user marks a parameter as `inout` but never writes to it in the function. A new intrinsic function `unmodified(out T)` has been added to explicitly indicate that an `inout` variable will not be modified in the function.

This is only one way to address the specific validation error in #4698. In general it seems that DXC does some more extensive checks on actual struct fields (as opposed to observing arbitrary struct writes), so that will be the next step.

1 files changed, 48 insertions, 0 deletions

diff --git a/tests/diagnostics/inout-never-written.slang b/tests/diagnostics/inout-never-written.slang
new file mode 100644
index 000000000..0bc59ecf3
--- /dev/null
+++ b/tests/diagnostics/inout-never-written.slang
@@ -0,0 +1,48 @@
+//TEST:SIMPLE(filecheck=CHK): -target spirv
+
+struct State
+{
+    float3 v;
+    float3 n;
+    int rnd;
+};
+
+//CHK-DAG: ([[# @LINE + 1]]): warning 41022: inout parameter 'x' is never written to
+void int_never_assigned(inout int x) {}
+
+//CHK-DAG: ([[# @LINE + 1]]): warning 41022: inout parameter 'state' is never written to
+void state_never_assigned(inout State state, inout float v)
+{
+    v = state.v.x;
+}
+
+void state_assigned(inout State state)
+{
+    state.rnd = (int) dot(state.v, state.n);
+}
+
+struct A
+{
+    int state;
+
+    //CHK-DAG: ([[# @LINE + 1]]): warning 41023: method marked `[mutable]` but never modifies `this`
+    [mutating] int next() { return state; }
+    
+    [mutating] int progress()
+    {
+        unmodified(state);
+        return state;
+    }
+};
+
+__generic <T>
+struct B
+{
+    int state;
+
+    //CHK-DAG: ([[# @LINE + 1]]): warning 41023: method marked `[mutable]` but never modifies `this`
+    [mutating] int next() { return state; }
+};
+
+//CHK-NOT: warning 41022
+//CHK-NOT: warning 41023
\ No newline at end of file