Add builtins.fetchClosure

This allows closures to be imported at evaluation time, without
requiring the user to configure substituters. E.g.

  builtins.fetchClosure {
    storePath = /nix/store/f89g6yi63m1ywfxj96whv5sxsm74w5ka-python3.9-sqlparse-0.4.2;
    from = "https://cache.ngi0.nixos.org";
  }

src/libexpr/primops/fetchClosure.cc

@@ -0,0 +1,62 @@
+#include "primops.hh"
+#include "store-api.hh"
+
+namespace nix {
+
+static void prim_fetchClosure(EvalState & state, const Pos & pos, Value * * args, Value & v)
+{
+    state.forceAttrs(*args[0], pos);
+
+    std::optional<StorePath> storePath;
+    std::optional<std::string> from;
+
+    for (auto & attr : *args[0]->attrs) {
+        if (attr.name == "storePath") {
+            PathSet context;
+            storePath = state.coerceToStorePath(*attr.pos, *attr.value, context);
+        }
+
+        else if (attr.name == "from")
+            from = state.forceStringNoCtx(*attr.value, *attr.pos);
+
+        else
+            throw Error({
+                .msg = hintfmt("attribute '%s' isn't supported in call to 'fetchClosure'", attr.name),
+                .errPos = pos
+            });
+    }
+
+    if (!storePath)
+        throw Error({
+            .msg = hintfmt("attribute '%s' is missing in call to 'fetchClosure'", "storePath"),
+            .errPos = pos
+        });
+
+    if (!from)
+        throw Error({
+            .msg = hintfmt("attribute '%s' is missing in call to 'fetchClosure'", "from"),
+            .errPos = pos
+        });
+
+    // FIXME: only allow some "trusted" store types (like BinaryCacheStore).
+    auto srcStore = openStore(*from);
+
+    copyClosure(*srcStore, *state.store, RealisedPath::Set { *storePath });
+
+    auto storePathS = state.store->printStorePath(*storePath);
+
+    v.mkString(storePathS, {storePathS});
+
+    // FIXME: in pure mode, require a CA path or a NAR hash of the
+    // top-level path.
+}
+
+static RegisterPrimOp primop_fetchClosure({
+    .name = "__fetchClosure",
+    .args = {"args"},
+    .doc = R"(
+    )",
+    .fun = prim_fetchClosure,
+});
+
+}

src/libexpr/primops/fetchTree.cc

@@ -145,7 +145,7 @@ static void fetchTree(
         if (!params.allowNameArgument)
             if (auto nameIter = attrs.find("name"); nameIter != attrs.end())
                 throw Error({
-                    .msg = hintfmt("attribute 'name' isn’t supported in call to 'fetchTree'"),
+                    .msg = hintfmt("attribute 'name' isn't supported in call to 'fetchTree'"),
                     .errPos = pos
                 });