Update Zig snapshot, Nix Zig version, and Zig install instructions

BUILDING_FROM_SOURCE.md

@@ -8,7 +8,7 @@ To build the compiler, you need these installed:
 * `libunwind` (macOS should already have this one installed)
 * `libc++-dev`
 * Python 2.7 (Windows only), `python-is-python3` (Ubuntu)
-* a particular version of Zig (see below)
+* Zig (0.7.0 or greater, see below)
 * a particular version of LLVM (see below)
 
 To run the test suite (via `cargo test`), you additionally need to install:
@@ -24,32 +24,10 @@ MacOS systems should already have `libunwind`, but other systems will need to in
 Some systems may already have `libc++-dev` on them, but if not, you may need to install it. (On Ubuntu, this can be done with `sudo apt-get install libc++-dev`.)
 
 ### Zig
-We use a specific version of Zig, a build off the the commit `0088efc4b`. The latest tagged version of Zig, 0.6.0, doesn't include the feature to emit LLVM ir, which is a core feature of how we use Zig. To download this specific version, you can:
-* use the following commands on Debian/Ubuntu (on other distros, steps should be essentially the same):
-  ```
-  cd /tmp
-  # download the files
-  wget https://ziglang.org/builds/zig-linux-x86_64-0.6.0+0088efc4b.tar.xz
-  # uncompress:
-  xz -d zig-linux-x86_64-0.6.0+0088efc4b.tar.xz
-  # untar:
-  tar xvf zig-linux-x86_64-0.6.0+0088efc4b.tar
-  # move the files into /opt:
-  sudo mkdir -p /opt/zig
-  sudo mv zig-linux-x86_64-0.6.0+0088efc4b/* /opt/zig/
-  ```
-  Then add `/opt/zig/` to your `PATH` (e.g. in `~/.bashrc`).
-  
-  Reload your `.bashrc` file: `source ~/.bashrc` and test that `zig` is
-  an available command.
 
-* [macOS](https://ziglang.org/builds/zig-macos-x86_64-0.6.0+0088efc4b.tar.xz)
-
-Alternatively, any recent master branch build should work. To install the latest master branch build you can use:
-* `brew install zig --HEAD` (on macos)
-* `snap install zig --classic --edge` (on ubunutu)
-
-Once 0.7.0 is released, we'll switch back to installing the tagged releases and this process will get easier.
+If you're on MacOS, you can install with `brew install zig`
+If you're on Ubuntu and use Snap, you can install with `snap install zig --classic --beta`
+For any other OS, checkout the [Zig installation page](https://github.com/ziglang/zig/wiki/Install-Zig-from-a-Package-Manager)
 
 ### LLVM
 

nix/zig.nix

@@ -1,42 +1,37 @@
 { pkgs, isMacOS, isAarch64 }:
 
-# We require at least specific commit of Zig after the latest tagged
-# release (0.6.0), so we just download the binaries for that commit
-
-let
-  version = "0.6.0+0088efc4b";
-  osName =
-    if isMacOS
-      then "macos"
-      else "linux";
-  archName =
-    if isAarch64
-      then "aarch64"
-      else "x86_64";
-  archiveName = "zig-${osName}-${archName}-${version}";
-  sha256 = 
-    if isMacOS
-      then "665c1a7f472cfc5e0715f0ddf6ff8409fb749ac91cbbae68c443b4a37ebd058e"
-    else if isAarch64
-      then "116ms44vx4xz57m9z9lsgrxd1g22qp00m5qbmklky8xdd2jmj24w"
-      else "bab70ae3bd0af538022bc3ef50d8f34fa8dceac39ba7d9e5d528eee7e6d5a1cf";
-in
-pkgs.stdenv.mkDerivation {
-  pname = "zig";
-  version = version;
-  buildInputs = [ pkgs.gzip ];
-  src = pkgs.fetchurl {
-    inherit sha256;
-    name = "${archiveName}.tar.xz";
-    url = "https://ziglang.org/builds/${archiveName}.tar.xz";
-  };
-  phases = [ "installPhase" ];
-  installPhase = ''
-    mkdir -p $out/bin
-    tar -xf $src
-    cp ${archiveName}/zig $out/zig
-    cp -r ${archiveName}/lib $out/lib
-    ln -s "$out/zig" "$out/bin/zig"
-    chmod +x $out/bin/zig
-  '';
-}
+if isMacOS then
+  let
+    version = "0.7.1";
+    osName =
+      if isMacOS
+        then "macos"
+        else "linux";
+    archName =
+      if isAarch64
+        then "aarch64"
+        else "x86_64";
+    archiveName = "zig-${osName}-${archName}-${version}";
+    sha256 = "845cb17562978af0cf67e3993f4e33330525eaf01ead9386df9105111e3bc519";
+  in
+  pkgs.stdenv.mkDerivation {
+    pname = "zig";
+    version = version;
+    buildInputs = [ pkgs.gzip ];
+    src = pkgs.fetchurl {
+      inherit sha256;
+      name = "${archiveName}.tar.xz";
+      url = "https://ziglang.org/builds/${archiveName}.tar.xz";
+    };
+    phases = [ "installPhase" ];
+    installPhase = ''
+      mkdir -p $out/bin
+      tar -xf $src
+      cp ${archiveName}/zig $out/zig
+      cp -r ${archiveName}/lib $out/lib
+      ln -s "$out/zig" "$out/bin/zig"
+      chmod +x $out/bin/zig
+    '';
+  }
+else
+  pkgs.zig

shell.nix

@@ -8,10 +8,10 @@ in with {
   # Look here for information about how pin version of nixpkgs
   #  → https://nixos.wiki/wiki/FAQ/Pinning_Nixpkgs
   pkgs = import (builtins.fetchGit {
-    name = "nixpkgs-2020-11-24";
+    name = "nixpkgs-2021-01-05";
     url = "https://github.com/nixos/nixpkgs/";
     ref = "refs/heads/nixpkgs-unstable";
-    rev = "6625284c397b44bc9518a5a1567c1b5aae455c08";
+    rev = "f53c431645da8e6760268092aa10f736b5cfb117";
   }) { };
 
   isMacOS = currentOS == "darwin";