Release X.Y (202?-??-??)

Nix can now be built with LTO by passing --enable-lto to configure. LTO is currently only supported when building with GCC.
nix repl now takes installables on the command line, unifying the usage with other commands that use --file and --expr. Primary breaking change is for the common usage of nix repl '<nixpkgs>' which can be recovered with nix repl --file '<nixpkgs>' or nix repl --expr 'import <nixpkgs>{}'