As of Rust 2018, macro_use is no longer required in most circumstances.
I think it is generally a good idea to remove these when not needed, to
stop them from polluting the crate's global namespace.
https://doc.rust-lang.org/edition-guide/rust-2018/macros/macro-changes.html#macro_rules-style-macros

1.81% -> 0.56%

From 1.03% to 0.19% of the runtime of 'nix-instantiate "<nixpkgs>" -A
texlive.combined.scheme-full --dry-run'.

Most functions now take a StorePath argument rather than a Path (which
is just an alias for std::string). The StorePath constructor ensures
that the path is syntactically correct (i.e. it looks like
<store-dir>/<base32-hash>-<name>). Similarly, functions like
buildPaths() now take a StorePathWithOutputs, rather than abusing Path
by adding a '!<outputs>' suffix.

Note that the StorePath type is implemented in Rust. This involves
some hackery to allow Rust values to be used directly in C++, via a
helper type whose destructor calls the Rust type's drop()
function. The main issue is the dynamic nature of C++ move semantics:
after we have moved a Rust value, we should not call the drop function
on the original value. So when we move a value, we set the original
value to bitwise zero, and the destructor only calls drop() if the
value is not bitwise zero. This should be sufficient for most types.

Also lots of minor cleanups to the C++ API to make it more modern
(e.g. using std::optional and std::string_view in some places).

https://hydra.nixos.org/build/107466992

In particular, these are emitted by 'git archive' (in fetchGit).

We can now convert Rust Errors to C++ exceptions. At the Rust->C++ FFI
boundary, Result<T, Error> will cause Error to be converted to and
thrown as a C++ exception.

This was the last function using a shell script, so this allows us to
get rid of tar, coreutils, bash etc.