common-eval-args: raise warning if --arg isn't a valid Nix identifier

See #496.

The core idea is to be able to do e.g.

    nix-instantiate -A some-nonfree-thing --arg config.allowUnfree true

which is currently not possible since `config.allowUnfree` is
interpreted as attribute name with a dot in it.

In order to change that (probably), Jade suggested to find out if there
are any folks out there relying on this behavior.

For such a use-case, it may still be possible to accept strings, i.e.
`--arg '"config.allowUnfree"'.

Change-Id: I986c73619fbd87a95b55e2f0ac03feaed3de2d2d

src/libcmd/common-eval-args.cc

@@ -9,8 +9,24 @@
 #include "store-api.hh"
 #include "command.hh"
 
+#include <regex>
+
 namespace nix {
 
+static std::regex const identifierRegex("^[A-Za-z_][A-Za-z0-9_'-]*$");
+static void warnInvalidNixIdentifier(const std::string & name)
+{
+    std::smatch match;
+    if (!std::regex_match(name, match, identifierRegex)) {
+        warn("This Nix invocation specifies a value for argument '%s' which isn't a valid \
+Nix identifier. The project is considering to drop support for this \
+or to require quotes around args that aren't valid Nix identifiers. \
+If you depend on this behvior, please reach out in \
+https://git.lix.systems/lix-project/lix/issues/496 so we can discuss \
+your use-case.", name);
+    }
+}
+
 MixEvalArgs::MixEvalArgs()
 {
     addFlag({
@@ -18,7 +34,10 @@ MixEvalArgs::MixEvalArgs()
         .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; }}
+        .handler = {[&](std::string name, std::string expr) {
+            warnInvalidNixIdentifier(name);
+            autoArgs[name] = 'E' + expr;
+        }}
     });
 
     addFlag({
@@ -26,7 +45,10 @@ MixEvalArgs::MixEvalArgs()
         .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; }},
+        .handler = {[&](std::string name, std::string s) {
+            warnInvalidNixIdentifier(name);
+            autoArgs[name] = 'S' + s;
+        }},
     });
 
     addFlag({