encoding: cleanup mbyte.c given fixed encoding=utf-8

Eliminate mb_init():
Set "enc_utf" and "has_mbyte" early. Eliminate "enc_unicode" and "enc_latin1like".
init_chartab() and screenalloc() are already invoked elsewhere
in the initialization process.
The EncodingChanged autocmd cannot be triggered.
At initialization, there is no spellfiles to reload

12 files changed, 108 insertions, 391 deletions

diff --git a/src/nvim/charset.c b/src/nvim/charset.c
index 61c5b10808..c501b7e83f 100644
--- a/src/nvim/charset.c
+++ b/src/nvim/charset.c
@@ -1612,9 +1612,7 @@ bool vim_islower(int c)
       return false;
     }
 
-    if (enc_latin1like) {
-      return (latin1flags[c] & LATIN1LOWER) == LATIN1LOWER;
-    }
+    return (latin1flags[c] & LATIN1LOWER) == LATIN1LOWER;
   }
   return islower(c);
 }
@@ -1643,9 +1641,7 @@ bool vim_isupper(int c)
       return false;
     }
 
-    if (enc_latin1like) {
-      return (latin1flags[c] & LATIN1UPPER) == LATIN1UPPER;
-    }
+    return (latin1flags[c] & LATIN1UPPER) == LATIN1UPPER;
   }
   return isupper(c);
 }
@@ -1670,9 +1666,7 @@ int vim_toupper(int c)
       return c;
     }
 
-    if (enc_latin1like) {
-      return latin1upper[c];
-    }
+    return latin1upper[c];
   }
   return TOUPPER_LOC(c);
 }
@@ -1697,9 +1691,7 @@ int vim_tolower(int c)
       return c;
     }
 
-    if (enc_latin1like) {
-      return latin1lower[c];
-    }
+    return latin1lower[c];
   }
   return TOLOWER_LOC(c);
 }
diff --git a/src/nvim/fileio.c b/src/nvim/fileio.c
index c0d4a71b35..0eb475d425 100644
--- a/src/nvim/fileio.c
+++ b/src/nvim/fileio.c
@@ -4165,9 +4165,8 @@ static bool need_conversion(const char_u *fenc)
     same_encoding = (enc_flags != 0 && fenc_flags == enc_flags);
   }
   if (same_encoding) {
-    /* Specified encoding matches with 'encoding'.  This requires
-     * conversion when 'encoding' is Unicode but not UTF-8. */
-    return enc_unicode != 0;
+    // Specified file encoding matches UTF-8.
+    return false;
   }
 
   /* Encodings differ.  However, conversion is not needed when 'enc' is any
diff --git a/src/nvim/globals.h b/src/nvim/globals.h
index 87fb928b30..e42382ad00 100644
--- a/src/nvim/globals.h
+++ b/src/nvim/globals.h
@@ -778,44 +778,18 @@ EXTERN int vr_lines_changed INIT(= 0);      /* #Lines changed by "gR" so far */
 # define DBCS_2BYTE     1       /* 2byte- */
 # define DBCS_DEBUG     -1
 
-EXTERN int enc_dbcs INIT(= 0);                  /* One of DBCS_xxx values if
-                                                   DBCS encoding */
-EXTERN int enc_unicode INIT(= 0);       /* 2: UCS-2 or UTF-16, 4: UCS-4 */
-EXTERN bool enc_utf8 INIT(= false);             /* UTF-8 encoded Unicode */
-EXTERN int enc_latin1like INIT(= TRUE);         /* 'encoding' is latin1 comp. */
-EXTERN int has_mbyte INIT(= 0);                 /* any multi-byte encoding */
+// mbyte flags that used to depend on 'encoding'. These are now deprecated, as
+// 'encoding' is always "utf-8". Code that use them can be refactored to
+// remove dead code.
+#define enc_dbcs false
+#define enc_utf8 true
+#define has_mbyte true
 
