rollup merge of #19215: aochagavia/pretty

Closes https://github.com/rust-lang/rust/issues/19077

I would appreciate any guidance on how to write a test for this. I saw some examples in `test/pretty`, but there are different ways to test... With or without `.pp` files, with a `pp-exact` comment, etc.

diff --cc src/libsyntax/print/pprust.rs
Simple merge