cargo-script or a similar tool could be extended to auto-generate a main wrapper if the script does not already contain a main function. This is what rustdoc already does for running code found in doc comments.
On a separate note, with my moderator’s hat on: @redradist, please note that the bar for changes to the Rust is very high. Before posting proposals, please consider looking more closely at alternatives and prior work, including ways of solving the problem outside of the core language. Thanks!