 /// Encoding used when 'fencs' is set to "default"
 EXTERN char_u *fenc_default INIT(= NULL);
 
-/*
- * To speed up BYTELEN() we fill a table with the byte lengths whenever
- * enc_utf8 or enc_dbcs changes.
- */
-EXTERN char mb_bytelen_tab[256];
-
-/*
- * Function pointers, used to quickly get to the right function.  Each has
- * three possible values: latin_ (8-bit), utfc_ or utf_ (utf-8) and dbcs_
- * (DBCS).
- * The value is set in mb_init();
- */
-/* length of char in bytes, including following composing chars */
-EXTERN int (*mb_ptr2len)(const char_u *p) INIT(= latin_ptr2len);
-/* idem, with limit on string length */
-EXTERN int (*mb_ptr2len_len)(const char_u *p, int size) INIT(= latin_ptr2len_len);
-/* byte length of char */
-EXTERN int (*mb_char2len)(int c) INIT(= latin_char2len);
-/* convert char to bytes, return the length */
-EXTERN int (*mb_char2bytes)(int c, char_u *buf) INIT(= latin_char2bytes);
-EXTERN int (*mb_ptr2cells)(const char_u *p) INIT(= latin_ptr2cells);
-EXTERN int (*mb_ptr2cells_len)(const char_u *p, int size) INIT(
-      = latin_ptr2cells_len);
-EXTERN int (*mb_char2cells)(int c) INIT(= latin_char2cells);
-EXTERN int (*mb_off2cells)(unsigned off, unsigned max_off) INIT(
-      = latin_off2cells);
-EXTERN int (*mb_ptr2char)(const char_u *p) INIT(= latin_ptr2char);
-EXTERN int (*mb_head_off)(const char_u *base, const char_u *p) INIT(= latin_head_off);
+// To speed up BYTELEN() we keep a table with the byte lengths for utf-8
+EXTERN char utf8len_tab[256];
 
 # if defined(USE_ICONV) && defined(DYNAMIC_ICONV)
 /* Pointers to functions and variables to be loaded at runtime */
diff --git a/src/nvim/macros.h b/src/nvim/macros.h
index 503daa9648..79e545771e 100644
--- a/src/nvim/macros.h
+++ b/src/nvim/macros.h
@@ -122,32 +122,29 @@
 /* Whether to draw the vertical bar on the right side of the cell. */
 # define CURSOR_BAR_RIGHT (curwin->w_p_rl && (!(State & CMDLINE) || cmdmsg_rl))
 
