I do wonder if it would be possible to put this code in a .rs file and compile it, to prevent deviations? Then link from the readme.

Well, we have the same example in lib.rs and there it's at least checked by Rust's doc tests. So far didn't bother adding a check to CI to keep them in-sync though^^

Rust doc tests do not check that it is valid code, or is it doing something like that?

Having it duplicated, is that necessary. Maybe lib.rs can just point to the readme or vv ?

Rust doc tests do not check that it is valid code, or is it doing something like that?

As no_run is enabled, it just compiles but doesn't run the code.

Having it duplicated, is that necessary.

Yup, we want the example to be in both places.