'inccommand': buftype=nofile, restore cursor/view

- Use a standard scratch buffer instead of a new 'buftype', functions
  like curbufIsChanged() already have special handling for scratch bufs.
- Cleanup some stuff from the previous merge.
- Add support for :smagic, :snomagic. Closes #5578

src/nvim/buffer.c

@@ -5273,14 +5273,20 @@ wipe_buffer (
   }
 }
 
-/// Creates or switches to a special-purpose buffer.
+/// Creates or switches to a scratch buffer. :h special-buffers
+/// Scratch buffer is:
+///   - buftype=nofile bufhidden=hide noswapfile
+///   - Always considered 'nomodified'
 ///
 /// @param bufnr     Buffer to switch to, or 0 to create a new buffer.
-void buf_open_special(handle_T bufnr, char *bufname, char *buftype)
+///
+/// @see curbufIsChanged()
+void buf_open_scratch(handle_T bufnr, char *bufname)
 {
   (void)do_ecmd((int)bufnr, NULL, NULL, NULL, ECMD_ONE, ECMD_HIDE, NULL);
   (void)setfname(curbuf, (char_u *)bufname, NULL, true);
-  set_option_value((char_u *)"bt", 0L, (char_u *)buftype, OPT_LOCAL);
+  set_option_value((char_u *)"bh", 0L, (char_u *)"hide", OPT_LOCAL);
+  set_option_value((char_u *)"bt", 0L, (char_u *)"nofile", OPT_LOCAL);
   set_option_value((char_u *)"swf", 0L, NULL, OPT_LOCAL);
   RESET_BINDING(curwin);
 }

src/nvim/ex_cmds.c

@@ -2188,7 +2188,6 @@ int do_ecmd(
     if (fnum) {
       buf = buflist_findnr(fnum);
     } else {
-      ILOG("here");
       if (flags & ECMD_ADDBUF) {
         linenr_T tlnum = 1L;
 
@@ -3275,8 +3274,7 @@ buf_T *do_sub(exarg_T *eap)
     return NULL;
   }
 
-  int search_options = eap->is_live ? 0 : SEARCH_HIS;
-  if (search_regcomp(pat, RE_SUBST, which_pat, search_options,
+  if (search_regcomp(pat, RE_SUBST, which_pat, (eap->is_live ? 0 : SEARCH_HIS),
                      &regmatch) == FAIL) {
     if (subflags.do_error) {
       EMSG(_(e_invcmd));
@@ -3951,13 +3949,10 @@ skip:
 
   // Show 'inccommand' preview if there are matched lines.
   buf_T *preview_buf = NULL;
-  if (eap->is_live && matched_lines.size != 0 && pat != NULL && *p_icm != NUL) {
+  if (eap->is_live && *p_icm != NUL && matched_lines.size != 0 && pat != NULL) {
     curbuf->b_changed = save_b_changed;  // preserve 'modified' during preview
     preview_buf = show_sub(old_cursor, pat, sub, eap->line1, eap->line2,
                            &matched_lines);
-
-  } else if (*p_icm != NUL && eap->is_live) {
-    curwin->w_cursor = old_cursor;  // don't move the cursor
   }
 
   for (MatchedLine m; kv_size(matched_lines);) {
@@ -6058,10 +6053,9 @@ static buf_T *show_sub(pos_T old_cusr, char_u *pat, char_u *sub, linenr_T line1,
   }
 
   if (split && win_split((int)p_cwh, WSP_BOT) != FAIL) {
-    buf_open_special(preview_buf ? bufnr : 0, "[Preview]", "incsub");
+    buf_open_scratch(preview_buf ? bufnr : 0, "[Preview]");
     buf_clear();
     preview_buf = curbuf;
-    set_option_value((char_u *)"bufhidden", 0L, (char_u *)"hide", OPT_LOCAL);
     bufnr = preview_buf->handle;
     curbuf->b_p_bl = false;
     curbuf->b_p_ma = true;
@@ -6092,7 +6086,7 @@ static buf_T *show_sub(pos_T old_cusr, char_u *pat, char_u *sub, linenr_T line1,
         old_line_size = line_size;
       }
 
-      // put " | lnum|line" into str and append it to the preview buffer
+      // Put "|lnum| line" into `str` and append it to the preview buffer.
       snprintf(str, line_size, "|%*ld| %s", col_width - 3, mat.lnum, mat.line);
       ml_append(line, (char_u *)str, (colnr_T)line_size, false);
 
@@ -6155,6 +6149,7 @@ void ex_substitute(exarg_T *eap)
   buf_T *preview_buf = do_sub(eap);
 
   if (save_changedtick != curbuf->b_changedtick) {
+    // Undo invisibly. This also moves the cursor!
     if (!u_undo_and_forget(1)) { abort(); }
     // Restore newhead. It is meaningless when curhead is valid, but we must
     // restore it so that undotree() is identical before/after the preview.

src/nvim/ex_cmds.h

@@ -14,7 +14,6 @@
 #define ECMD_OLDBUF          0x04    // use existing buffer if it exists
 #define ECMD_FORCEIT         0x08    // ! used in Ex command
 #define ECMD_ADDBUF          0x10    // don't edit, just add to buffer list
-#define ECMD_RESERVED_BUFNR  0x20    // bufnr argument is reserved bufnr
 
 
 /* for lnum argument in do_ecmd() */

src/nvim/ex_docmd.c

@@ -7295,15 +7295,13 @@ void ex_may_print(exarg_T *eap)
   }
 }
 
-/*
- * ":smagic" and ":snomagic".
- */
+/// ":smagic" and ":snomagic".
 static void ex_submagic(exarg_T *eap)
 {
   int magic_save = p_magic;
 
   p_magic = (eap->cmdidx == CMD_smagic);
-  do_sub(eap);
+  ex_substitute(eap);
   p_magic = magic_save;
 }
 
@@ -9669,5 +9667,7 @@ bool cmd_is_live(char_u *cmd)
   }
   find_command(&ea, NULL);
 
-  return (ea.cmdidx == CMD_substitute);
+  return ea.cmdidx == CMD_substitute
+      || ea.cmdidx == CMD_smagic
+      || ea.cmdidx == CMD_snomagic;
 }

src/nvim/ex_getln.c

@@ -5147,7 +5147,9 @@ static int ex_window(void)
   cmdwin_type = get_cmdline_type();
 
   // Create empty command-line buffer.
-  buf_open_special(0, "[Command Line]", "nofile");
+  buf_open_scratch(0, "[Command Line]");
+  // Command-line buffer has bufhidden=wipe, unlike a true "scratch" buffer.
+  set_option_value((char_u *)"bh", 0L, (char_u *)"wipe", OPT_LOCAL);
   curwin->w_p_rl = cmdmsg_rl;
   cmdmsg_rl = false;
   curbuf->b_p_ma = true;

src/nvim/undo.c

@@ -1685,7 +1685,8 @@ void u_redo(int count)
   u_doit(count, false);
 }
 
-/// undo, and remove the undo branch from the undo tree.
+/// Undo and remove the branch from the undo tree.
+/// Also moves the cursor (as a "normal" undo would).
 bool u_undo_and_forget(int count)
 {
   if (curbuf->b_u_synced == false) {