-/*
- * mb_ptr_adv(): advance a pointer to the next character, taking care of
- * multi-byte characters if needed.
- * mb_ptr_back(): backup a pointer to the previous character, taking care of
- * multi-byte characters if needed.
- * MB_COPY_CHAR(f, t): copy one char from "f" to "t" and advance the pointers.
- * PTR2CHAR(): get character from pointer.
- */
-/* Get the length of the character p points to */
-# define MB_PTR2LEN(p)          (has_mbyte ? (*mb_ptr2len)(p) : 1)
-/* Advance multi-byte pointer, skip over composing chars. */
-# define mb_ptr_adv(p)      (p += has_mbyte ? (*mb_ptr2len)((char_u *)p) : 1)
-/* Advance multi-byte pointer, do not skip over composing chars. */
-# define mb_cptr_adv(p)     (p += \
-  enc_utf8 ? utf_ptr2len(p) : has_mbyte ? (*mb_ptr2len)(p) : 1)
-/* Backup multi-byte pointer. Only use with "p" > "s" ! */
-# define mb_ptr_back(s, p)  (p -= has_mbyte ? ((*mb_head_off)((char_u *)s, (char_u *)p - 1) + 1) : 1)
-/* get length of multi-byte char, not including composing chars */
-# define mb_cptr2len(p)     (enc_utf8 ? utf_ptr2len(p) : (*mb_ptr2len)(p))
-
-# define MB_COPY_CHAR(f, t) \
-  if (has_mbyte) mb_copy_char((const char_u **)(&f), &t); \
-  else *t++ = *f++
-# define MB_CHARLEN(p)      (has_mbyte ? mb_charlen(p) : (int)STRLEN(p))
-# define MB_CHAR2LEN(c)     (has_mbyte ? mb_char2len(c) : 1)
-# define PTR2CHAR(p)        (has_mbyte ? mb_ptr2char(p) : (int)*(p))
+// mb_ptr_adv(): advance a pointer to the next character, taking care of
+// multi-byte characters if needed.
+// mb_ptr_back(): backup a pointer to the previous character, taking care of
+// multi-byte characters if needed.
+// MB_COPY_CHAR(f, t): copy one char from "f" to "t" and advance the pointers.
+// PTR2CHAR(): get character from pointer.
+
+// Get the length of the character p points to
+# define MB_PTR2LEN(p)          mb_ptr2len(p)
+// Advance multi-byte pointer, skip over composing chars.
+# define mb_ptr_adv(p)      (p += mb_ptr2len((char_u *)p))
+// Advance multi-byte pointer, do not skip over composing chars.
+# define mb_cptr_adv(p)     (p += utf_ptr2len(p))
+// Backup multi-byte pointer. Only use with "p" > "s" !
+# define mb_ptr_back(s, p)  (p -= mb_head_off((char_u *)s, (char_u *)p - 1) + 1)
+// get length of multi-byte char, not including composing chars
+# define mb_cptr2len(p)     utf_ptr2len(p)
+
+# define MB_COPY_CHAR(f, t) mb_copy_char((const char_u **)(&f), &t);
+
+# define MB_CHARLEN(p)      mb_charlen(p)
+# define MB_CHAR2LEN(c)     mb_char2len(c)
+# define PTR2CHAR(p)        mb_ptr2char(p)
 
 # define RESET_BINDING(wp)  (wp)->w_p_scb = FALSE; (wp)->w_p_crb = FALSE
 
diff --git a/src/nvim/main.c b/src/nvim/main.c
index eb67483d08..ffd9353252 100644
--- a/src/nvim/main.c
+++ b/src/nvim/main.c
@@ -177,7 +177,6 @@ void early_init(void)
   fs_init();
   handle_init();
 
-  (void)mb_init();      // init mb_bytelen_tab[] to ones
   eval_init();          // init global variables
 
