concat.zig

const std = @import("std");
const print = std.debug.print;
const Allocator = std.mem.Allocator;

pub fn main() !void {
    var arena = std.heap.ArenaAllocator.init(std.heap.page_allocator);
    defer arena.deinit();
    var A = &arena.allocator;

    var a1 = [_][]const u8{ "a", "b", "c" };
    var a2 = [_][]const u8{ "d", "e", "f" };
    var a3 = try std.mem.concat(A, u8, &[_][][]const u8{ a1, a2 });
    print("a3: {}", .{a3});
}

$ zig run scratch.zig
./scratch.zig:13:58: error: expected type '[][]const u8', found '[3][]const u8'
    var a3 = try std.mem.concat(A, u8, &[_][][]const u8{ a1, a2 });
                                                         ^

var a3 = try std.mem.concat(A, u8, &[_][][]const u8{ &a1, &a2 });

gives:

$ zig run scratch.zig
./scratch.zig:11:5: error: expected type '[]const u8', found '*[3][]const u8'
    var a1 = [_][]const u8{ "a", "b", "c" };
    ^
./scratch.zig:13:57: note: referenced here
    var a3 = try std.mem.concat(A, u8, &[_][]const u8{ &a1, &a2 });
                                                        ^