feat(options)!: disallow setting hidden options #28400

Problem:
There are three different ways of marking an option as hidden, `enable_if
= false`, `hidden = true` and `immutable = true`. These also have different
behaviors. Options hidden with `enable_if = false` can't have their value
fetched using Vim script or the API, but options hidden with `hidden = true` or
`immutable = true` can. On the other hand, options with `hidden = true` do not
error when trying to set their value, but options with `immutable = true` do.

Solution:
Remove `enable_if = false`, remove the `hidden` property for options, and use
`immutable = true` to mark an option as hidden instead. Also make hidden option
variable pointers always point to the default value, which allows fetching the
value of every hidden option using Vim script and the API. This does also mean
that trying to set a hidden option will now give an error instead of just being
ignored.

4 files changed, 16 insertions, 11 deletions

diff --git a/runtime/autoload/zip.vim b/runtime/autoload/zip.vim
index 172de98d36..a8ea6a3fab 100644
--- a/runtime/autoload/zip.vim
+++ b/runtime/autoload/zip.vim
@@ -408,7 +408,9 @@ fun! s:SetSaneOpts()
   let dict.shellslash = &shellslash
 
   let &report = 10
-  let &shellslash = 0
+  if exists('+shellslash')
+    let &shellslash = 0
+  endif
 
   return dict
 endfun
diff --git a/runtime/doc/news.txt b/runtime/doc/news.txt
index 3ed1442a96..3a8277f566 100644
--- a/runtime/doc/news.txt
+++ b/runtime/doc/news.txt
@@ -113,6 +113,8 @@ OPTIONS
   of just string |global-local| options.
 • `:setlocal {option}<` copies the global value to the local value for number
   and boolean |global-local| options instead of removing the local value.
+• Setting |hidden-options| now gives an error. In particular, setting
+  'noshellslash' is now only allowed on Windows.
 
 PLUGINS
 
diff --git a/runtime/doc/options.txt b/runtime/doc/options.txt
index 8877054b77..3217f5c565 100644
--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -642,11 +642,12 @@ Hidden options						*hidden-options*
 
 Not all options are supported in all versions.  This depends on the supported
 features and sometimes on the system.  A remark about this is in curly braces
-below.  When an option is not supported it may still be set without getting an
-error, this is called a hidden option.  You can't get the value of a hidden
-option though, it is not stored.
+below.  When an option is not supported, it is called a hidden option.  Trying
+to get the value of a hidden option will not give an error, it will return the
+default value for that option instead. You can't change the value of a hidden
+option.
 
-To test if option "foo" can be used with ":set" use something like this: >
+To test if "foo" is a valid option name, use something like this: >
 	if exists('&foo')
 This also returns true for a hidden option.  To test if option "foo" is really
 supported use something like this: >
@@ -1573,7 +1574,7 @@ A jump table for the options with a short description can be found at |Q_op|.
 						*'completeslash'* *'csl'*
 'completeslash' 'csl'	string	(default "")
 			local to buffer
-			only for MS-Windows
+			only modifiable in MS-Windows
 	When this option is set it overrules 'shellslash' for completion:
 	- When this option is set to "slash", a forward slash is used for path
 	  completion in insert mode. This is useful when editing HTML tag, or
@@ -5277,9 +5278,9 @@ A jump table for the options with a short description can be found at |Q_op|.
 	security reasons.
 
 			*'shellslash'* *'ssl'* *'noshellslash'* *'nossl'*
-'shellslash' 'ssl'	boolean	(default off)
+'shellslash' 'ssl'	boolean	(default on, Windows: off)
 			global
-			only for MS-Windows
+			only modifiable in MS-Windows
 	When set, a forward slash is used when expanding file names.  This is
 	useful when a Unix-like shell is used instead of cmd.exe.  Backward
 	slashes can still be typed, but they are changed to forward slashes by
diff --git a/runtime/lua/vim/_meta/options.lua b/runtime/lua/vim/_meta/options.lua
index f9886957a7..45ab14a774 100644
--- a/runtime/lua/vim/_meta/options.lua
+++ b/runtime/lua/vim/_meta/options.lua
@@ -1106,7 +1106,7 @@ vim.bo.cot = vim.bo.completeopt
 vim.go.completeopt = vim.o.completeopt
 vim.go.cot = vim.go.completeopt
 
---- 		only for MS-Windows
+--- 		only modifiable in MS-Windows
 --- When this option is set it overrules 'shellslash' for completion:
 --- - When this option is set to "slash", a forward slash is used for path
 ---   completion in insert mode. This is useful when editing HTML tag, or
@@ -5547,7 +5547,7 @@ vim.o.srr = vim.o.shellredir
 vim.go.shellredir = vim.o.shellredir
 vim.go.srr = vim.go.shellredir
 
---- 		only for MS-Windows
+--- 		only modifiable in MS-Windows
 --- When set, a forward slash is used when expanding file names.  This is
 --- useful when a Unix-like shell is used instead of cmd.exe.  Backward
 --- slashes can still be typed, but they are changed to forward slashes by
@@ -5564,7 +5564,7 @@ vim.go.srr = vim.go.shellredir
 --- Also see 'completeslash'.
 ---
 --- @type boolean
-vim.o.shellslash = false
+vim.o.shellslash = true
 vim.o.ssl = vim.o.shellslash
 vim.go.shellslash = vim.o.shellslash
 vim.go.ssl = vim.go.shellslash