a pattern using comptime fields that might be nice for applications that want to use a global allocator

global_allocator_example.zig

const std = @import("std");

var gpa_instance: std.heap.GeneralPurposeAllocator(.{}) = .{};
const global_allocator = gpa_instance.allocator();

const S = struct {
    comptime gpa: std.mem.Allocator = global_allocator,
    x: i32,
};

pub fn main() !void {
    defer _ = gpa_instance.deinit();

    var s: S = .{ .x = 1234 };
    try foo(&s);
}

fn foo(s: *S) !void {
    const ptr = try s.gpa.create(i32);
    defer s.gpa.destroy(ptr);

    ptr.* = s.x;
    s.x += 1;
}