Hide public implementation of `rb_io`. (#9568)

Remove `struct rb_io {...}`.

include/ruby/io.h

@@ -137,124 +137,7 @@ struct rb_io_encoding {
     VALUE ecopts;
 };
 
-#ifndef HAVE_RB_IO_T
-#define HAVE_RB_IO_T 1
-/** Ruby's IO, metadata and buffers. */
-struct rb_io {
-    /** The IO's Ruby level counterpart. */
-    RBIMPL_ATTR_DEPRECATED(("with no replacement"))
-    VALUE self;
-
-    /** stdio ptr for read/write, if available. */
-    RBIMPL_ATTR_DEPRECATED(("with no replacement"))
-    FILE *stdio_file;
-
-    /** file descriptor. */
-    RBIMPL_ATTR_DEPRECATED(("rb_io_descriptor"))
-    int fd;
-
-    /** mode flags: FMODE_XXXs */
-    RBIMPL_ATTR_DEPRECATED(("rb_io_mode"))
-    int mode;
-
-    /** child's pid (for pipes) */
-    RBIMPL_ATTR_DEPRECATED(("with no replacement"))
-    rb_pid_t pid;
-
-    /** number of lines read */
-    RBIMPL_ATTR_DEPRECATED(("with no replacement"))
-    int lineno;
-
-    /** pathname for file */
-    RBIMPL_ATTR_DEPRECATED(("rb_io_path"))
-    VALUE pathv;
-
-    /** finalize proc */
-    RBIMPL_ATTR_DEPRECATED(("with no replacement"))
-    void (*finalize)(struct rb_io*,int);
-
-    /** Write buffer. */
-    RBIMPL_ATTR_DEPRECATED(("with no replacement"))
-    rb_io_buffer_t wbuf;
-
-    /**
-     * (Byte)  read   buffer.   Note  also   that  there  is  a   field  called
-     * ::rb_io_t::cbuf, which also concerns read IO.
-     */
-    RBIMPL_ATTR_DEPRECATED(("with no replacement"))
-    rb_io_buffer_t rbuf;
-
-    /**
-     * Duplex IO object, if set.
-     *
-     * @see rb_io_set_write_io()
-     */
-    RBIMPL_ATTR_DEPRECATED(("rb_io_get_write_io"))
-    VALUE tied_io_for_writing;
-
-    RBIMPL_ATTR_DEPRECATED(("with no replacement"))
-    struct rb_io_encoding encs; /**< Decomposed encoding flags. */
-
-    /** Encoding converter used when reading from this IO. */
-    RBIMPL_ATTR_DEPRECATED(("with no replacement"))
-    rb_econv_t *readconv;
-
-    /**
-     * rb_io_ungetc()  destination.   This  buffer   is  read  before  checking
-     * ::rb_io_t::rbuf
-     */
-    RBIMPL_ATTR_DEPRECATED(("with no replacement"))
-    rb_io_buffer_t cbuf;
-
-    /** Encoding converter used when writing to this IO. */
-    RBIMPL_ATTR_DEPRECATED(("with no replacement"))
-    rb_econv_t *writeconv;
-
-    /**
-     * This is, when set, an instance  of ::rb_cString which holds the "common"
-     * encoding.   Write  conversion  can  convert strings  twice...   In  case
-     * conversion from encoding  X to encoding Y does not  exist, Ruby finds an
-     * encoding Z that bridges the two, so that X to Z to Y conversion happens.
-     */
-    RBIMPL_ATTR_DEPRECATED(("with no replacement"))
-    VALUE writeconv_asciicompat;
-
-    /** Whether ::rb_io_t::writeconv is already set up. */
-    RBIMPL_ATTR_DEPRECATED(("with no replacement"))
-    int writeconv_initialized;
-
-    /**
-     * Value   of    ::rb_io_t::rb_io_enc_t::ecflags   stored    right   before
-     * initialising ::rb_io_t::writeconv.
-     */
-    RBIMPL_ATTR_DEPRECATED(("with no replacement"))
-    int writeconv_pre_ecflags;
-
-    /**
-     * Value of ::rb_io_t::rb_io_enc_t::ecopts stored right before initialising
-     * ::rb_io_t::writeconv.
-     */
-    RBIMPL_ATTR_DEPRECATED(("with no replacement"))
-    VALUE writeconv_pre_ecopts;
-
-    /**
-     * This is a Ruby  level mutex.  It avoids multiple threads  to write to an
-     * IO at  once; helps  for instance rb_io_puts()  to ensure  newlines right
-     * next to its arguments.
-     *
-     * This of course doesn't help inter-process IO interleaves, though.
-     */
-    RBIMPL_ATTR_DEPRECATED(("with no replacement"))
-    VALUE write_lock;
-
-    /**
-     * The timeout associated with this IO when performing blocking operations.
-     */
-    RBIMPL_ATTR_DEPRECATED(("rb_io_timeout/rb_io_set_timeout"))
-    VALUE timeout;
-};
-#endif
-
+struct rb_io;
 typedef struct rb_io rb_io_t;
 
 /** @alias{rb_io_enc_t} */