* Revived the old "nix-store --delete" operation that deletes the

specified paths from the Nix store.  However, this operation is
  safe: it refuses to delete anything that the garbage collector
  wouldn't delete.

src/nix-store/help.txt

@@ -7,6 +7,7 @@ Operations:
   --realise / -r: ensure path validity; if a derivation, ensure that
       validity of the outputs
   --add / -A: copy a path to the Nix store
+  --delete: safely delete paths from the Nix store
   --query / -q: query information
 
   --register-substitutes: register a substitute expression (dangerous!)

src/nix-store/main.cc

@@ -518,7 +518,7 @@ static void opGC(Strings opFlags, Strings opArgs)
 
     PathSet result;
     PrintFreed freed(action == gcDeleteDead);
-    collectGarbage(action, result, freed.bytesFreed);
+    collectGarbage(action, PathSet(), result, freed.bytesFreed);
 
     if (action != gcDeleteDead) {
         for (PathSet::iterator i = result.begin(); i != result.end(); ++i)
@@ -527,6 +527,25 @@ static void opGC(Strings opFlags, Strings opArgs)
 }
 
 
+/* Remove paths from the Nix store if possible (i.e., if they do not
+   have any remaining referrers and are not reachable from any GC
+   roots). */
+static void opDelete(Strings opFlags, Strings opArgs)
+{
+    if (!opFlags.empty()) throw UsageError("unknown flag");
+
+    PathSet pathsToDelete;
+    for (Strings::iterator i = opArgs.begin();
+         i != opArgs.end(); ++i)
+        pathsToDelete.insert(fixPath(*i));
+    
+    PathSet dummy;
+    PrintFreed freed(true);
+    collectGarbage(gcDeleteSpecific, pathsToDelete,
+        dummy, freed.bytesFreed);
+}
+
+
 /* A sink that writes dump output to stdout. */
 struct StdoutSink : DumpSink
 {
@@ -621,6 +640,8 @@ void run(Strings args)
             op = opAddFixed;
         else if (arg == "--print-fixed-path")
             op = opPrintFixedPath;
+        else if (arg == "--delete")
+            op = opDelete;
         else if (arg == "--query" || arg == "-q")
             op = opQuery;
         else if (arg == "--register-substitutes")