Use nixpkgs 21.05

[nomeata]

to see what breaks, and also to get
NixOS/nixpkgs@5a923e5
which may allow us to use rustc from nixpkgs again (see #2519)

Issues encountered:

• in nixpks, various cargoSHA256 were wrong. (rav1e: fix cargoSha256 NixOS/nixpkgs#123522, cargo-c: Fix cargoSha256 NixOS/nixpkgs#123348, rust-bindgen: Update cargoSha256 NixOS/nixpkgs#123349)

• Ocaml built with musl, used for static builds, is currently broken.

  Reported at pkgsMusl.ocamlPackages.ocaml: -r and -pie may not be used together NixOS/nixpkgs#124476.

  Found a possible fix, submitted at pkgsMusl.ocamlPackages.ocaml: Disable pie hardening on musl NixOS/nixpkgs#124498 and currently included in this branch.

• New ocamlformat version

• Running new formatter on these files

• The RTS fails to build or link the C files (stddef.h missing)
  This stuff was very hairy before, and that doesn’t get better :-(

  If I use the wrapped wasm32-unknown-wasi-clang, the build makes some progress, but then fails with https://bugs.llvm.org/show_bug.cgi?id=43393 (as it used to before, and then somehow we fixed that)

  In the end, surgically adding an include to CLANG_FLAGS works, it seems.

• Some upgraded tool produces new names subsections, presumably from this Wasm proposal: https://github.com/WebAssembly/extended-name-section/blob/master/proposals/extended-name-section/Overview.md
  Worked around by jumping over them, but otherwise ignoring them. Should be refined.

• Haskell code ought to be adjusted to latest GHC

• Bump the niv that we install into the shell (also because of Haskell changes)

• The darwin rebuild of rustc and git keeps timing out on hydra.

[dfinity-ci]

This PR does not affect the produced WebAssembly code.

[nomeata]

Claudio wants to test this, so don’t merge before. Might be nice if someone on OSX would test using the nix-shell?

[ggreif]

Hmm, my Mac exhibits the same failure mode as the hydra:

/nix/store/9xawf09c90761xkxv8r1b55ja46x4wr3-cctools-binutils-darwin-949.0.1/bin/ranlib: file: libclang_rt.cc_kext_x86_64_osx.a(trunctfsf2.c.o) has no symbols
[ 66%] Built target clang_rt.cc_kext_x86_64_osx
[ 66%] Built target RTfuzzer.osx
make: *** [Makefile:149: all] Error 2
builder for '/nix/store/s7d1y3nsvnazhwh85gfskp42wcr22k9n-compiler-rt-libc-12.0.0.drv' failed with exit code 2
cannot build derivation '/nix/store/52vnh7i8n3igfi2pgx1nk02zfx47mh3p-clang-wrapper-12.0.0.drv': 1 dependencies couldn't be built
error: build of '/nix/store/52vnh7i8n3igfi2pgx1nk02zfx47mh3p-clang-wrapper-12.0.0.drv' failed

I have seen this happening on the nixpkgs PR too. Not sure how to deal with it, I have no time to investigate/fix right now.

[nomeata]

I am surprised you have to build that at all - isn't it part of our nix cache by now?

[crusso]

Testing it now...

(just by running nix-shell - lemme know if you want more).

[crusso]

Works for me!

[crusso]

Approved, but see suggestions for typos and issue filing