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, 55 insertions, 8 deletions

diff --git a/source/slang/diff.meta.slang b/source/slang/diff.meta.slang
index c912e026c..a4c468ef7 100644
--- a/source/slang/diff.meta.slang
+++ b/source/slang/diff.meta.slang
@@ -273,7 +273,12 @@ struct TensorView
     __subscript(uint index) -> T
     {
         [ForceInline] [__NoSideEffect] get { return load(index); }
-        [ForceInline] set { store(index, newValue); }
+        
+        [ForceInline] set
+        {
+            unmodified(this);
+            store(index, newValue);
+        }
 
         [__NoSideEffect]
         ref
@@ -288,7 +293,13 @@ struct TensorView
     __subscript(uint i1, uint i2) -> T
     {
         [ForceInline] [__NoSideEffect] get { return load(i1, i2); }
-        [ForceInline] set { store(i1, i2, newValue); }
+
+        [ForceInline] set
+        {
+            unmodified(this);
+            store(i1, i2, newValue);
+        }
+        
         [__NoSideEffect]
         ref
         {
@@ -302,7 +313,13 @@ struct TensorView
     __subscript(uint2 i) -> T
     {
         [ForceInline] [__NoSideEffect] get { return load(i.x, i.y); }
-        [ForceInline] set { store(i.x, i.y, newValue); }
+
+        [ForceInline] set
+        {
+            unmodified(this);
+            store(i.x, i.y, newValue);
+        }
+
         [__NoSideEffect]
         ref
         {
@@ -316,7 +333,13 @@ struct TensorView
     __subscript(uint i1, uint i2, uint i3) -> T
     {
         [ForceInline] [__NoSideEffect] get { return load(i1, i2, i3); }
-        [ForceInline] set { store(i1, i2, i3, newValue); }
+        
+        [ForceInline] set
+        {
+            unmodified(this);
+            store(i1, i2, i3, newValue);
+        }
+
         [__NoSideEffect]
         ref
         {
@@ -330,7 +353,13 @@ struct TensorView
     __subscript(uint3 i) -> T
     {
         [ForceInline] [__NoSideEffect] get { return load(i.x, i.y, i.z); }
-        [ForceInline] set { store(i.x, i.y, i.z, newValue); }
+
+        [ForceInline] set
+        {
+            unmodified(this);
+            store(i.x, i.y, i.z, newValue);
+        }
+
         [__NoSideEffect]
         ref
         {
@@ -344,7 +373,13 @@ struct TensorView
     __subscript(uint i1, uint i2, uint i3, uint i4) -> T
     {
         [ForceInline] [__NoSideEffect] get { return load(i1, i2, i3, i4); }
-        [ForceInline] set { store(i1, i2, i3, i4, newValue); }
+
+        [ForceInline] set
+        {
+            unmodified(this);
+            store(i1, i2, i3, i4, newValue);
+        }
+
         [__NoSideEffect]
         ref
         {
@@ -358,7 +393,13 @@ struct TensorView
     __subscript(uint4 i) -> T
     {
         [__NoSideEffect][ForceInline] get { return load(i.x, i.y, i.z, i.w); }
-        [ForceInline] set { store(i.x, i.y, i.z, i.w, newValue); }
+
+        [ForceInline] set
+        {
+            unmodified(this);
+            store(i.x, i.y, i.z, i.w, newValue);
+        }
+
         [__NoSideEffect]
         ref
         {
@@ -372,7 +413,13 @@ struct TensorView
     __subscript(uint i1, uint i2, uint i3, uint i4, uint i5) -> T
     {
         [ForceInline] [__NoSideEffect] get { return load(i1, i2, i3, i4, i5); }
-        [ForceInline] set { store(i1, i2, i3, i4, i5, newValue); }
+        
+        [ForceInline] set
+        {
+            unmodified(this);
+            store(i1, i2, i3, i4, i5, newValue);
+        }
+
         [__NoSideEffect]
         ref
         {