Constant cargo::core::compiler::build_runner::compilation_files::METADATA_VERSION

const METADATA_VERSION: u8 = 2;

This is a generic version number that can be changed to make backwards-incompatible changes to any file structures in the output directory. For example, the fingerprint files or the build-script output files.

Normally cargo updates ship with rustc updates which will cause a new hash due to the rustc version changing, but this allows cargo to be extra careful to deal with different versions of cargo that use the same rustc version.