When using C libraries, C99 designator list initialization is often
times used to initialize data structure.
While `std.mem.zeroes` and manually assigning to each field can
achieve the same result, it is much more verbose then the equivalent
C code:
```zig
usingnamespace @cImport({
@cInclude("sokol_app.h");
});
// Using `std.mem.zeroes` and manual assignment.
var app_desc = std.mem.zeroes(sapp_desc);
app_desc.init_cb = init;
app_desc.frame_cb = frame;
app_desc.cleanup_cb = cleanup;
app_desc.width = 400;
app_desc.height = 300;
app_desc.window_name = "no default init";
// Using `std.mem.defaultInit`.
var app_desc = std.mem.defaultInit(sapp_desc, .{
.init_cb = init,
.frame_cb = frame,
.cleanup_cb = cleanup,
.width = 400,
.height = 300,
.window_name = "default init"
});
```
The `std.mem.defaultInit` aims to solve this problem by zero
initializing all fields of the given struct to their zero, or default
value if any. Each field mentionned in the `init` variable is then
assigned to the corresponding field in the struct.
If a field is a struct, and an initializer for it is present, it is
recursively initialized.
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