onboarding friction log

[nikomatsakis]

Logging friction I encountered trying to onboard...

• An example of a working binary that uses run and run_with_tcx would be nice.
• The docs didn't make it clear that I should import use rustc_smir::{run, rustc_internal}.
• My function should return ControlFlow-- what does Continue vs Break mean in this context? What type arguments should I give? (I think Continue means "no error" and Break lets me pass back an error value? unclear.)
• How do I setup my rust-toolchain.toml file? What is this build.rs I see?
• Why am I getting weird link errors? (cannot satisfy dependencies so object only shows up once)
• Where exactly do I need feature(rustc_private)? On the crate? What if this is a library dependency of another crate?

It'd be really nice to have a simpler entry point, maybe even on crates.io, that automatically compiled to some fallback when nightly is not available.