patch 9.1.1329: cannot get information about command line completion

Problem:  cannot get information about command line completion
Solution: add CmdlineLeavePre autocommand and cmdcomplete_info() Vim
          script function (Girish Palya)

This commit introduces two features to improve introspection and control
over command-line completion in Vim:

- Add CmdlineLeavePre autocmd event:

  A new event triggered just before leaving the command line and before
  CmdlineLeave. It allows capturing completion-related state that is
  otherwise cleared by the time CmdlineLeave fires.

- Add cmdcomplete_info() Vim script function:

  Returns a Dictionary with details about the current command-line
  completion state.

These are similar in spirit to InsertLeavePre and complete_info(),
but focused on command-line mode.

**Use case:**

In [[PR #16759](https://github.com/vim/vim/pull/16759)], two examples
demonstrate command-line completion: one for live grep, and another for
fuzzy file finding. However, both examples share two key limitations:

1. **Broken history recall (`<Up>`)**
   When selecting a completion item via `<Tab>` or `<C-n>`, the original
pattern used for searching (e.g., a regex or fuzzy string) is
overwritten in the command-line history. This makes it impossible to
recall the original query later.
   This is especially problematic for interactive grep workflows, where
it’s useful to recall a previous search and simply select a different
match from the menu.

2. **Lack of default selection on `<CR>`**
   Often, it’s helpful to allow `<CR>` (Enter) to accept the first match
in the completion list, even when no item is explicitly selected. This
behavior is particularly useful in fuzzy file finding.

----
Below are the updated examples incorporating these improvements:

**Live grep, fuzzy find file, fuzzy find buffer:**

```vim
command! -nargs=+ -complete=customlist,GrepComplete Grep VisitFile()
def GrepComplete(arglead: string, cmdline: string, cursorpos: number):
list<any>
    return arglead->len() > 1 ? systemlist($'grep -REIHns "{arglead}"' ..
       ' --exclude-dir=.git --exclude=".*" --exclude="tags" --exclude="*.swp"') : []
enddef
def VisitFile()
    if (selected_match != null_string)
        var qfitem = getqflist({lines: [selected_match]}).items[0]
        if qfitem->has_key('bufnr') && qfitem.lnum > 0
            var pos = qfitem.vcol > 0 ? 'setcharpos' : 'setpos'
            exec $':b +call\ {pos}(".",\ [0,\ {qfitem.lnum},\ {qfitem.col},\ 0]) {qfitem.bufnr}'
            setbufvar(qfitem.bufnr, '&buflisted', 1)
        endif
    endif
enddef
nnoremap <leader>g :Grep<space>
nnoremap <leader>G :Grep <c-r>=expand("<cword>")<cr>
command! -nargs=* -complete=customlist,FuzzyFind Find
execute(selected_match != '' ? $'edit {selected_match}' : '')
var allfiles: list<string>
autocmd CmdlineEnter : allfiles = null_list
def FuzzyFind(arglead: string, _: string, _: number): list<string>
    if allfiles == null_list
        allfiles = systemlist($'find {get(g:, "fzfind_root", ".")} \! \(
-path "*/.git" -prune -o -name "*.swp" \) -type f -follow')
    endif
    return arglead == '' ? allfiles : allfiles->matchfuzzy(arglead)
enddef
nnoremap <leader><space> :<c-r>=execute('let
fzfind_root="."')\|''<cr>Find<space><c-@>
nnoremap <leader>fv :<c-r>=execute('let
fzfind_root="$HOME/.vim"')\|''<cr>Find<space><c-@>
nnoremap <leader>fV :<c-r>=execute('let
fzfind_root="$VIMRUNTIME"')\|''<cr>Find<space><c-@>
command! -nargs=* -complete=customlist,FuzzyBuffer Buffer execute('b '
.. selected_match->matchstr('\d\+'))
def FuzzyBuffer(arglead: string, _: string, _: number): list<string>
    var bufs = execute('buffers', 'silent!')->split("\n")
    var altbuf = bufs->indexof((_, v) => v =~ '^\s*\d\+\s\+#')
    if altbuf != -1
        [bufs[0], bufs[altbuf]] = [bufs[altbuf], bufs[0]]
    endif
    return arglead == '' ? bufs : bufs->matchfuzzy(arglead)
enddef
nnoremap <leader><bs> :Buffer <c-@>
var selected_match = null_string
autocmd CmdlineLeavePre : SelectItem()
def SelectItem()
    selected_match = ''
    if getcmdline() =~ '^\s*\%(Grep\|Find\|Buffer\)\s'
        var info = cmdcomplete_info()
        if info != {} && info.pum_visible && !info.matches->empty()
            selected_match = info.selected != -1 ? info.matches[info.selected] : info.matches[0]
            setcmdline(info.cmdline_orig). # Preserve search pattern in history
        endif
    endif
enddef
```

