ability to @ptrCast to *void

fixes #960

src/codegen.cpp

@@ -2655,6 +2655,9 @@ static LLVMValueRef ir_render_ptr_cast(CodeGen *g, IrExecutable *executable,
         IrInstructionPtrCast *instruction)
 {
     TypeTableEntry *wanted_type = instruction->base.value.type;
+    if (!type_has_bits(wanted_type)) {
+        return nullptr;
+    }
     LLVMValueRef ptr = ir_llvm_value(g, instruction->ptr);
     return LLVMBuildBitCast(g->builder, ptr, wanted_type->type_ref, "");
 }

src/ir.cpp

@@ -19757,6 +19757,8 @@ static IrInstruction *ir_align_cast(IrAnalyze *ira, IrInstruction *target, uint3
 }
 
 static TypeTableEntry *ir_analyze_instruction_ptr_cast(IrAnalyze *ira, IrInstructionPtrCast *instruction) {
+    Error err;
+
     IrInstruction *dest_type_value = instruction->dest_type->other;
     TypeTableEntry *dest_type = ir_resolve_type(ira, dest_type_value);
     if (type_is_invalid(dest_type))
@@ -19810,9 +19812,13 @@ static TypeTableEntry *ir_analyze_instruction_ptr_cast(IrAnalyze *ira, IrInstruc
             instruction->base.source_node, nullptr, ptr);
     casted_ptr->value.type = dest_type;
 
-    // keep the bigger alignment, it can only help
+    // Keep the bigger alignment, it can only help-
+    // unless the target is zero bits.
+    if ((err = type_ensure_zero_bits_known(ira->codegen, dest_type)))
+        return ira->codegen->builtin_types.entry_invalid;
+
     IrInstruction *result;
-    if (src_align_bytes > dest_align_bytes) {
+    if (src_align_bytes > dest_align_bytes && type_has_bits(dest_type)) {
         result = ir_align_cast(ira, casted_ptr, src_align_bytes, false);
         if (type_is_invalid(result->value.type))
             return ira->codegen->builtin_types.entry_invalid;

test/cases/cast.zig

@@ -519,3 +519,9 @@ fn incrementVoidPtrArray(array: ?*c_void, len: usize) void {
         @ptrCast([*]u8, array.?)[n] += 1;
     }
 }
+
+test "*usize to *void" {
+    var i = usize(0);
+    var v = @ptrCast(*void, &i);
+    v.* = {};
+}