Add a matrix of stdenvs to the flake

For a (currently hardcoded and limited) list of stdenvs,
make `.#$nix-${stdenvName}` correspond to a Nix built with the
corresponding stdenv.

For example, `.#nix-${clang11Stdenv}` is Nix built with clang11.

Likewise, `devShells.x86_64-linux.clang11StdenvPackages` is a development
shell for Nix with clang11, that can be used with

```shell
nix develop .#clang11StdenvPackages
```

Fix #4129

/cc @Pamplemousse

flake.nix

@@ -22,15 +22,36 @@
 
  crossSystems = [ "armv6l-linux" "armv7l-linux" ];
 
+ stdenvs = [ "gccStdenv" "clangStdenv" "clang11Stdenv" "stdenv" ];
+
  forAllSystems = f: nixpkgs.lib.genAttrs systems (system: f system);
+ forAllSystemsAndStdenvs = f: forAllSystems (system:
+ nixpkgs.lib.listToAttrs
+ (map
+ (n:
+ nixpkgs.lib.nameValuePair "${n}Packages" (
+ f system n
+ )) stdenvs
+ )
+ );
+
+ forAllStdenvs = stdenvs: f: nixpkgs.lib.genAttrs stdenvs (stdenv: f stdenv);
 
  # Memoize nixpkgs for different platforms for efficiency.
- nixpkgsFor = forAllSystems (system:
- import nixpkgs {
- inherit system;
- overlays = [ self.overlay ];
- }
- );
+ nixpkgsFor =
+ let stdenvsPackages = forAllSystemsAndStdenvs
+ (system: stdenv:
+ import nixpkgs {
+ inherit system;
+ overlays = [
+ (overlayFor (p: p.${stdenv}))
+ ];
+ }
+ );
+ in
+ # Add the `stdenvPackages` at toplevel, both because these are the ones
+ # we want most of the time and for backwards compatibility
+ forAllSystems (system: stdenvsPackages.${system} // stdenvsPackages.${system}.stdenvPackages);
 
  commonDeps = pkgs: with pkgs; rec {
  # Use "busybox-sandbox-shell" if present,
@@ -255,18 +276,15 @@
  $(cat ${installerClosureInfo}/store-paths)
  '';
 
- in {
-
- # A Nixpkgs overlay that overrides the 'nix' and
- # 'nix.perl-bindings' packages.
- overlay = final: prev: {
-
+ overlayFor = getStdenv: final: prev:
+ let currentStdenv = getStdenv final; in
+ {
  nixStable = prev.nix;
 
  # Forward from the previous stage as we don’t want it to pick the lowdown override
  nixUnstable = prev.nixUnstable;
 
- nix = with final; with commonDeps pkgs; stdenv.mkDerivation {
+ nix = with final; with commonDeps pkgs; currentStdenv.mkDerivation {
  name = "nix-${version}";
  inherit version;
 
@@ -288,9 +306,9 @@
  mkdir -p $out/lib
  cp -pd ${boost}/lib/{libboost_context*,libboost_thread*,libboost_system*} $out/lib
  rm -f $out/lib/*.a
- ${lib.optionalString stdenv.isLinux ''
+ ${lib.optionalString currentStdenv.isLinux ''
  chmod u+w $out/lib/*.so.*
- patchelf --set-rpath $out/lib:${stdenv.cc.cc.lib}/lib $out/lib/libboost_thread.so.*
+ patchelf --set-rpath $out/lib:${currentStdenv.cc.cc.lib}/lib $out/lib/libboost_thread.so.*
  ''}
  '';
 
@@ -317,7 +335,7 @@
 
  strictDeps = true;
 
- passthru.perl-bindings = with final; stdenv.mkDerivation {
+ passthru.perl-bindings = with final; currentStdenv.mkDerivation {
  name = "nix-perl-${version}";
 
  src = self;
@@ -336,8 +354,8 @@
  pkgs.perl
  boost
  ]
- ++ lib.optional (stdenv.isLinux || stdenv.isDarwin) libsodium
- ++ lib.optional stdenv.isDarwin darwin.apple_sdk.frameworks.Security;
+ ++ lib.optional (currentStdenv.isLinux || currentStdenv.isDarwin) libsodium
+ ++ lib.optional currentStdenv.isDarwin darwin.apple_sdk.frameworks.Security;
 
  configureFlags = ''
  --with-dbi=${perlPackages.DBI}/${pkgs.perl.libPrefix}
@@ -351,7 +369,7 @@
 
  };
 
- lowdown-nix = with final; stdenv.mkDerivation rec {
+ lowdown-nix = with final; currentStdenv.mkDerivation rec {
  name = "lowdown-0.9.0";
 
  src = lowdown-src;
@@ -361,15 +379,20 @@
  nativeBuildInputs = [ buildPackages.which ];
 
  configurePhase = ''
- ${if (stdenv.isDarwin && stdenv.isAarch64) then "echo \"HAVE_SANDBOX_INIT=false\" > configure.local" else ""}
+ ${if (currentStdenv.isDarwin && currentStdenv.isAarch64) then "echo \"HAVE_SANDBOX_INIT=false\" > configure.local" else ""}
  ./configure \
  PREFIX=${placeholder "dev"} \
  BINDIR=${placeholder "bin"}/bin
-  '';
+ '';
  };
-
  };
 
+ in {
+
+ # A Nixpkgs overlay that overrides the 'nix' and
+ # 'nix.perl-bindings' packages.
+ overlay = overlayFor (p: p.stdenv);
+
  hydraJobs = {
 
  # Binary package for various platforms.
@@ -610,15 +633,22 @@
  doInstallCheck = true;
  installCheckFlags = "sysconfdir=$(out)/etc";
  };
- }) crossSystems)));
+ }) crossSystems)) // (builtins.listToAttrs (map (stdenvName:
+ nixpkgsFor.${system}.lib.nameValuePair
+ "nix-${stdenvName}"
+ nixpkgsFor.${system}."${stdenvName}Packages".nix
+ ) stdenvs))
+ );
 
  defaultPackage = forAllSystems (system: self.packages.${system}.nix);
 
- devShell = forAllSystems (system:
+ devShell = forAllSystems (system: self.devShells.${system}.stdenvPackages);
+
+ devShells = forAllSystemsAndStdenvs (system: stdenv:
  with nixpkgsFor.${system};
  with commonDeps pkgs;
 
- stdenv.mkDerivation {
+ nixpkgsFor.${system}.${stdenv}.mkDerivation {
  name = "nix";
 
  outputs = [ "out" "dev" "doc" ];