**Auto-completion snippet:**

```vim
set wim=noselect:lastused,full wop=pum wcm=<C-@> wmnu
autocmd CmdlineChanged : CmdComplete()
def CmdComplete()
    var [cmdline, curpos] = [getcmdline(), getcmdpos()]
    if getchar(1, {number: true}) == 0  # Typehead is empty (no more pasted input)
            && !pumvisible() && curpos == cmdline->len() + 1
            && cmdline =~ '\%(\w\|[*/:.-]\)$' && cmdline !~ '^\d\+$'  # Reduce noise
        feedkeys("\<C-@>", "ti")
        SkipCmdlineChanged()  # Suppress redundant completion attempts
        # Remove <C-@> that get inserted when no items are available
        timer_start(0, (_) => getcmdline()->substitute('\%x00', '', 'g')->setcmdline())
    endif
enddef
cnoremap <expr> <up> SkipCmdlineChanged("\<up>")
cnoremap <expr> <down> SkipCmdlineChanged("\<down>")
autocmd CmdlineEnter : set bo+=error
autocmd CmdlineLeave : set bo-=error
def SkipCmdlineChanged(key = ''): string
    set ei+=CmdlineChanged
    timer_start(0, (_) => execute('set ei-=CmdlineChanged'))
    return key != '' ? ((pumvisible() ? "\<c-e>" : '') .. key) : ''
enddef
```

These customizable snippets can serve as *lightweight* and *native*
alternatives to picker plugins like **FZF** or **Telescope** for common,
everyday workflows. Also, live grep snippet can replace **cscope**
without the overhead of building its database.

closes: #17115

Signed-off-by: Girish Palya <girishji@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/doc/autocmd.txt

@@ -1,4 +1,4 @@
-*autocmd.txt*   For Vim version 9.1.  Last change: 2025 Apr 04
+*autocmd.txt*   For Vim version 9.1.  Last change: 2025 Apr 21
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar
@@ -398,6 +398,7 @@ Name			triggered by ~
 |CmdlineChanged|	after a change was made to the command-line text
 |CmdlineEnter|		after the cursor moves to the command line
 |CmdlineLeave|		before the cursor leaves the command line
+|CmdlineLeavePre|	before preparing to leave the command line
 
 |InsertEnter|		starting Insert mode
 |InsertChange|		when typing <Insert> while in Insert or Replace mode
@@ -639,6 +640,18 @@ CmdlineLeave			Before leaving the command line; including
 				<afile> is set to a single character,
 				indicating the type of command-line.
 				|cmdwin-char|
+							*CmdlineLeavePre*
+CmdlineLeavePre			Just before leaving the command line, and
+				before |CmdlineLeave|.  Useful for capturing
+				completion info with |cmdcomplete_info()|, as
+				this information is cleared before
+				|CmdlineLeave| is triggered.  Triggered for
+				non-interactive use of ":" in a mapping, but
+				not when using |<Cmd>|.  Also triggered when
+				abandoning the command line by typing CTRL-C
+				or <Esc>.  <afile> is set to a single
+				character indicating the command-line type.
+				See |cmdwin-char| for details.
 							*CmdwinEnter*
 CmdwinEnter			After entering the command-line window.
 				Useful for setting options specifically for

runtime/doc/builtin.txt

@@ -1,4 +1,4 @@
-*builtin.txt*	For Vim version 9.1.  Last change: 2025 Apr 18
+*builtin.txt*	For Vim version 9.1.  Last change: 2025 Apr 21
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -129,6 +129,8 @@ charidx({string}, {idx} [, {countcc} [, {utf16}]])
 chdir({dir})			String	change current working directory
 cindent({lnum})			Number	C indent for line {lnum}
 clearmatches([{win}])		none	clear all matches
+cmdcomplete_info()		Dict	get current cmdline completion
+					information
 col({expr} [, {winid}])		Number	column byte index of cursor or mark
 complete({startcol}, {matches}) none	set Insert mode completion
 complete_add({expr})		Number	add completion match
@@ -1832,6 +1834,29 @@ clearmatches([{win}])					*clearmatches()*
 		Return type: |Number|
 
