feat(editor)!: insert-mode ctrl-r should work like paste #35477

Problem:
insert-mode ctrl-r input is treated like raw user input, which is almost
never useful. This means any newlines in the input are affected by
autoindent, etc., which is:
- slow
- usually breaks the formatting of the input

Solution:
- ctrl-r should be treated like a paste, not user-input.
- does not affect `<c-r>=`, so `<c-r>=@x` can still be used to get the
  old behavior.

Co-authored-by: Justin M. Keyes <justinkz@gmail.com>
Co-authored-by: zeertzjq <zeertzjq@outlook.com>

src/nvim/edit.c

@@ -2854,6 +2854,8 @@ static void ins_reg(void)
     vim_beep(kOptBoFlagRegister);
     need_redraw = true;  // remove the '"'
   } else {
+    yankreg_T *reg = get_yank_register(regname, YREG_PASTE);
+
     if (literally == Ctrl_O || literally == Ctrl_P) {
       // Append the command to the redo buffer.
       AppendCharToRedobuff(Ctrl_R);
@@ -2862,7 +2864,11 @@ static void ins_reg(void)
 
       do_put(regname, NULL, BACKWARD, 1,
              (literally == Ctrl_P ? PUT_FIXINDENT : 0) | PUT_CURSEND);
-    } else if (insert_reg(regname, NULL, literally) == FAIL) {
+    } else if (reg->y_size > 1 && is_literal_register(regname)) {
+      AppendCharToRedobuff(Ctrl_R);
+      AppendCharToRedobuff(regname);
+      do_put(regname, NULL, BACKWARD, 1, PUT_CURSEND);
+    } else if (insert_reg(regname, NULL, !!literally) == FAIL) {
       vim_beep(kOptBoFlagRegister);
       need_redraw = true;  // remove the '"'
     } else if (stop_insert_mode) {

src/nvim/ops.h

@@ -152,7 +152,7 @@ static inline int op_reg_index(const int regname)
 static inline bool is_literal_register(const int regname)
   FUNC_ATTR_CONST
 {
-  return regname == '*' || regname == '+';
+  return regname == '*' || regname == '+' || ASCII_ISALNUM(regname);
 }
 
 EXTERN LuaRef repeat_luaref INIT( = LUA_NOREF);  ///< LuaRef for "."