nix-env/nix-instantiate/nix-build: Support URIs

For instance, you can install Firefox from a specific Nixpkgs revision
like this:

  $ nix-env -f https://github.com/NixOS/nixpkgs/archive/63def04891a0abc328b1b0b3a78ec02c58f48583.tar.gz -iA firefox

Or build a package from the latest nixpkgs-unstable channel:

  $ nix-build https://nixos.org/channels/nixpkgs-unstable/nixexprs.tar.xz -A hello

src/libexpr/common-opts.cc

@@ -1,5 +1,6 @@
 #include "common-opts.hh"
-#include "../libmain/shared.hh"
+#include "shared.hh"
+#include "download.hh"
 #include "util.hh"
 
 
@@ -53,7 +54,9 @@ bool parseSearchPathArg(Strings::iterator & i,
 
 Path lookupFileArg(EvalState & state, string s)
 {
-    if (s.size() > 2 && s.at(0) == '<' && s.at(s.size() - 1) == '>') {
+    if (isUri(s))
+        return downloadFileCached(s, true);
+    else if (s.size() > 2 && s.at(0) == '<' && s.at(s.size() - 1) == '>') {
         Path p = s.substr(1, s.size() - 2);
         return state.findFile(p);
     } else

src/libexpr/download.cc

@@ -223,4 +223,13 @@ Path downloadFileCached(const string & url, bool unpack)
 }
 
 
+bool isUri(const string & s)
+{
+    size_t pos = s.find("://");
+    if (pos == string::npos) return false;
+    string scheme(s, 0, pos);
+    return scheme == "http" || scheme == "https";
+}
+
+
 }

src/libexpr/download.hh

@@ -17,4 +17,6 @@ Path downloadFileCached(const string & url, bool unpack);
 
 MakeError(DownloadError, Error)
 
+bool isUri(const string & s);
+
 }

src/libexpr/parser.y

@@ -601,15 +601,6 @@ Expr * EvalState::parseExprFromString(const string & s, const Path & basePath)
 }
 
 
-bool isUri(const string & s)
-{
-    size_t pos = s.find("://");
-    if (pos == string::npos) return false;
-    string scheme(s, 0, pos);
-    return scheme == "http" || scheme == "https";
-}
-
-
 void EvalState::addToSearchPath(const string & s, bool warn)
 {
     size_t pos = s.find('=');

src/nix-instantiate/nix-instantiate.cc

@@ -183,10 +183,10 @@ int main(int argc, char * * argv)
         } else if (files.empty() && !fromArgs)
             files.push_back("./default.nix");
 
-        foreach (Strings::iterator, i, files) {
+        for (auto & i : files) {
             Expr * e = fromArgs
-                ? state.parseExprFromString(*i, absPath("."))
-                : state.parseExprFromFile(resolveExprPath(lookupFileArg(state, *i)));
+                ? state.parseExprFromString(i, absPath("."))
+                : state.parseExprFromFile(resolveExprPath(lookupFileArg(state, i)));
             processExpr(state, attrPaths, parseOnly, strict, autoArgs,
                 evalOnly, outputKind, xmlOutputSourceLocation, e);
         }