rustc_private

The tracking issue for this feature is: #27812

This feature allows access to unstable internal compiler crates such as rustc_driver.

The presence of this feature changes the way the linkage format for dylibs is calculated in a way that is necessary for linking against dylibs that statically link std (such as rustc_driver). This makes this feature "viral" in linkage; its use in a given crate makes its use required in dependent crates which link to it (including integration tests, which are built as separate crates).