zlib: simplify arguments & explain --shared & --static (#490608)

pkgs/development/libraries/zlib/default.nix

@@ -3,25 +3,31 @@
   stdenv,
   fetchurl,
   shared ? !stdenv.hostPlatform.isStatic,
-  static ? true,
-  # If true, a separate .static output is created and the .a is moved there.
-  # If false, and if `{ static = true; }`, the .a stays in the main output.
-  splitStaticOutput ? shared && static && !stdenv.hostPlatform.isWindows,
+  # zlib's hand-written `./configure` script is a bit buggy, and it always
+  # builds the static library. The `--enable-shared` argument (alias to
+  # `--shared`) makes it build the shared library too. The `--static` argument
+  # is particularly confusing - if used after `--shared` it disables the build
+  # of the shared library. See also:
+  #
+  # - https://github.com/madler/zlib/pull/394
+  #
+  # Since --static is aliased to --disable-shared, we use this flag in the
+  # `configureFlags` below, along with a few more relevant arguments to
+  # `stdenv.mkDerivation`.
+  #
+  # We avoid the closure size penalty of this buggy upstream behavior by using
+  # a dedicated output for the static library, which is fortuenatly also
+  # supported by the `--libdir` argument to `./configure`.
+  splitStaticOutput ? shared && !stdenv.hostPlatform.isWindows,
   testers,
   minizip,
 }:
 
-# Without either the build will actually still succeed because the build
-# system makes an arbitrary choice, but we shouldn't be so indecisive.
-assert shared || static;
-
 # Note: this package is used for bootstrapping fetchurl, and thus
 # cannot use fetchpatch! All mutable patches (generated by GitHub or
 # cgit) that are needed here should be included directly in Nixpkgs as
 # files.
 
-assert splitStaticOutput -> static;
-
 stdenv.mkDerivation (finalAttrs: {
   pname = "zlib";
   version = "1.3.2";
@@ -40,17 +46,14 @@ stdenv.mkDerivation (finalAttrs: {
       hash = "sha256-uzKaCizQJ00FUZ1hxmfAYuBpkNcuEl7i36jeZPARnRY=";
     };
 
-  postPatch =
-    lib.optionalString stdenv.hostPlatform.isDarwin ''
-      substituteInPlace configure \
-        --replace '/usr/bin/libtool' '${stdenv.cc.targetPrefix}ar' \
-        --replace 'AR="libtool"' 'AR="${stdenv.cc.targetPrefix}ar"' \
-        --replace 'ARFLAGS="-o"' 'ARFLAGS="-r"'
-    ''
-    + lib.optionalString stdenv.hostPlatform.isCygwin ''
-      substituteInPlace win32/zlib.def \
-        --replace-fail 'gzopen_w' ""
-    '';
+  postPatch = ''
+    substituteInPlace configure \
+      --replace-fail '/usr/bin/libtool' '${stdenv.cc.targetPrefix}ar' \
+      --replace-fail 'AR="libtool"' 'AR="${stdenv.cc.targetPrefix}ar"' \
+      --replace-fail 'ARFLAGS="-o"' 'ARFLAGS="-r"'
+    substituteInPlace win32/zlib.def \
+      --replace-fail 'gzopen_w' ""
+  '';
 
   strictDeps = true;
   outputs = [
@@ -71,22 +74,9 @@ stdenv.mkDerivation (finalAttrs: {
     "--includedir=${placeholder "dev"}/include"
     "--sharedlibdir=${placeholder "out"}/lib"
     "--libdir=${placeholder (if splitStaticOutput then "static" else "out")}/lib"
-  ]
-  # For zlib's ./configure (as of version 1.2.11), the order
-  # of --static/--shared flags matters!
-  # `--shared --static` builds only static libs, while
-  # `--static --shared` builds both.
-  # So we use the latter order to be able to build both.
-  # Also, giving just `--shared` builds both,
-  # giving just `--static` builds only static,
-  # and giving nothing builds both.
-  # So we have 3 possible ways to build both:
-  # `--static --shared`, `--shared` and giving nothing.
-  # Of these, we choose `--static --shared`, for clarity and simpler
-  # conditions.
-  ++ lib.optional static "--static"
-  ++ lib.optional shared "--shared";
-  # We do the right thing manually, above, so don't need these.
+    # See comment near splitStaticOutput argument
+    (lib.enableFeature shared "shared")
+  ];
   dontDisableStatic = true;
   dontAddStaticConfigureFlags = true;
 
@@ -130,7 +120,7 @@ stdenv.mkDerivation (finalAttrs: {
 
   # We don't strip on static cross-compilation because of reports that native
   # stripping corrupted the target library; see commit 12e960f5 for the report.
-  dontStrip = stdenv.hostPlatform != stdenv.buildPlatform && static;
+  dontStrip = stdenv.hostPlatform != stdenv.buildPlatform;
   configurePlatforms = [ ];
 
   installFlags = lib.optionals (stdenv.hostPlatform.isMinGW || stdenv.hostPlatform.isCygwin) [