   // Init the table of Normal mode commands.
diff --git a/src/nvim/mbyte.c b/src/nvim/mbyte.c
index e6312f9c00..da00b565db 100644
--- a/src/nvim/mbyte.c
+++ b/src/nvim/mbyte.c
@@ -1,68 +1,27 @@
-/*
- * mbyte.c: Code specifically for handling multi-byte characters.
- * Multibyte extensions partly by Sung-Hoon Baek
- *
- * The encoding used in the core is set with 'encoding'.  When 'encoding' is
- * changed, the following four variables are set (for speed).
- * Currently these types of character encodings are supported:
- *
- * "enc_dbcs"	    When non-zero it tells the type of double byte character
- *		    encoding (Chinese, Korean, Japanese, etc.).
- *		    The cell width on the display is equal to the number of
- *		    bytes.  (exception: DBCS_JPNU with first byte 0x8e)
- *		    Recognizing the first or second byte is difficult, it
- *		    requires checking a byte sequence from the start.
- * "enc_utf8"	    When TRUE use Unicode characters in UTF-8 encoding.
- *		    The cell width on the display needs to be determined from
- *		    the character value.
- *		    Recognizing bytes is easy: 0xxx.xxxx is a single-byte
- *		    char, 10xx.xxxx is a trailing byte, 11xx.xxxx is a leading
- *		    byte of a multi-byte character.
- *		    To make things complicated, up to six composing characters
- *		    are allowed.  These are drawn on top of the first char.
- *		    For most editing the sequence of bytes with composing
- *		    characters included is considered to be one character.
- * "enc_unicode"    When 2 use 16-bit Unicode characters (or UTF-16).
- *		    When 4 use 32-but Unicode characters.
- *		    Internally characters are stored in UTF-8 encoding to
- *		    avoid NUL bytes.  Conversion happens when doing I/O.
- *		    "enc_utf8" will also be TRUE.
- *
- * "has_mbyte" is set when "enc_dbcs" or "enc_utf8" is non-zero.
- *
- * If none of these is TRUE, 8-bit bytes are used for a character.  The
- * encoding isn't currently specified (TODO).
- *
- * 'encoding' specifies the encoding used in the core.  This is in registers,
- * text manipulation, buffers, etc.  Conversion has to be done when characters
- * in another encoding are received or send:
- *
- *		       clipboard
- *			   ^
- *			   | (2)
- *			   V
- *		   +---------------+
- *	      (1)  |		   | (3)
- *  keyboard ----->|	 core	   |-----> display
- *		   |		   |
- *		   +---------------+
- *			   ^
- *			   | (4)
- *			   V
- *			 file
- *
- * (1) Typed characters arrive in the current locale.
- * (2) Text will be made available with the encoding specified with
- *     'encoding'.  If this is not sufficient, system-specific conversion
- *     might be required.
- * (3) For the GUI the correct font must be selected, no conversion done.
- * (4) The encoding of the file is specified with 'fileencoding'.  Conversion
- *     is to be done when it's different from 'encoding'.
- *
- * The ShaDa file is a special case: Only text is converted, not file names.
- * Vim scripts may contain an ":encoding" command.  This has an effect for
- * some commands, like ":menutrans"
- */
+/// mbyte.c: Code specifically for handling multi-byte characters.
+/// Multibyte extensions partly by Sung-Hoon Baek
+///
+/// The encoding used in nvim is always UTF-8. "enc_utf8" and "has_mbyte" is
+/// thus always true. "enc_dbcs" is always zero. The 'encoding' option is
+/// read-only and always reads "utf-8".
+///
+/// The cell width on the display needs to be determined from the character
+/// value. Recognizing UTF-8 bytes is easy: 0xxx.xxxx is a single-byte char,
+/// 10xx.xxxx is a trailing byte, 11xx.xxxx is a leading byte of a multi-byte
+/// character. To make things complicated, up to six composing characters
+/// are allowed. These are drawn on top of the first char. For most editing
+/// the sequence of bytes with composing characters included is considered to
+/// be one character.
+///
+/// UTF-8 is used everywhere in the core. This is in registers, text
+/// manipulation, buffers, etc. Nvim core communicates with external plugins
+/// and GUIs in this encoding.
+///
+/// The encoding of a file is specified with 'fileencoding'.  Conversion
+/// is to be done when it's different from "utf-8".
+///
+/// Vim scripts may contain an ":scriptencoding" command. This has an effect
+/// for some commands, like ":menutrans".
 
 #include <inttypes.h>
 #include <stdbool.h>
@@ -115,7 +74,7 @@ struct interval {
  * Bytes which are illegal when used as the first byte have a 1.
  * The NUL byte has length 1.
  */
-static char utf8len_tab[256] =
+char utf8len_tab[256] =
 {
   1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,
   1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,
@@ -385,207 +344,6 @@ int enc_canon_props(const char_u *name)
 }
 
