Sema: preserve volatileness when constructing field pointers

Closes #12928

src/Sema.zig

@@ -22027,6 +22027,7 @@ fn structFieldPtrByIndex(
     var ptr_ty_data: Type.Payload.Pointer.Data = .{
         .pointee_type = field.ty,
         .mutable = struct_ptr_ty_info.mutable,
+        .@"volatile" = struct_ptr_ty_info.@"volatile",
         .@"addrspace" = struct_ptr_ty_info.@"addrspace",
     };
 
@@ -22246,6 +22247,7 @@ fn unionFieldPtr(
     const ptr_field_ty = try Type.ptr(arena, sema.mod, .{
         .pointee_type = field.ty,
         .mutable = union_ptr_ty.ptrIsMutable(),
+        .@"volatile" = union_ptr_ty.isVolatilePtr(),
         .@"addrspace" = union_ptr_ty.ptrAddressSpace(),
     });
     const enum_field_index = @intCast(u32, union_obj.tag_ty.enumFieldIndex(field_name).?);
@@ -22568,6 +22570,7 @@ fn tupleFieldPtr(
     const ptr_field_ty = try Type.ptr(sema.arena, sema.mod, .{
         .pointee_type = field_ty,
         .mutable = tuple_ptr_ty.ptrIsMutable(),
+        .@"volatile" = tuple_ptr_ty.isVolatilePtr(),
         .@"addrspace" = tuple_ptr_ty.ptrAddressSpace(),
     });
 

test/behavior.zig

@@ -93,6 +93,7 @@ test {
     _ = @import("behavior/bugs/12794.zig");
     _ = @import("behavior/bugs/12801-1.zig");
     _ = @import("behavior/bugs/12801-2.zig");
+    _ = @import("behavior/bugs/12928.zig");
     _ = @import("behavior/byteswap.zig");
     _ = @import("behavior/byval_arg_var.zig");
     _ = @import("behavior/call.zig");

test/behavior/bugs/12928.zig

@@ -0,0 +1,26 @@
+const std = @import("std");
+const expect = std.testing.expect;
+const A = extern struct {
+    value: *volatile B,
+};
+const B = extern struct {
+    a: u32,
+    b: i32,
+};
+test {
+    var a: *A = undefined;
+    try expect(@TypeOf(&a.value.a) == *volatile u32);
+    try expect(@TypeOf(&a.value.b) == *volatile i32);
+}
+const C = extern struct {
+    value: *volatile D,
+};
+const D = extern union {
+    a: u32,
+    b: i32,
+};
+test {
+    var c: *C = undefined;
+    try expect(@TypeOf(&c.value.a) == *volatile u32);
+    try expect(@TypeOf(&c.value.b) == *volatile i32);
+}