Auto merge of #53314 - nikomatsakis:nll-invert-liveness, r=pnkfelix

NLL: experiment with inverting liveness

I got inspired to see what would happen here.

Fixes #52460

r? @pnkfelix

diff --cc src/librustc_data_structures/lib.rs
Simple merge

diff --cc src/librustc_mir/lib.rs
index da29c900b8f2d5b873c82187b09cce21b2406881,44a731399064e79cc59ba8bc7c181f94d12bfe92..cfa84264b1f5a6a56d9cdffe80b190c7dae829c6
--- 1/src/librustc_mir/lib.rs
--- 2/src/librustc_mir/lib.rs
+++ b/src/librustc_mir/lib.rs
@@@ -15,8 -15,9 +15,9 @@@ Rust MIR: a lowered representation of R
  */
  
  #![cfg_attr(not(stage0), feature(nll))]
 -#![feature(infer_outlives_requirements)]
 +#![cfg_attr(not(stage0), feature(infer_outlives_requirements))]
  #![feature(in_band_lifetimes)]
+ #![feature(impl_header_lifetime_elision)]
  #![feature(slice_patterns)]
  #![feature(slice_sort_by_cached_key)]
  #![feature(box_patterns)]