 /*
- * Set up for using multi-byte characters.
- * Called in three cases:
- * - by main() to initialize (p_enc == NULL)
- * - by set_init_1() after 'encoding' was set to its default.
- * - by do_set() when 'encoding' has been set.
- * p_enc must have been passed through enc_canonize() already.
- * Sets the "enc_unicode", "enc_utf8", "enc_dbcs" and "has_mbyte" flags.
- * Fills mb_bytelen_tab[] and returns NULL when there are no problems.
- * When there is something wrong: Returns an error message and doesn't change
- * anything.
- */
-char_u * mb_init(void)
-{
-  int i;
-  int idx;
-  int n;
-  int enc_dbcs_new = 0;
-#if defined(USE_ICONV) && !defined(WIN3264) && !defined(WIN32UNIX) \
-  && !defined(MACOS)
-# define LEN_FROM_CONV
-  vimconv_T vimconv;
-  char_u      *p;
-#endif
-
-  if (p_enc == NULL) {
-    /* Just starting up: set the whole table to one's. */
-    for (i = 0; i < 256; ++i)
-      mb_bytelen_tab[i] = 1;
-    return NULL;
-  } else if (STRNCMP(p_enc, "8bit-", 5) == 0
-      || STRNCMP(p_enc, "iso-8859-", 9) == 0) {
-    /* Accept any "8bit-" or "iso-8859-" name. */
-    enc_unicode = 0;
-    enc_utf8 = false;
-  } else if (STRNCMP(p_enc, "2byte-", 6) == 0) {
-    /* Unix: accept any "2byte-" name, assume current locale. */
-    enc_dbcs_new = DBCS_2BYTE;
-  } else if ((idx = enc_canon_search(p_enc)) >= 0) {
-    i = enc_canon_table[idx].prop;
-    if (i & ENC_UNICODE) {
-      /* Unicode */
-      enc_utf8 = true;
-      if (i & (ENC_2BYTE | ENC_2WORD))
-        enc_unicode = 2;
-      else if (i & ENC_4BYTE)
-        enc_unicode = 4;
-      else
-        enc_unicode = 0;
-    } else if (i & ENC_DBCS) {
-      /* 2byte, handle below */
-      enc_dbcs_new = enc_canon_table[idx].codepage;
-    } else {
-      /* Must be 8-bit. */
-      enc_unicode = 0;
-      enc_utf8 = false;
-    }
-  } else    /* Don't know what encoding this is, reject it. */
-    return e_invarg;
-
-  if (enc_dbcs_new != 0) {
-    enc_unicode = 0;
-    enc_utf8 = false;
-  }
-  enc_dbcs = enc_dbcs_new;
-  has_mbyte = (enc_dbcs != 0 || enc_utf8);
-
-
-  /* Detect an encoding that uses latin1 characters. */
-  enc_latin1like = (enc_utf8 || STRCMP(p_enc, "latin1") == 0
-      || STRCMP(p_enc, "iso-8859-15") == 0);
-
-  /*
-   * Set the function pointers.
-   */
-  if (enc_utf8) {
-    mb_ptr2len = utfc_ptr2len;
-    mb_ptr2len_len = utfc_ptr2len_len;
-    mb_char2len = utf_char2len;
-    mb_char2bytes = utf_char2bytes;
-    mb_ptr2cells = utf_ptr2cells;
-    mb_ptr2cells_len = utf_ptr2cells_len;
-    mb_char2cells = utf_char2cells;
-    mb_off2cells = utf_off2cells;
-    mb_ptr2char = utf_ptr2char;
-    mb_head_off = utf_head_off;
-  } else if (enc_dbcs != 0) {
-    mb_ptr2len = dbcs_ptr2len;
-    mb_ptr2len_len = dbcs_ptr2len_len;
-    mb_char2len = dbcs_char2len;
-    mb_char2bytes = dbcs_char2bytes;
-    mb_ptr2cells = dbcs_ptr2cells;
-    mb_ptr2cells_len = dbcs_ptr2cells_len;
-    mb_char2cells = dbcs_char2cells;
-    mb_off2cells = dbcs_off2cells;
-    mb_ptr2char = dbcs_ptr2char;
-    mb_head_off = dbcs_head_off;
-  } else {
-    mb_ptr2len = latin_ptr2len;
-    mb_ptr2len_len = latin_ptr2len_len;
-    mb_char2len = latin_char2len;
-    mb_char2bytes = latin_char2bytes;
-    mb_ptr2cells = latin_ptr2cells;
-    mb_ptr2cells_len = latin_ptr2cells_len;
-    mb_char2cells = latin_char2cells;
-    mb_off2cells = latin_off2cells;
-    mb_ptr2char = latin_ptr2char;
-    mb_head_off = latin_head_off;
-  }
-
-  /*
-   * Fill the mb_bytelen_tab[] for MB_BYTE2LEN().
-   */
-#ifdef LEN_FROM_CONV
-  /* When 'encoding' is different from the current locale mblen() won't
-   * work.  Use conversion to "utf-8" instead. */
-  vimconv.vc_type = CONV_NONE;
-  if (enc_dbcs) {
-    p = enc_locale();
-    if (p == NULL || STRCMP(p, p_enc) != 0) {
-      convert_setup(&vimconv, p_enc, (char_u *)"utf-8");
-      vimconv.vc_fail = true;
-    }
-    xfree(p);
-  }
-#endif
-
-  for (i = 0; i < 256; ++i) {
-    /* Our own function to reliably check the length of UTF-8 characters,
-     * independent of mblen(). */
-    if (enc_utf8)
-      n = utf8len_tab[i];
-    else if (enc_dbcs == 0)
-      n = 1;
-    else {
-      char buf[MB_MAXBYTES + 1];
-      if (i == NUL)             /* just in case mblen() can't handle "" */
-        n = 1;
-      else {
-        buf[0] = i;
-        buf[1] = 0;
-#ifdef LEN_FROM_CONV
-        if (vimconv.vc_type != CONV_NONE) {
-          /*
-           * string_convert() should fail when converting the first
-           * byte of a double-byte character.
-           */
-          p = string_convert(&vimconv, (char_u *)buf, NULL);
-          if (p != NULL) {
-            xfree(p);
-            n = 1;
-          } else
-            n = 2;
-        } else
-#endif
-        {
-          /*
-           * mblen() should return -1 for invalid (means the leading
-           * multibyte) character.  However there are some platforms
-           * where mblen() returns 0 for invalid character.
-           * Therefore, following condition includes 0.
-           */
-          ignored = mblen(NULL, 0);             /* First reset the state. */
-          if (mblen(buf, (size_t)1) <= 0)
-            n = 2;
-          else
-            n = 1;
-        }
-      }
-    }
-    mb_bytelen_tab[i] = n;
-  }
-
-#ifdef LEN_FROM_CONV
-  convert_setup(&vimconv, NULL, NULL);
-#endif
-
-  /* The cell width depends on the type of multi-byte characters. */
-  (void)init_chartab();
-
-  /* When enc_utf8 is set or reset, (de)allocate ScreenLinesUC[] */
-  screenalloc(false);
-
-#ifdef HAVE_WORKING_LIBINTL
-  /* GNU gettext 0.10.37 supports this feature: set the codeset used for
-   * translated messages independently from the current locale. */
-  (void)bind_textdomain_codeset(PROJECT_NAME,
-                                enc_utf8 ? "utf-8" : (char *)p_enc);
-#endif
-
-
-  /* Fire an autocommand to let people do custom font setup. This must be
-   * after Vim has been setup for the new encoding. */
-  apply_autocmds(EVENT_ENCODINGCHANGED, NULL, (char_u *)"", FALSE, curbuf);
-
-  /* Need to reload spell dictionaries */
-  spell_reload();
-
-  return NULL;
-}
-
-/*
  * Return the size of the BOM for the current buffer:
  * 0 - no BOM
  * 2 - UCS-2 or UTF-16 BOM
@@ -597,20 +355,15 @@ int bomb_size(void)
   int n = 0;
 
