Add shell completion for --override-flake

Requires moving the MixEvalArgs class from libexpr to libcmd because
that's where completeFlakeRef is.

src/libcmd/common-eval-args.cc

@@ -0,0 +1,105 @@
+#include "common-eval-args.hh"
+#include "shared.hh"
+#include "filetransfer.hh"
+#include "util.hh"
+#include "eval.hh"
+#include "fetchers.hh"
+#include "registry.hh"
+#include "flake/flakeref.hh"
+#include "store-api.hh"
+#include "command.hh"
+
+namespace nix {
+
+MixEvalArgs::MixEvalArgs()
+{
+    auto category = "Common evaluation options";
+
+    addFlag({
+        .longName = "arg",
+        .description = "Pass the value *expr* as the argument *name* to Nix functions.",
+        .category = category,
+        .labels = {"name", "expr"},
+        .handler = {[&](std::string name, std::string expr) { autoArgs[name] = 'E' + expr; }}
+    });
+
+    addFlag({
+        .longName = "argstr",
+        .description = "Pass the string *string* as the argument *name* to Nix functions.",
+        .category = category,
+        .labels = {"name", "string"},
+        .handler = {[&](std::string name, std::string s) { autoArgs[name] = 'S' + s; }},
+    });
+
+    addFlag({
+        .longName = "include",
+        .shortName = 'I',
+        .description = "Add *path* to the list of locations used to look up `<...>` file names.",
+        .category = category,
+        .labels = {"path"},
+        .handler = {[&](std::string s) { searchPath.push_back(s); }}
+    });
+
+    addFlag({
+        .longName = "impure",
+        .description = "Allow access to mutable paths and repositories.",
+        .category = category,
+        .handler = {[&]() {
+            evalSettings.pureEval = false;
+        }},
+    });
+
+    addFlag({
+        .longName = "override-flake",
+        .description = "Override the flake registries, redirecting *original-ref* to *resolved-ref*.",
+        .category = category,
+        .labels = {"original-ref", "resolved-ref"},
+        .handler = {[&](std::string _from, std::string _to) {
+            auto from = parseFlakeRef(_from, absPath("."));
+            auto to = parseFlakeRef(_to, absPath("."));
+            fetchers::Attrs extraAttrs;
+            if (to.subdir != "") extraAttrs["dir"] = to.subdir;
+            fetchers::overrideRegistry(from.input, to.input, extraAttrs);
+        }},
+        .completer = {[&](size_t, std::string_view prefix) {
+            completeFlakeRef(openStore(), prefix);
+        }}
+    });
+
+    addFlag({
+        .longName = "eval-store",
+        .description = "The Nix store to use for evaluations.",
+        .category = category,
+        .labels = {"store-url"},
+        .handler = {&evalStoreUrl},
+    });
+}
+
+Bindings * MixEvalArgs::getAutoArgs(EvalState & state)
+{
+    auto res = state.buildBindings(autoArgs.size());
+    for (auto & i : autoArgs) {
+        auto v = state.allocValue();
+        if (i.second[0] == 'E')
+            state.mkThunk_(*v, state.parseExprFromString(i.second.substr(1), absPath(".")));
+        else
+            v->mkString(((std::string_view) i.second).substr(1));
+        res.insert(state.symbols.create(i.first), v);
+    }
+    return res.finish();
+}
+
+Path lookupFileArg(EvalState & state, std::string_view s)
+{
+    if (isUri(s)) {
+        return state.store->toRealPath(
+            fetchers::downloadTarball(
+                state.store, resolveUri(s), "source", false).first.storePath);
+    } 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
+        return absPath(std::string(s));
+}
+
+}