Add real completions for ./configure

This relies on the new `read --line/-L` support as an entire parser for
the output of `./configure --help`  was written in fishscript. Also
doesn't work without 72f32e6d8a7905b064680ec4b578c41dea62bf84.

The completion script is slow... a function of both the autotools
configure script itself being written in a shell script combined with a
fishscript output parser.

fish's own `./configure --help` takes around 350ms to execute, while
`__fish_parse_configure ./configure` (which runs that behind the scenes)
takes around 660ms to run, all-in-all - a not insignificant overhead.

Output can be cached (based off of ./configure hash or mtime) in the
future if this is a big deal.

CHANGELOG.md

@@ -59,17 +59,18 @@ This section is for changes merged to the `major` branch that are not also merge
 ## Other significant changes
 - Command substitution output is now limited to 10 MB by default (#3822).
 - Added completions for
-  - `j` (autojump #4344)
   - `bd` (#4472)
+  - `configure`
+  - `j` (autojump #4344)
   - `jhipster` (#4472)
   - `ngrok` (#4642)
   - `port`
 - Improved completions for
-  - `git` (#4395, #4396, #4592)
   - `brew`
   - `diskutil`
-  - `yarn`
+  - `git` (#4395, #4396, #4592)
   - `ssh` (#4344)
+  - `yarn`
 
 --
 

share/completions/configure.fish

@@ -10,3 +10,7 @@ complete -c configure -l exec-prefix -d "Architecture-dependent install director
 complete -c configure -l build -d "Configure for building on BUILD" -x
 complete -c configure -l host -d "Cross-compile to build programs to run on HOST" -x
 complete -c configure -l target -d "Configure for building compilers for TARGET" -x
+
+# use autoconf's --help to generate completions:
+echo "sourcing configure completions"
+complete -c 'configure' -a '(__fish_parse_configure (commandline | string replace -r "(.*configure) .*" "\$1"))'

share/functions/__fish_parse_configure.fish

@@ -0,0 +1,62 @@
+function __fish_parse_configure
+	if test (count $argv) -ne 1
+		echo "Usage: parse_configure path/to/configure" 1>&2
+		return 1
+	end
+
+	# `complete` parses `./configure` as `configure` so we have to handle all paths, not just ./
+	if not test -x $argv[1]
+		printf "Cannot find or execute '%s'\n" $argv[1] 1>&2
+		return 1
+	end
+
+# Must support output along the lines of
+#  -h, --help              display this help and exit
+#      --help=short        display options specific to this package
+#      --help=recursive    display the short help of all the included packages
+#  -V, --version           display version information and exit
+#  -q, --quiet, --silent   do not print `checking ...' messages
+
+	set -l next_line
+	set -l line
+	set -l buffer
+	eval $argv[1] --help 2>/dev/null | while test (string length -- "$next_line") -gt 0 || read -lL next_line
+		# In autoconfigure scripts, the first column wraps at 26 chars
+		# echo next_line: $next_line
+		# echo old_line: $line
+		if test (string length -- "$line") -eq 0
+			set line $next_line
+			set next_line "" # mark it as consumed
+			continue
+		else if string match -qr '^(   |\t){2,}[^-]\S*' -- $next_line
+			# echo "continuation line found. Old value of line: " \"$line\"
+			set line "$line "(string trim $next_line)
+			set next_line "" # mark it as consumed
+			continue
+		end
+
+		# echo line: $line
+
+		# Search for one or more strings starting with `-` separated by commas
+		if string replace -fr '^\s+(-.*?)\s+([^\s\-].*)' '$1\n$2' -- $line | read -lL opts description
+			for opt in (string split -n , -- $opts | string trim)
+
+				if string match -qr -- '--.*=\[.*\]' $opt
+					# --option=[OPTIONAL_VALUE]
+					string replace -r -- '(--.*)=.*' '$1' $opt | read opt
+				else if string match -qr -- '--.*=[A-Z]+' $opt
+					# --option=CLASS_OF_VALUE (eg FILE or DIR)
+					string replace -r -- '(--.*)=.*' '$1' $opt | read opt
+				else if string match -qr -- '--.*=\S+' $opt
+					# --option=literal_value, leave as-is
+				else if string match -qr -- '-[^-]\b' $opt
+					# short option, leave as-is
+				end
+
+				echo "$opt"\t"$description" # parsed by `complete` as value and description
+			end
+		end
+
+		set line ""
+	end
+end