   if (curbuf->b_p_bomb && !curbuf->b_p_bin) {
-    if (*curbuf->b_p_fenc == NUL) {
-      if (enc_utf8) {
-        if (enc_unicode != 0)
-          n = enc_unicode;
-        else
-          n = 3;
-      }
-    } else if (STRCMP(curbuf->b_p_fenc, "utf-8") == 0)
+    if (*curbuf->b_p_fenc == NUL
+        || STRCMP(curbuf->b_p_fenc, "utf-8") == 0) {
       n = 3;
-    else if (STRNCMP(curbuf->b_p_fenc, "ucs-2", 5) == 0
-        || STRNCMP(curbuf->b_p_fenc, "utf-16", 6) == 0)
+    } else if (STRNCMP(curbuf->b_p_fenc, "ucs-2", 5) == 0
+               || STRNCMP(curbuf->b_p_fenc, "utf-16", 6) == 0) {
       n = 2;
-    else if (STRNCMP(curbuf->b_p_fenc, "ucs-4", 5) == 0)
+    } else if (STRNCMP(curbuf->b_p_fenc, "ucs-4", 5) == 0) {
       n = 4;
+    }
   }
   return n;
 }
@@ -1010,7 +763,7 @@ int latin_ptr2cells_len(const char_u *p, int size)
   return 1;
 }
 
