libutil/args: dont crash completion when receiving incorrect number of arguments

When using completion, the number of the word for which the shell
requests completion is provided in the environment variable
`NIX_GET_COMPLETIONS`. When the number smaller than 1 is or larger
than the number of arguments nix coredumps as a assert is violated.

This change removes the assert and instead throws an exception informing
the user that their autocomplete is most likely misconfigured.

Change-Id: I821719e470e576b6f63c06beb097338b53d183e0

lix/libutil/args.cc

@@ -87,7 +87,8 @@ void RootArgs::parseCmdline(const Strings & _cmdline)
 
     if (auto s = getEnv("NIX_GET_COMPLETIONS")) {
         size_t n = std::stoi(*s);
-        assert(n > 0 && n <= cmdline.size());
+        if (!(n > 0 && n <= cmdline.size()))
+            throw UsageError("Invalid word number to get completion for: %zu\n. Your autocompletions might be misconfigured", n);
         *std::next(cmdline.begin(), n - 1) += completionMarker;
         completions = std::make_shared<Completions>();
         verbosity = lvlError;

tests/functional/completions.sh

@@ -37,6 +37,12 @@ EOF
 [[ "$(NIX_GET_COMPLETIONS=1 nix buil)" == $'normal\nbuild\t' ]]
 [[ "$(NIX_GET_COMPLETIONS=2 nix flake metad)" == $'normal\nmetadata\t' ]]
 
+# Test how completion  fails if the number is not a valid index for the number of arguments
+NIX_GET_COMPLETIONS=0 expectStderr 1 nix \
+    | grepQuiet "error: Invalid word number to get completion for:"
+NIX_GET_COMPLETIONS=4 expectStderr 1  nix build a \
+    | grepQuiet "error: Invalid word number to get completion for:"
+
 # Filename completion
 [[ "$(NIX_GET_COMPLETIONS=2 nix build ./f)" == $'filenames\n./foo\t' ]]
 [[ "$(NIX_GET_COMPLETIONS=2 nix build ./nonexistent)" == $'filenames' ]]