How `.len` field of tuples work?

I am puzzled by the output of the following program:

const std = @import("std");

pub fn main() void {
    const s = .{9, 2};
    const T = @TypeOf(s);
    const S = @typeInfo(T).Struct;
    inline for (S.fields) |f| {
        std.debug.print("field {}\n", .{f});
    inline for (S.decls) |d| {
        std.debug.print("decl {}\n", .{d});
    std.debug.print("{}\n", .{@field(s, "len")});

It prints two fields (@"0" and @"1"), zero decls, but somehow len is also a field.

Ah, looks like in my attempt to ask the question I nerd-sniped myself. Turns out this is hard-coded on a pretty deep level: