Once upon a time I briefly looked at symlinking rubygem install dirs from global to project-specific directories (because a project-specific $GEM_HOME avoided f

Once upon a time I briefly looked at symlinking rubygem install dirs from global to project-specific directories (because a project-specific $GEM_HOME avoided functionally all the problems with just about any other approach, and is still what I use today). Functionally it worked, it just needed some tooling to make it easy. It sounds like what you might want is a shared global dir that uses hashes of feature flags to separate crate installs in the same target directory, then some after-the-fact GC to hardlink matching files across different builds of the same crate. Then you can symlink from there into your local project target.