MSL: fix robust struct zero initialization

reference/opt/shaders-msl/comp/robust-buffer-access2-struct.robust-buffer-access2.comp

@@ -0,0 +1,31 @@
+#include <metal_stdlib>
+#include <simd/simd.h>
+
+using namespace metal;
+
+struct Record
+{
+    uint a;
+    uint b;
+    uint c;
+    uint d;
+};
+
+struct WriteSSBO
+{
+    Record result;
+};
+
+struct ReadSSBO
+{
+    Record records[1];
+};
+
+constant uint3 gl_WorkGroupSize [[maybe_unused]] = uint3(1u);
+
+kernel void main0(constant uint* spvBufferSizeConstants [[buffer(25)]], device WriteSSBO& _10 [[buffer(0)]], device ReadSSBO& _16 [[buffer(1)]], uint3 gl_GlobalInvocationID [[thread_position_in_grid]])
+{
+    constant uint& _10BufferSize = spvBufferSizeConstants[0];
+    constant uint& _16BufferSize = spvBufferSizeConstants[1];
+    _10.result = (uint(gl_GlobalInvocationID.x) < (_16BufferSize / 4u) ? _16.records[min(uint(gl_GlobalInvocationID.x), uint(max(int((_16BufferSize / 4u)) - 1, 0)))] : Record{});
+}

reference/shaders-msl/comp/robust-buffer-access2-struct.robust-buffer-access2.comp

@@ -0,0 +1,31 @@
+#include <metal_stdlib>
+#include <simd/simd.h>
+
+using namespace metal;
+
+struct Record
+{
+    uint a;
+    uint b;
+    uint c;
+    uint d;
+};
+
+struct WriteSSBO
+{
+    Record result;
+};
+
+struct ReadSSBO
+{
+    Record records[1];
+};
+
+constant uint3 gl_WorkGroupSize [[maybe_unused]] = uint3(1u);
+
+kernel void main0(constant uint* spvBufferSizeConstants [[buffer(25)]], device WriteSSBO& _10 [[buffer(0)]], device ReadSSBO& _16 [[buffer(1)]], uint3 gl_GlobalInvocationID [[thread_position_in_grid]])
+{
+    constant uint& _10BufferSize = spvBufferSizeConstants[0];
+    constant uint& _16BufferSize = spvBufferSizeConstants[1];
+    _10.result = (uint(gl_GlobalInvocationID.x) < (_16BufferSize / 4u) ? _16.records[min(uint(gl_GlobalInvocationID.x), uint(max(int((_16BufferSize / 4u)) - 1, 0)))] : Record{});
+}

shaders-msl/comp/robust-buffer-access2-struct.robust-buffer-access2.comp

@@ -0,0 +1,25 @@
+#version 450
+layout(local_size_x = 1) in;
+
+struct Record
+{
+    uint a;
+    uint b;
+    uint c;
+    uint d;
+};
+
+layout(std430, binding = 0) readonly buffer ReadSSBO
+{
+    Record records[];
+};
+
+layout(std430, binding = 1) writeonly buffer WriteSSBO
+{
+    Record result;
+};
+
+void main()
+{
+    result = records[gl_GlobalInvocationID.x];
+}

spirv_msl.cpp

@@ -10336,12 +10336,14 @@ void CompilerMSL::emit_instruction(const Instruction &instruction)
 			if (expr)
 			{
 				auto &loaded_type = get<SPIRType>(result_type);
-				// Generate a proper zero expression using type constructor
-				// to_zero_initialized_expression returns {} which doesn't work in ternary
-				string zero_expr = join(type_to_glsl(loaded_type), "(0)");
+				// MSL structs do not have a scalar constructor. Use typed aggregate
+				// initialization so the expression remains valid in a ternary.
+				string zero_expr = loaded_type.basetype == SPIRType::Struct ? join(type_to_glsl(loaded_type), "{}") :
+				                                                              join(type_to_glsl(loaded_type), "(0)");
 
 				// Wrap the expression with a select: (in_bounds) ? loaded_value : zero
-				expr->expression = join("(", info.bounds_check_condition, " ? ", expr->expression, " : ", zero_expr, ")");
+				expr->expression =
+				    join("(", info.bounds_check_condition, " ? ", expr->expression, " : ", zero_expr, ")");
 			}
 			// Remove from map to avoid double-processing
 			robust_access_chains.erase(it);