The lazy part comes from the use of OnceCell. Because OnceCell uses interior mutability LazyAst has to be marked not_covariant, which in turn means we have to use the with_dependent accessor function.

Run this example with cargo run, it should output:

lazy_ast.get_code() -> a[i * x[y]] * sin(z)
lazy_ast.is_parsed() -> false
[parsing code]
lazy_ast.fmt_ast() -> Ast(["a[i", "*", "x[y]]", "*", "sin(z)"])
lazy_ast.is_parsed() -> true
lazy_ast.fmt_ast() -> Ast(["a[i", "*", "x[y]]", "*", "sin(z)"])

Notice how at the beginning is_parsed returns false because we haven't accessed the Ast yet. Once we call fmt_ast for the first time 'Parsing code' shows up. However on the second call to fmt_ast this doesn't show up anymore. This enables us to create a struct with internal lifetime that doesn't leak outside, is cheap to create and only does the parsing work only once when needed.