Rollup of 8 pull requests

[Manishearth]

• Successful merges: trans: Add early-out when translating unreachable controlflow expressions. #23066, Closure region hierarchy #23109, Blanket impls for &F/&mut F where F: Fn/F:FnMut respectively #23895, Pretty printing ids for assoc items and some tidying up in resolve/ast conv #23924, Improvements to PhantomData<T>'s docs 👻 #23925, Simplify match branches in iter.rs example #23927, Fix rust book error-handling.md for new std::io. #23933, Remove variances from traits and deprecate PhantomFn/MarkerTrait #23938
• Failed merges:

[rust-highfive]

r? @eddyb

(rust_highfive has picked a reviewer for you, use r? to override)

[Manishearth]

@bors: r+ p=10

[bors]

📌 Commit 6fb7b62 has been approved by Manishearth

[bors]

🔒 Merge conflict