4 files changed, 101 insertions, 3 deletions
diff --git a/src/nvim/eval.lua b/src/nvim/eval.lua
index 4863d8013d..3e89489459 100644
--- a/src/nvim/eval.lua
+++ b/src/nvim/eval.lua
@@ -332,6 +332,7 @@ return {
     setcharpos={args=2, base=2},
     setcharsearch={args=1, base=1},
     setcmdpos={args=1, base=1},
+    setcmdline={args={1, 2}, base=1},
     setcursorcharpos={args={1, 3}, base=1},
     setenv={args=2, base=2},
     setfperm={args=2, base=1},
diff --git a/src/nvim/eval/funcs.c b/src/nvim/eval/funcs.c
index 581f187140..2ed9ed34f4 100644
--- a/src/nvim/eval/funcs.c
+++ b/src/nvim/eval/funcs.c
@@ -7619,6 +7619,31 @@ static void f_setcharsearch(typval_T *argvars, typval_T *rettv, EvalFuncData fpt
   }
 }
 
+/// "setcmdline()" function
+static void f_setcmdline(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
+{
+  if (argvars[0].v_type != VAR_STRING || argvars[0].vval.v_string == NULL) {
+    emsg(_(e_stringreq));
+    return;
+  }
+
+  int pos = -1;
+  if (argvars[1].v_type != VAR_UNKNOWN) {
+    bool error = false;
+
+    pos = (int)tv_get_number_chk(&argvars[1], &error) - 1;
+    if (error) {
+      return;
+    }
+    if (pos < 0) {
+      emsg(_(e_positive));
+      return;
+    }
+  }
+
+  rettv->vval.v_number = set_cmdline_str(argvars[0].vval.v_string, pos);
+}
+
 /// "setcmdpos()" function
 static void f_setcmdpos(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
 {
diff --git a/src/nvim/ex_getln.c b/src/nvim/ex_getln.c
index 081aa80778..84eacf3997 100644
--- a/src/nvim/ex_getln.c
+++ b/src/nvim/ex_getln.c
@@ -2373,9 +2373,9 @@ end:
   return cmdpreview_type != 0;
 }
 
-static int command_line_changed(CommandLineState *s)
+/// Trigger CmdlineChanged autocommands.
+static void do_autocmd_cmdlinechanged(int firstc)
 {
-  // Trigger CmdlineChanged autocommands.
   if (has_event(EVENT_CMDLINECHANGED)) {
     TryState tstate;
     Error err = ERROR_INIT;
@@ -2383,7 +2383,7 @@ static int command_line_changed(CommandLineState *s)
     dict_T *dict = get_v_event(&save_v_event);
 
     char firstcbuf[2];
-    firstcbuf[0] = (char)(s->firstc > 0 ? s->firstc : '-');
+    firstcbuf[0] = (char)firstc;
     firstcbuf[1] = 0;
 
     // set v:event to a dictionary with information about the commandline
@@ -2403,6 +2403,12 @@ static int command_line_changed(CommandLineState *s)
       redrawcmd();
     }
   }
+}
+
+static int command_line_changed(CommandLineState *s)
+{
+  // Trigger CmdlineChanged autocommands.
+  do_autocmd_cmdlinechanged(s->firstc > 0 ? s->firstc : '-');
 
   if (s->firstc == ':'
       && current_sctx.sc_sid == 0    // only if interactive
@@ -3991,6 +3997,32 @@ int get_cmdline_screen_pos(void)
   return p->cmdspos;
 }
 
+/// Set the command line str to "str".
+/// @return  1 when failed, 0 when OK.
+int set_cmdline_str(const char *str, int pos)
+{
+  CmdlineInfo *p = get_ccline_ptr();
+
+  if (p == NULL) {
+    return 1;
+  }
+
+  int len = (int)STRLEN(str);
+  realloc_cmdbuff(len + 1);
+  p->cmdlen = len;
+  STRCPY(p->cmdbuff, str);
+
+  p->cmdpos = pos < 0 || pos > p->cmdlen ? p->cmdlen : pos;
+  new_cmdpos = p->cmdpos;
+
+  redrawcmd();
+
+  // Trigger CmdlineChanged autocommands.
+  do_autocmd_cmdlinechanged(ccline.cmdfirstc == NUL ? '-' : ccline.cmdfirstc);
+
+  return 0;
+}
+
 /*
  * Set the command line byte position to "pos".  Zero is the first position.
  * Only works when the command line is being edited.
diff --git a/src/nvim/testdir/test_cmdline.vim b/src/nvim/testdir/test_cmdline.vim
index 517509004a..a5a30b7953 100644
--- a/src/nvim/testdir/test_cmdline.vim
+++ b/src/nvim/testdir/test_cmdline.vim
@@ -2185,4 +2185,44 @@ func Test_wildmenu_pum_disable_while_shown()
   set wildoptions& wildmenu&
 endfunc
 
+func Test_setcmdline()
+  func SetText(text, pos)
+    call assert_equal(0, setcmdline(a:text))
+    call assert_equal(a:text, getcmdline())
+    call assert_equal(len(a:text) + 1, getcmdpos())
+
+    call assert_equal(0, setcmdline(a:text, a:pos))
+    call assert_equal(a:text, getcmdline())
+    call assert_equal(a:pos, getcmdpos())
+
+    call assert_fails('call setcmdline("' .. a:text .. '", -1)', 'E487:')
+    call assert_fails('call setcmdline({}, 0)', 'E928:')
+    call assert_fails('call setcmdline("' .. a:text .. '", {})', 'E728:')
+
+    return ''
+  endfunc
+
+  call feedkeys(":\<C-R>=SetText('set rtp?', 2)\<CR>\<CR>", 'xt')
+  call assert_equal('set rtp?', @:)
+
+  " setcmdline() returns 1 when not editing the command line.
+  call assert_equal(1, 'foo'->setcmdline())
+
+  " Called in custom function
+  func CustomComplete(A, L, P)
+    call assert_equal(0, setcmdline("DoCmd "))
+    return "January\nFebruary\nMars\n"
+  endfunc
+
+  com! -nargs=* -complete=custom,CustomComplete DoCmd :
+  call feedkeys(":DoCmd \<C-A>\<C-B>\"\<CR>", 'tx')
+  call assert_equal('"DoCmd January February Mars', @:)
+
+  " Called in <expr>
+  cnoremap <expr>a setcmdline('let foo=')
+  call feedkeys(":a\<CR>", 'tx')
+  call assert_equal('let foo=0', @:)
+  cunmap a
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab