fix(editor): respect [+cmd] when executing :drop #33339

Problem:
Normally, `:drop +41 foo.txt` will open foo.txt with the cursor on line
41. But if foo.txt is already open, it instead is a no-op, even if the
cursor is on a different line.

Steps to reproduce:

    nvim --clean foo.txt
    :drop +30 foo.txt

Solution:
Handle +cmd in ex_drop().

(cherry picked from commit 3647b821ea)

src/nvim/ex_cmds.c

@@ -2064,6 +2064,29 @@ theend:
   return retval;
 }
 
+/// set v:swapcommand for the SwapExists autocommands.
+///
+/// @param command  [+cmd] to be executed (e.g. +10).
+/// @param newlnum  if > 0: put cursor on this line number (if possible)
+//
+/// @return 1 if swapcommand was actually set, 0 otherwise
+bool set_swapcommand(char *command, linenr_T newlnum)
+{
+  if ((command == NULL && newlnum <= 0) || *get_vim_var_str(VV_SWAPCOMMAND) != NUL) {
+    return false;
+  }
+  const size_t len = (command != NULL) ? strlen(command) + 3 : 30;
+  char *const p = xmalloc(len);
+  if (command != NULL) {
+    vim_snprintf(p, len, ":%s\r", command);
+  } else {
+    vim_snprintf(p, len, "%" PRId64 "G", (int64_t)newlnum);
+  }
+  set_vim_var_string(VV_SWAPCOMMAND, p, -1);
+  xfree(p);
+  return true;
+}
+
 /// start editing a new file
 ///
 /// @param fnum     file number; if zero use ffname/sfname
@@ -2197,20 +2220,7 @@ int do_ecmd(int fnum, char *ffname, char *sfname, exarg_T *eap, linenr_T newlnum
     oldwin = NULL;
   }
 
-  if ((command != NULL || newlnum > 0)
-      && *get_vim_var_str(VV_SWAPCOMMAND) == NUL) {
-    // Set v:swapcommand for the SwapExists autocommands.
-    const size_t len = (command != NULL) ? strlen(command) + 3 : 30;
-    char *const p = xmalloc(len);
-    if (command != NULL) {
-      vim_snprintf(p, len, ":%s\r", command);
-    } else {
-      vim_snprintf(p, len, "%" PRId64 "G", (int64_t)newlnum);
-    }
-    set_vim_var_string(VV_SWAPCOMMAND, p, -1);
-    did_set_swapcommand = true;
-    xfree(p);
-  }
+  did_set_swapcommand = set_swapcommand(command, newlnum);
 
   // If we are starting to edit another file, open a (new) buffer.
   // Otherwise we re-use the current buffer.

src/nvim/ex_cmds2.c

@@ -888,6 +888,17 @@ void ex_drop(exarg_T *eap)
       if (curbuf->b_ml.ml_flags & ML_EMPTY) {
         ex_rewind(eap);
       }
+
+      // execute [+cmd]
+      if (eap->do_ecmd_cmd) {
+        bool did_set_swapcommand = set_swapcommand(eap->do_ecmd_cmd, 0);
+        do_cmdline(eap->do_ecmd_cmd, NULL, NULL, DOCMD_VERBOSE);
+        if (did_set_swapcommand) {
+          set_vim_var_string(VV_SWAPCOMMAND, NULL, -1);
+        }
+      }
+
+      // no need to execute [++opts] - they only apply for newly loaded buffers.
       return;
     }
   }

test/functional/ex_cmds/drop_spec.lua

@@ -75,4 +75,25 @@ describe(':drop', function()
       :drop Xdrop_modified.txt           |
     ]])
   end)
+
+  it('jumps to line number when passed +line', function()
+    exec([[
+      edit Xdrop_line.txt
+      call append(0, "I just miss doing art. Don't you?")
+      call append(1, "It is not so hard as we have supposed.")
+      call append(2, "We are propelled by disaster. We are moving swiftly.")
+    ]])
+    feed_command('drop +2 Xdrop_line.txt')
+    screen:expect([[
+      I just miss doing art. Don't you?  |
+      ^It is not so hard as we have suppos|
+      ed.                                |
+      We are propelled by disaster. We ar|
+      e moving swiftly.                  |
+                                         |
+      {0:~                                  }|*2
+      {1:Xdrop_line.txt [+]                 }|
+      :drop +2 Xdrop_line.txt            |
+    ]])
+  end)
 end)