completions: Show description in zsh completions

Uses the second part of NIX_GET_COMPLETIONS if available.

Change-Id: I35efaaea3f35eea25a8de723ab32ceecd054e7df
Fixes: https://git.lix.systems/lix-project/lix/issues/910

doc/manual/rl-next/zsh-completions-desc.md

@@ -0,0 +1,10 @@
+---
+synopsis: add description to zsh completions
+issues: [fj#910]
+cls: [3632]
+category: "Fixes"
+credits: [matthewbauer]
+---
+
+Emit descriptions when completing args in zsh completions. This uses the descriptions we already
+provided in NIX\_GET\_COMPLETIONS.

misc/zsh/completion.zsh

@@ -1,24 +1,28 @@
 #compdef nix
 
 function _nix() {
-  local ifs_bk="$IFS"
   local input=("${(Q)words[@]}")
-  IFS=$'\n'
-  local res=($(NIX_GET_COMPLETIONS=$((CURRENT - 1)) "$input[@]" 2>/dev/null))
+  local -a suggestions # what value to actually use on completion
+  local -a suggestions_display # what to display to the user
+  local tpe suggestion description
+  local ifs_bk="$IFS"
+  IFS=$'\t'
+  NIX_GET_COMPLETIONS=$((CURRENT - 1)) "$input[@]" 2>/dev/null | {read tpe; while IFS=$'\t' read -r suggestion description; do
+    suggestions+=("$suggestion")
+    if [ -n "$description" ]; then
+      suggestions_display+=("$suggestion -- $description")
+    else
+      suggestions_display+=("$suggestion")
+    fi
+  done}
   IFS="$ifs_bk"
-  local tpe="${${res[1]}%%>	*}"
-  local -a suggestions
-  declare -a suggestions
-  for suggestion in ${res:1}; do
-    suggestions+=("${suggestion%%	*}")
-  done
   local -a args
   if [[ "$tpe" == filenames ]]; then
     args+=('-f')
   elif [[ "$tpe" == attrs ]]; then
     args+=('-S' '')
   fi
-  compadd -J nix "${args[@]}" -a suggestions
+  compadd -J nix "${args[@]}" -d suggestions_display -a suggestions
 }
 
 _nix "$@"