vim-patch:9.0.1958: cannot complete option values

Problem:  cannot complete option values
Solution: Add completion functions for several options

Add cmdline tab-completion for setting string options

Add tab-completion for setting string options on the cmdline using
`:set=` (along with `:set+=` and `:set-=`).

The existing tab completion for setting options currently only works
when nothing is typed yet, and it only fills in with the existing value,
e.g. when the user does `:set diffopt=<Tab>` it will be completed to
`set diffopt=internal,filler,closeoff` and nothing else. This isn't too
useful as a user usually wants auto-complete to suggest all the possible
values, such as 'iblank', or 'algorithm:patience'.

For set= and set+=, this adds a new optional callback function for each
option that can be invoked when doing completion. This allows for each
option to have control over how completion works. For example, in
'diffopt', it will suggest the default enumeration, but if `algorithm:`
is selected, it will further suggest different algorithm types like
'meyers' and 'patience'. When using set=, the existing option value will
be filled in as the first choice to preserve the existing behavior. When
using set+= this won't happen as it doesn't make sense.

For flag list options (e.g. 'mouse' and 'guioptions'), completion will
take into account existing typed values (and in the case of set+=, the
existing option value) to make sure it doesn't suggest duplicates.

For set-=, there is a new `ExpandSettingSubtract` function which will
handle flag list and comma-separated options smartly, by only suggesting
values that currently exist in the option.

Note that Vim has some existing code that adds special handling for
'filetype', 'syntax', and misc dir options like 'backupdir'. This change
preserves them as they already work, instead of converting to the new
callback API for each option.

closes: vim/vim#13182

https://github.com/vim/vim/commit/900894b09a95398dfc75599e9f0aa2ea25723384

Co-authored-by: Yee Cheng Chin <ychin.git@gmail.com>

2 files changed, 22 insertions, 7 deletions

diff --git a/runtime/doc/cmdline.txt b/runtime/doc/cmdline.txt
index 5ee1a2af13..c586b30863 100644
--- a/runtime/doc/cmdline.txt
+++ b/runtime/doc/cmdline.txt
@@ -493,16 +493,26 @@ example, to match only files that end in ".c": >
 	:e *.c$
 This will not match a file ending in ".cpp".  Without the "$" it does match.
 
-The old value of an option can be obtained by hitting 'wildchar' just after
-the '='.  For example, typing 'wildchar' after ":set dir=" will insert the
-current value of 'dir'.  This overrules file name completion for the options
-that take a file name.
-
 If you would like using <S-Tab> for CTRL-P in an xterm, put this command in
 your .cshrc: >
 	xmodmap -e "keysym Tab = Tab Find"
 And this in your vimrc: >
 	:cmap <Esc>[1~ <C-P>
+<						*complete-set-option*
+When setting an option using |:set=|, the old value of an option can be
+obtained by hitting 'wildchar' just after the '='.  For example, typing
+'wildchar' after ":set dir=" will insert the current value of 'dir'.  This
+overrules file name completion for the options that take a file name.
+
+When using |:set=|, |:set+=|, or |:set^=|, string options that have
+pre-defined names or syntax (e.g. 'diffopt', 'listchars') or are a list of
+single-character flags (e.g. 'shortmess') will also present a list of possible
+values for completion when using 'wildchar'.
+
+When using |:set-=|, comma-separated options like 'diffopt' or 'backupdir'
+will show each item separately.  Flag list options like 'shortmess' will show
+both the entire old value and the individual flags.  Otherwise completion will
+just fill in with the entire old value.
 
 ==============================================================================
 3. Ex command-lines					*cmdline-lines*
diff --git a/runtime/doc/options.txt b/runtime/doc/options.txt
index 84cff775f6..35fb08a9a4 100644
--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -52,14 +52,16 @@ achieve special effects.  These options come in three forms:
 			  'lines'
 			Warning: This may have a lot of side effects.
 
-						*:set-args* *E487* *E521*
+						*:set-args* *:set=* *E487* *E521*
 :se[t] {option}={value}		or
 :se[t] {option}:{value}
 			Set string or number option to {value}.
 			For numeric options the value can be given in decimal,
 			hex (preceded with 0x) or octal (preceded with '0').
 			The old value can be inserted by typing 'wildchar' (by
-			default this is a <Tab>).  See |cmdline-completion|.
+			default this is a <Tab>).  Many string options with
+			fixed syntax also support completing known values.
+			See |cmdline-completion| and |complete-set-option|.
 			White space between {option} and '=' is allowed and
 			will be ignored.  White space between '=' and {value}
 			is not allowed.
@@ -93,6 +95,9 @@ achieve special effects.  These options come in three forms:
 			When the option is a list of flags, {value} must be
 			exactly as they appear in the option.  Remove flags
 			one by one to avoid problems.
+			The individual values from a comma separated list or
+			list of flags can be inserted by typing 'wildchar'.
+			See |complete-set-option|.
 			Also see |:set-args| above.
 
 The {option} arguments to ":set" may be repeated.  For example: >