 
+cmdcomplete_info([{what}])				*cmdcomplete_info()*
+		Returns a |Dictionary| with information about cmdline
+		completion.  See |cmdline-completion|.
+		The items are:
+		   cmdline_orig	The original command-line string before
+				completion began.
+		   pum_visible	|TRUE| if popup menu is visible.
+				See |pumvisible()|.
+		   matches	List of all completion candidates. Each item
+				is a string.
+		   selected	Selected item index.  First index is zero.
+				Index is -1 if no item is selected (showing
+				typed text only, or the last completion after
+				no item is selected when using the <Up> or
+				<Down> keys)
+
+		Returns an empty |Dictionary| if no completion was attempted,
+		if there was only one candidate and it was fully completed, or
+		if an error occurred.
+
+		Return type: dict<any>
+
+
 col({expr} [, {winid}])					*col()*
 		The result is a Number, which is the byte index of the column
 		position given with {expr}.

runtime/doc/tags

@@ -4079,6 +4079,7 @@ Cmdline-mode	cmdline.txt	/*Cmdline-mode*
 CmdlineChanged	autocmd.txt	/*CmdlineChanged*
 CmdlineEnter	autocmd.txt	/*CmdlineEnter*
 CmdlineLeave	autocmd.txt	/*CmdlineLeave*
+CmdlineLeavePre	autocmd.txt	/*CmdlineLeavePre*
 CmdwinEnter	autocmd.txt	/*CmdwinEnter*
 CmdwinLeave	autocmd.txt	/*CmdwinLeave*
 ColorScheme	autocmd.txt	/*ColorScheme*
@@ -6568,6 +6569,7 @@ close_cb	channel.txt	/*close_cb*
 closure	eval.txt	/*closure*
 cmdarg-variable	eval.txt	/*cmdarg-variable*
 cmdbang-variable	eval.txt	/*cmdbang-variable*
+cmdcomplete_info()	builtin.txt	/*cmdcomplete_info()*
 cmdline-arguments	vi_diff.txt	/*cmdline-arguments*
 cmdline-changed	version5.txt	/*cmdline-changed*
 cmdline-completion	cmdline.txt	/*cmdline-completion*

runtime/doc/usr_41.txt

@@ -1,4 +1,4 @@
-*usr_41.txt*	For Vim version 9.1.  Last change: 2025 Mar 30
+*usr_41.txt*	For Vim version 9.1.  Last change: 2025 Apr 21
 
 		     VIM USER MANUAL - by Bram Moolenaar
 
@@ -1111,6 +1111,7 @@ Command line:					*command-line-functions*
 	getcmdwintype()		return the current command-line window type
 	getcompletion()		list of command-line completion matches
 	fullcommand()		get full command name
+	cmdcomplete_info()	get current completion information
 
 Quickfix and location lists:			*quickfix-functions*
 	getqflist()		list of quickfix errors

runtime/doc/version9.txt

@@ -1,4 +1,4 @@
-*version9.txt*  For Vim version 9.1.  Last change: 2025 Apr 18
+*version9.txt*  For Vim version 9.1.  Last change: 2025 Apr 21
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar
@@ -41685,6 +41685,7 @@ Functions: ~
 |base64_encode()|	encode a blob into a base64 string
 |blob2str()|		convert a blob into a List of strings
 |bindtextdomain()|	set message lookup translation base path
+|cmdcomplete_info()|	get current cmdline completion info
 |diff()|		diff two Lists of strings
 |filecopy()|		copy a file {from} to {to}
 |foreach()|		apply function to List items
@@ -41708,6 +41709,7 @@ Functions: ~
 
 Autocommands: ~
 
+|CmdlineLeavePre|	before preparing to leave the command line
 |CursorMovedC|		after the cursor was moved in the command-line
 |KeyInputPre|		before processing any key event in any mode
 |SessionWritePost|	after writing the session file |:mksession|