perf(bash-v2): standard completion optimizations (#1683)

Refactor to remove two loops over the entire list of candidates.

Format descriptions only for completions that are actually going to be
displayed, instead of for all candidates.

Format descriptions inline in completions array, removing need for a
command substitution/subshell and a printf escape per displayed
completion.

bash_completionsV2.go

@@ -177,40 +177,28 @@ __%[1]s_handle_standard_completion_case() {
     local tab=$'\t' comp
 
     local longest=0
+    local compline
     # Look for the longest completion so that we can format things nicely
-    while IFS='' read -r comp; do
+    while IFS='' read -r compline; do
         # Strip any description before checking the length
-        comp=${comp%%%%$tab*}
+        comp=${compline%%%%$tab*}
         # Only consider the completions that match
         comp=$(compgen -W "$comp" -- "$cur")
+        [[ -z $comp ]] && continue
+        COMPREPLY+=("$compline")
         if ((${#comp}>longest)); then
             longest=${#comp}
         fi
     done < <(printf "%%s\n" "${out}")
 
-    local completions=()
-    while IFS='' read -r comp; do
-        if [ -z "$comp" ]; then
-            continue
-        fi
-
-        __%[1]s_debug "Original comp: $comp"
-        comp="$(__%[1]s_format_comp_descriptions "$comp" "$longest")"
-        __%[1]s_debug "Final comp: $comp"
-        completions+=("$comp")
-    done < <(printf "%%s\n" "${out}")
-
-    while IFS='' read -r comp; do
-        COMPREPLY+=("$comp")
-    done < <(compgen -W "${completions[*]}" -- "$cur")
-
     # If there is a single completion left, remove the description text
     if [ ${#COMPREPLY[*]} -eq 1 ]; then
         __%[1]s_debug "COMPREPLY[0]: ${COMPREPLY[0]}"
-        comp="${COMPREPLY[0]%%%% *}"
+        comp="${COMPREPLY[0]%%%%$tab*}"
         __%[1]s_debug "Removed description from single completion, which is now: ${comp}"
-        COMPREPLY=()
-        COMPREPLY+=("$comp")
+        COMPREPLY[0]=$comp
+    else # Format the descriptions
+        __%[1]s_format_comp_descriptions $longest
     fi
 }
 
@@ -230,43 +218,47 @@ __%[1]s_handle_special_char()
 __%[1]s_format_comp_descriptions()
 {
     local tab=$'\t'
-    local comp="$1"
-    local longest=$2
-
-    # Properly format the description string which follows a tab character if there is one
-    if [[ "$comp" == *$tab* ]]; then
-        desc=${comp#*$tab}
-        comp=${comp%%%%$tab*}
-
-        # $COLUMNS stores the current shell width.
-        # Remove an extra 4 because we add 2 spaces and 2 parentheses.
-        maxdesclength=$(( COLUMNS - longest - 4 ))
-
-        # Make sure we can fit a description of at least 8 characters
-        # if we are to align the descriptions.
-        if [[ $maxdesclength -gt 8 ]]; then
-            # Add the proper number of spaces to align the descriptions
-            for ((i = ${#comp} ; i < longest ; i++)); do
-                comp+=" "
-            done
-        else
-            # Don't pad the descriptions so we can fit more text after the completion
-            maxdesclength=$(( COLUMNS - ${#comp} - 4 ))
-        fi
+    local comp desc maxdesclength
+    local longest=$1
+
+    local i ci
+    for ci in ${!COMPREPLY[*]}; do
+        comp=${COMPREPLY[ci]}
+        # Properly format the description string which follows a tab character if there is one
+        if [[ "$comp" == *$tab* ]]; then
+            __%[1]s_debug "Original comp: $comp"
+            desc=${comp#*$tab}
+            comp=${comp%%%%$tab*}
 
-        # If there is enough space for any description text,
-        # truncate the descriptions that are too long for the shell width
-        if [ $maxdesclength -gt 0 ]; then
-            if [ ${#desc} -gt $maxdesclength ]; then
-                desc=${desc:0:$(( maxdesclength - 1 ))}
-                desc+="…"
+            # $COLUMNS stores the current shell width.
+            # Remove an extra 4 because we add 2 spaces and 2 parentheses.
+            maxdesclength=$(( COLUMNS - longest - 4 ))
+
+            # Make sure we can fit a description of at least 8 characters
+            # if we are to align the descriptions.
+            if [[ $maxdesclength -gt 8 ]]; then
+                # Add the proper number of spaces to align the descriptions
+                for ((i = ${#comp} ; i < longest ; i++)); do
+                    comp+=" "
+                done
+            else
+                # Don't pad the descriptions so we can fit more text after the completion
+                maxdesclength=$(( COLUMNS - ${#comp} - 4 ))
             fi
-            comp+="  ($desc)"
-        fi
-    fi
 
-    # Must use printf to escape all special characters
-    printf "%%q" "${comp}"
+            # If there is enough space for any description text,
+            # truncate the descriptions that are too long for the shell width
+            if [ $maxdesclength -gt 0 ]; then
+                if [ ${#desc} -gt $maxdesclength ]; then
+                    desc=${desc:0:$(( maxdesclength - 1 ))}
+                    desc+="…"
+                fi
+                comp+="  ($desc)"
+            fi
+            COMPREPLY[ci]=$comp
+            __%[1]s_debug "Final comp: $comp"
+        fi
+    done
 }
 
 __start_%[1]s()