1 files changed, 53 insertions, 2 deletions

diff --git a/src/libutil/args.cc b/src/libutil/args.cc
index f829415d1..ceaabcfca 100644
--- a/src/libutil/args.cc
+++ b/src/libutil/args.cc
@@ -13,6 +13,19 @@ void Args::addFlag(Flag && flag_)
     if (flag->shortName) shortFlags[flag->shortName] = flag;
 }
 
+std::shared_ptr<std::set<std::string>> completions;
+
+std::string completionMarker = "___COMPLETE___";
+
+std::optional<std::string> needsCompletion(std::string_view s)
+{
+    if (!completions) return {};
+    auto i = s.find(completionMarker);
+    if (i != std::string::npos)
+        return std::string(s.begin(), i);
+    return {};
+}
+
 void Args::parseCmdline(const Strings & _cmdline)
 {
     Strings pendingArgs;
@@ -20,6 +33,13 @@ void Args::parseCmdline(const Strings & _cmdline)
 
     Strings cmdline(_cmdline);
 
+    if (auto s = getEnv("NIX_GET_COMPLETIONS")) {
+        size_t n = std::stoi(*s);
+        assert(n > 0 && n <= cmdline.size());
+        *std::next(cmdline.begin(), n - 1) += completionMarker;
+        completions = std::make_shared<decltype(completions)::element_type>();
+    }
+
     for (auto pos = cmdline.begin(); pos != cmdline.end(); ) {
 
         auto arg = *pos;
@@ -99,18 +119,32 @@ bool Args::processFlag(Strings::iterator & pos, Strings::iterator end)
     auto process = [&](const std::string & name, const Flag & flag) -> bool {
         ++pos;
         std::vector<std::string> args;
+        bool anyNeedsCompletion = false;
         for (size_t n = 0 ; n < flag.handler.arity; ++n) {
             if (pos == end) {
                 if (flag.handler.arity == ArityAny) break;
-                throw UsageError("flag '%s' requires %d argument(s)", name, flag.handler.arity);
+                if (anyNeedsCompletion)
+                    args.push_back("");
+                else
+                    throw UsageError("flag '%s' requires %d argument(s)", name, flag.handler.arity);
+            } else {
+                if (needsCompletion(*pos))
+                    anyNeedsCompletion = true;
+                args.push_back(*pos++);
             }
-            args.push_back(*pos++);
         }
         flag.handler.fun(std::move(args));
         return true;
     };
 
     if (string(*pos, 0, 2) == "--") {
+        if (auto prefix = needsCompletion(*pos)) {
+            for (auto & [name, flag] : longFlags) {
+                if (!hiddenCategories.count(flag->category)
+                    && hasPrefix(name, std::string(*prefix, 2)))
+                    completions->insert("--" + name);
+            }
+        }
         auto i = longFlags.find(string(*pos, 2));
         if (i == longFlags.end()) return false;
         return process("--" + i->first, *i->second);
@@ -123,6 +157,14 @@ bool Args::processFlag(Strings::iterator & pos, Strings::iterator end)
         return process(std::string("-") + c, *i->second);
     }
 
+    if (auto prefix = needsCompletion(*pos)) {
+        if (prefix == "-") {
+            completions->insert("--");
+            for (auto & [flag, _] : shortFlags)
+                completions->insert(std::string("-") + flag);
+        }
+    }
+
     return false;
 }
 
@@ -161,6 +203,10 @@ Args::Flag Args::Flag::mkHashTypeFlag(std::string && longName, HashType * ht)
         .description = "hash algorithm ('md5', 'sha1', 'sha256', or 'sha512')",
         .labels = {"hash-algo"},
         .handler = {[ht](std::string s) {
+            if (auto prefix = needsCompletion(s))
+                for (auto & type : hashTypes)
+                    if (hasPrefix(type, *prefix))
+                        completions->insert(type);
             *ht = parseHashType(s);
             if (*ht == htUnknown)
                 throw UsageError("unknown hash type '%1%'", s);
@@ -217,6 +263,11 @@ MultiCommand::MultiCommand(const Commands & commands)
 {
     expectedArgs.push_back(ExpectedArg{"command", 1, true, [=](std::vector<std::string> ss) {
         assert(!command);
+        if (auto prefix = needsCompletion(ss[0])) {
+            for (auto & [name, command] : commands)
+                if (hasPrefix(name, *prefix))
+                    completions->insert(name);
+        }
         auto i = commands.find(ss[0]);
         if (i == commands.end())
             throw UsageError("'%s' is not a recognised command", ss[0]);