NLL can be very slow #47597
Description
In servo/html5ever@3d5e24b, RUSTFLAGS="-Ztime-passes" cargo +nightly-2018-01-19 build -j1 (with -j1 to make time-passes readable) ends in 31 seconds.
With -Znll added, it takes 233 seconds, a ~7× slowdown.
Each log contains 41 MIR borrow checking lines. Without NLL they’re all under 1 second. With it, the ones over 1 second are:
time: 18.400; rss: 252MB MIR borrow checking
time: 87.612; rss: 211MB MIR borrow checking
time: 60.710; rss: 120MB MIR borrow checking
time: 37.651; rss: 188MB MIR borrow checking
They add up to roughly all of the time difference. They are for the serde, syn, serde_derive_internals, and serde_derive crates respectively.