* Allow redirections in search path entries. E.g. if you have a

directory

    /home/eelco/src/stdenv-updates

  that you want to use as the directory for import such as

    with (import <nixpkgs> { });

  then you can say

    $ nix-build -I nixpkgs=/home/eelco/src/stdenv-updates

src/libexpr/eval.hh

@@ -213,8 +213,9 @@ private:
 
     std::map<Path, Expr *> parseTrees;
 
-    Paths searchPath;
-    Paths::iterator searchPathInsertionPoint;
+    typedef list<std::pair<string, Path> > SearchPath;
+    SearchPath searchPath;
+    SearchPath::iterator searchPathInsertionPoint;
 
 public:
     

src/libexpr/parser.y

@@ -530,18 +530,36 @@ Expr * EvalState::parseExprFromString(const string & s, const Path & basePath)
 
 void EvalState::addToSearchPath(const string & s)
 {
-    Path path = absPath(s);
+    size_t pos = s.find('=');
+    string prefix;
+    Path path;
+    if (pos == string::npos) {
+        path = s;
+    } else {
+        prefix = string(s, 0, pos);
+        path = string(s, pos + 1);
+    }
+    
+    path = absPath(path);
     if (pathExists(path)) {
         debug(format("adding path `%1%' to the search path") % path);
-        searchPath.insert(searchPathInsertionPoint, path);
+        searchPath.insert(searchPathInsertionPoint, std::pair<string, Path>(prefix, path));
     }
 }
 
 
 Path EvalState::findFile(const string & path)
 {
-    foreach (Paths::iterator, i, searchPath) {
-        Path res = *i + "/" + path;
+    foreach (SearchPath::iterator, i, searchPath) {
+        Path res;
+        if (i->first.empty()) 
+            res = i->second + "/" + path;
+        else {
+            if (path.compare(0, i->first.size(), i->first) != 0 ||
+                (path.size() > i->first.size() && path[i->first.size()] != '/'))
+                continue;
+            res = i->second + "/" + string(path, i->first.size());
+        }
         if (pathExists(res)) return canonPath(res);
     }
     return "";

tests/lang/eval-okay-search-path.exp

@@ -1 +1 @@
-"abc"
+"abcc"

tests/lang/eval-okay-search-path.flags

@@ -1 +1 @@
--I lang/dir1 -I lang/dir2
+-I lang/dir1 -I lang/dir2 -I dir5=lang/dir3

tests/lang/eval-okay-search-path.nix

@@ -1,3 +1,3 @@
-import <a.nix> + import <b.nix> + import <c.nix>
+import <a.nix> + import <b.nix> + import <c.nix> + import <dir5/c.nix>