fetchurl: cleanup, better errors

Also fix what seems like bugs in uncommon `stdenv`s.

pkgs/build-support/fetchurl/default.nix

···
-{ stdenvNoCC, curl }: # Note that `curl' may be `null', in case of the native stdenvNoCC.
+{ lib, stdenvNoCC, curl }: # Note that `curl' may be `null', in case of the native stdenvNoCC.
 
 let
 
···
   # "gnu", etc.).
   sites = builtins.attrNames mirrors;
 
-  impureEnvVars = stdenvNoCC.lib.fetchers.proxyImpureEnvVars ++ [
+  impureEnvVars = lib.fetchers.proxyImpureEnvVars ++ [
     # This variable allows the user to pass additional options to curl
     "NIX_CURL_FLAGS"
 
···
 , passthru ? {}
 }:
 
-assert builtins.isList urls;
-assert (urls == []) != (url == "");
 assert sha512 != "" -> builtins.compareVersions "1.11" builtins.nixVersion <= 0;
 
+let
 
-let
+  urls_ =
+    if urls != [] && url == "" then
+      (if lib.isList urls then urls
+       else throw "`urls` is not a list")
+    else if urls == [] && url != "" then [url]
+    else throw "fetchurl requires either `url` or `urls` to be set";
 
-  hasHash = showURLs || (outputHash != "" && outputHashAlgo != "")
-    || sha1 != "" || sha256 != "" || sha512 != "";
-  urls_ = if urls != [] then urls else [url];
+  hash_ =
+    if md5 != "" then throw "fetchurl does not support md5 anymore, please use sha256 or sha512"
+    else if (outputHash != "" && outputHashAlgo != "") then { inherit outputHashAlgo outputHash; }
+    else if sha512 != "" then { outputHashAlgo = "sha512"; outputHash = sha512; }
+    else if sha256 != "" then { outputHashAlgo = "sha256"; outputHash = sha256; }
+    else if sha1   != "" then { outputHashAlgo = "sha1";   outputHash = sha1; }
+    else throw "fetchurl requires a hash for fixed-output derivation: ${lib.concatStringsSep ", " urls_}";
 
 in
 
-if md5 != "" then throw "fetchurl does not support md5 anymore, please use sha256 or sha512"
-else if (!hasHash) then throw "Specify hash for fetchurl fixed-output derivation: ${stdenvNoCC.lib.concatStringsSep ", " urls_}"
-else stdenvNoCC.mkDerivation {
+stdenvNoCC.mkDerivation {
   name =
     if showURLs then "urls"
     else if name != "" then name
···
   preferHashedMirrors = true;
 
   # New-style output content requirements.
-  outputHashAlgo = if outputHashAlgo != "" then outputHashAlgo else
-      if sha512 != "" then "sha512" else if sha256 != "" then "sha256" else "sha1";
-  outputHash = if outputHash != "" then outputHash else
-      if sha512 != "" then sha512 else if sha256 != "" then sha256 else sha1;
+  inherit (hash_) outputHashAlgo outputHash;
 
   outputHashMode = if (recursiveHash || executable) then "recursive" else "flat";
 

pkgs/stdenv/darwin/default.nix

···
         initialPath  = [ bootstrapTools ];
 
         fetchurlBoot = import ../../build-support/fetchurl {
+          inherit lib;
           stdenvNoCC = stage0.stdenv;
           curl = bootstrapTools;
         };

pkgs/stdenv/freebsd/default.nix

···
     inherit bootstrapTools;
 
     fetchurl = import ../../build-support/fetchurl {
-      inherit stdenv;
+      inherit lib;
+      stdenvNoCC = stdenv;
       curl = bootstrapTools;
     };
 

pkgs/stdenv/native/default.nix

···
     };
 
     fetchurl = import ../../build-support/fetchurl {
-      inherit stdenv;
+      inherit lib stdenvNoCC;
       # Curl should be in /usr/bin or so.
       curl = null;
     };

pkgs/top-level/all-packages.nix

···
 
   # `fetchurl' downloads a file from the network.
   fetchurl = import ../build-support/fetchurl {
-    inherit stdenvNoCC;
+    inherit lib stdenvNoCC;
     # On darwin, libkrb5 needs bootstrap_cmds which would require
     # converting many packages to fetchurl_boot to avoid evaluation cycles.
     curl = buildPackages.curl.override (lib.optionalAttrs stdenv.isDarwin { gssSupport = false; });