Pos objects are somewhat wasteful as they duplicate the origin file name and
input type for each object. on files that produce more than one Pos when parsed
this a sizeable waste of memory (one pointer per Pos). the same goes for
ptr<Pos> on 64 bit machines: parsing enough source to require 8 bytes to locate
a position would need at least 8GB of input and 64GB of expression memory. it's
not likely that we'll hit that any time soon, so we can use a uint32_t index to
locate positions instead.
In particular, this means that 'nix eval` (which uses toValue()) no
longer auto-calls functions or functors (because
AttrCursor::findAlongAttrPath() doesn't).
Fixes#6152.
Also use ref<> in a few places, and don't return attrpaths from
getCursor() because cursors already have a getAttrPath() method.
This is needed to get the path of a derivation that might not exist
(e.g. for 'nix store copy-log').
InstallableStorePath::toDerivedPaths() cannot be used for this because
it calls readDerivation(), so it fails if the store doesn't have the
derivation.
In particular, this now works:
$ nix path-info --eval-store auto --store https://cache.nixos.org nixpkgs#hello
Previously this would fail as it would try to upload the hello .drv to
cache.nixos.org. Now the .drv is instantiated in the local store, and
then we check for the existence of the outputs in cache.nixos.org.
