list.zig

const std = @import("std");
const mem = std.mem;

fn LinkedList(comptime T: type) type {
    const Self = struct {
        const Node = struct {
            next: ?*Node,
            data: T,
        };

        first: ?*Node,
        allocator: *mem.Allocator,

        pub fn init(allocator: *mem.Allocator) !*Self {
            var self = try allocator.create(Self);
            self.first = null;
            self.allocator = allocator;
            return self;
        }
    };

    return Self;
}


test "LinkedList" {
    var arena = std.heap.ArenaAllocator.init(std.heap.direct_allocator);
    defer arena.deinit();

    const allocator = &arena.allocator;

    var list = try LinkedList(i32).init(allocator);
}

log.txt

list.zig:14:50: error: use of undeclared identifier 'Self'
list.zig:32:35: note: referenced here

Do it this way, you need to return the struct expression instead.

fn LinkedList(comptime T: type) type {
    return struct {
        const Self = @This();

        const Node = struct {
            next: ?*Node,
            data: T,
        };

        first: ?*Node,
        allocator: *mem.Allocator,

        pub fn init(allocator: *mem.Allocator) !*Self {
            var self = try allocator.create(Self);
            self.first = null;
            self.allocator = allocator;
            return self;
        }
    };
}