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

900894b09a

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

src/nvim/option_vars.h

@@ -16,6 +16,7 @@
                                     ///< the same.
 #define P_EXPAND       0x10U        ///< environment expansion.  NOTE: P_EXPAND can
                                     ///< never be used for local or hidden options
+#define P_NO_DEF_EXP   0x20U        ///< do not expand default value
 #define P_NODEFAULT    0x40U        ///< don't set to default value
 #define P_DEF_ALLOCED  0x80U        ///< default value is in allocated memory, must
                                     ///< use free() when assigning new value
@@ -51,8 +52,7 @@
 #define P_RWINONLY     0x10000000U  ///< only redraw current window
 #define P_MLE          0x20000000U  ///< under control of 'modelineexpr'
 #define P_FUNC         0x40000000U  ///< accept a function reference or a lambda
-
-#define P_NO_DEF_EXP   0x80000000U  ///< Do not expand default value.
+#define P_COLON        0x80000000U  ///< values use colons to create sublists
 
 #define HIGHLIGHT_INIT \
   "8:SpecialKey,~:EndOfBuffer,z:TermCursor,Z:TermCursorNC,@:NonText,d:Directory,e:ErrorMsg," \
@@ -183,7 +183,7 @@
 #define CPO_VI          "aAbBcCdDeEfFiIJKlLmMnoOpPqrRsStuvWxXyZ$!%+>;_"
 
 // characters for p_ww option:
-#define WW_ALL          "bshl<>[],~"
+#define WW_ALL          "bshl<>[]~"
 
 // characters for p_mouse option:
 #define MOUSE_NORMAL    'n'             // use mouse in Normal mode
@@ -757,9 +757,9 @@ extern char *p_vfile;           ///< 'verbosefile'
 EXTERN int p_warn;              ///< 'warn'
 EXTERN char *p_wop;             ///< 'wildoptions'
 EXTERN unsigned wop_flags;
-#define WOP_TAGFILE             0x01
-#define WOP_PUM                 0x02
-#define WOP_FUZZY               0x04
+#define WOP_FUZZY               0x01
+#define WOP_TAGFILE             0x02
+#define WOP_PUM                 0x04
 EXTERN OptInt p_window;         ///< 'window'
 EXTERN char *p_wak;             ///< 'winaltkeys'
 EXTERN char *p_wig;             ///< 'wildignore'