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.

10 files changed, 68 insertions, 14 deletions

diff --git a/tests/autodiff/reverse-inout-param-custom-derivative.slang b/tests/autodiff/reverse-inout-param-custom-derivative.slang
index 8769a33c7..c4549e37b 100644
--- a/tests/autodiff/reverse-inout-param-custom-derivative.slang
+++ b/tests/autodiff/reverse-inout-param-custom-derivative.slang
@@ -5,7 +5,7 @@
 //TEST_INPUT:ubuffer(data=[0], stride=4):out,name=outputBuffer
 RWStructuredBuffer<float> outputBuffer;
 
-float rng(inout int state, float x)
+float rng(int state, float x)
 {
     return state + x;
 }
diff --git a/tests/bugs/gl-33-ext.slang b/tests/bugs/gl-33-ext.slang
index fccde99a3..6df9f286a 100644
--- a/tests/bugs/gl-33-ext.slang
+++ b/tests/bugs/gl-33-ext.slang
@@ -4,5 +4,5 @@
 public struct A
 {
     public int state;
-    [mutating] public int next() { return state; }
+    public int next() { return state; }
 };
diff --git a/tests/bugs/optional.slang b/tests/bugs/optional.slang
index 3512ba29f..7a220b23d 100644
--- a/tests/bugs/optional.slang
+++ b/tests/bugs/optional.slang
@@ -14,7 +14,7 @@ struct P
 }
 struct Tr
 {
-    int test<T:IArithmetic>(T t, inout P p)
+    int test<T:IArithmetic>(T t, P p)
     {
         const IFoo hit = p.f;
         let castResult = hit as S;
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
diff --git a/tests/hlsl-intrinsic/matrix-double.slang b/tests/hlsl-intrinsic/matrix-double.slang
index 31ec54f8e..6ed1b42bf 100644
--- a/tests/hlsl-intrinsic/matrix-double.slang
+++ b/tests/hlsl-intrinsic/matrix-double.slang
@@ -46,6 +46,8 @@ IntMatrix makeIntMatrix(int v)
 
 void test1(inout FloatMatrix ft, inout FloatMatrix f, int idx)
 {
+    unmodified(f);
+
     // fmod
     ft += FloatMatrix(IntMatrix(((f % makeFloatMatrix(0.11f)) * makeFloatMatrix(100)) + makeFloatMatrix(0.5)));
 
@@ -112,6 +114,8 @@ void test1(inout FloatMatrix ft, inout FloatMatrix f, int idx)
 
 void test2(inout FloatMatrix ft, inout FloatMatrix f)
 {
+    unmodified(f);
+
     ft += pow(makeFloatMatrix(0.5), f);
 
     ft += smoothstep(makeFloatMatrix(0.2), makeFloatMatrix(0.7), f);
diff --git a/tests/hlsl-intrinsic/size-of/align-of-3.slang b/tests/hlsl-intrinsic/size-of/align-of-3.slang
index 28665b5ab..079de9241 100644
--- a/tests/hlsl-intrinsic/size-of/align-of-3.slang
+++ b/tests/hlsl-intrinsic/size-of/align-of-3.slang
@@ -10,7 +10,7 @@
 RWStructuredBuffer<int> outputBuffer;
 
 
-int getParamSize(inout uint3 v)
+int getParamSize(uint3 v)
 {
     return alignof(v);
 }
@@ -19,7 +19,7 @@ int getParamSize(inout uint3 v)
 // With "natural" layout size must be the same across all targets.
 // The parameter passing semantic is "value in/value out", so the size 
 // will always be the size of the *value* regardless. So uint3 in this case.
-int getParamSizeWithDirection(inout uint3 v)
+int getParamSizeWithDirection(uint3 v)
 {
     return alignof(v);
 }
diff --git a/tests/hlsl-intrinsic/size-of/size-of-3.slang b/tests/hlsl-intrinsic/size-of/size-of-3.slang
index 8a211867f..7e55bd320 100644
--- a/tests/hlsl-intrinsic/size-of/size-of-3.slang
+++ b/tests/hlsl-intrinsic/size-of/size-of-3.slang
@@ -10,7 +10,7 @@
 RWStructuredBuffer<int> outputBuffer;
 
 
-int getParamSize(inout uint3 v)
+int getParamSize(uint3 v)
 {
     return sizeof(v);
 }
@@ -19,7 +19,7 @@ int getParamSize(inout uint3 v)
 // With "natural" layout size must be the same across all targets.
 // The parameter passing semantic is "value in/value out", so the size 
 // will always be the size of the *value* regardless. So uint3 in this case.
-int getParamSizeWithDirection(inout uint3 v)
+int getParamSizeWithDirection(uint3 v)
 {
     return sizeof(v);
 }
diff --git a/tests/language-feature/struct-field-initializers/struct-field-no-initializer-complex-types.slang b/tests/language-feature/struct-field-initializers/struct-field-no-initializer-complex-types.slang
index a55c4725d..66a3a2c64 100644
--- a/tests/language-feature/struct-field-initializers/struct-field-no-initializer-complex-types.slang
+++ b/tests/language-feature/struct-field-initializers/struct-field-no-initializer-complex-types.slang
@@ -6,7 +6,7 @@
 //TEST_INPUT:ubuffer(data=[0 0], stride=4):out,name=outputBuffer
 RWStructuredBuffer<int> outputBuffer;
 
-struct DefaultData
+struct DefaultData : IDefaultInitializable
 {
     static const int2 val = int2(0, 1);
     float2 size;
@@ -16,13 +16,10 @@ struct DefaultData
 
 extension DefaultData
 {
-    int someGet()
-    {
-        return val.x;
-    }
+    int someGet() { return val.x; }
 }
 
-int loadDefaultData(inout DefaultData noInit)
+int loadDefaultData(DefaultData noInit)
 {
     outputBuffer[1] = 1;
     return noInit.someGet();
diff --git a/tests/language-feature/unsized-array.slang b/tests/language-feature/unsized-array.slang
index c4479867e..19ab0bd63 100644
--- a/tests/language-feature/unsized-array.slang
+++ b/tests/language-feature/unsized-array.slang
@@ -17,6 +17,7 @@ int test(int arr[])
 
 int test2<T>(inout T arr[])
 {
+    unmodified(arr);
     return arr.getCount();
 }
 
diff --git a/tests/pipeline/ray-tracing/trace-ray-inline.slang b/tests/pipeline/ray-tracing/trace-ray-inline.slang
index 009dffbc7..b8688c2f5 100644
--- a/tests/pipeline/ray-tracing/trace-ray-inline.slang
+++ b/tests/pipeline/ray-tracing/trace-ray-inline.slang
@@ -57,6 +57,7 @@ void myTriangleClosestHit(inout MyRayPayload payload)
 //
 bool myTriangleAnyHit(inout MyRayPayload payload)
 {
+    unmodified(payload);
 	return true;
 }
 
@@ -75,6 +76,7 @@ void myProceduralClosestHit(inout MyRayPayload payload, MyProceduralHitAttrs att
 }
 bool myProceduralAnyHit(inout MyRayPayload payload)
 {
+    unmodified(payload);
 	return true;
 }
 
@@ -87,6 +89,8 @@ bool myProceduralAnyHit(inout MyRayPayload payload)
 //
 bool myProceduralIntersection(inout float tHit, inout MyProceduralHitAttrs hitAttrs)
 {
+    unmodified(tHit);
+    unmodified(hitAttrs);
 	return true;
 }
 
@@ -180,4 +184,4 @@ void main(uint3 tid : SV_DispatchThreadID)
 	}
 
     resultBuffer[index] = payload.value;
-}
\ No newline at end of file
+}