-static int utf_ptr2cells_len(const char_u *p, int size)
+int utf_ptr2cells_len(const char_u *p, int size)
 {
   int c;
 
@@ -2232,26 +1985,20 @@ int mb_tail_off(char_u *base, char_u *p)
   if (*p == NUL)
     return 0;
 
-  if (enc_utf8) {
-    /* Find the last character that is 10xx.xxxx */
-    for (i = 0; (p[i + 1] & 0xc0) == 0x80; ++i)
-      ;
-    /* Check for illegal sequence. */
-    for (j = 0; p - j > base; ++j)
-      if ((p[-j] & 0xc0) != 0x80)
-        break;
-    if (utf8len_tab[p[-j]] != i + j + 1)
-      return 0;
-    return i;
+  // Find the last character that is 10xx.xxxx
+  for (i = 0; (p[i + 1] & 0xc0) == 0x80; i++) {}
+
+  // Check for illegal sequence.
+  for (j = 0; p - j > base; j++) {
+    if ((p[-j] & 0xc0) != 0x80) {
+      break;
+    }
   }
 
-  /* It can't be the first byte if a double-byte when not using DBCS, at the
-   * end of the string or the byte can't start a double-byte. */
-  if (enc_dbcs == 0 || p[1] == NUL || MB_BYTE2LEN(*p) == 1)
+  if (utf8len_tab[p[-j]] != i + j + 1) {
     return 0;
-
-  /* Return 1 when on the lead byte, 0 when on the tail byte. */
-  return 1 - dbcs_head_off(base, p);
+  }
+  return i;
 }
 
 /*
@@ -2466,13 +2213,10 @@ int mb_fix_col(int col, int row)
 {
   col = check_col(col);
   row = check_row(row);
-  if (has_mbyte && ScreenLines != NULL && col > 0
-      && ((enc_dbcs
-          && ScreenLines[LineOffset[row] + col] != NUL
-          && dbcs_screen_head_off(ScreenLines + LineOffset[row],
-            ScreenLines + LineOffset[row] + col))
-        || (enc_utf8 && ScreenLines[LineOffset[row] + col] == 0)))
+  if (ScreenLines != NULL && col > 0
+      && ScreenLines[LineOffset[row] + col] == 0) {
     return col - 1;
+  }
   return col;
 }
 
diff --git a/src/nvim/mbyte.h b/src/nvim/mbyte.h
index 0cfe2c4bab..2c92a0fbb2 100644
--- a/src/nvim/mbyte.h
+++ b/src/nvim/mbyte.h
@@ -9,8 +9,8 @@
  * MB_BYTE2LEN_CHECK() can be used to count a special key as one byte.
  * Don't call MB_BYTE2LEN(b) with b < 0 or b > 255!
  */
-#define MB_BYTE2LEN(b)         mb_bytelen_tab[b]
-#define MB_BYTE2LEN_CHECK(b)   (((b) < 0 || (b) > 255) ? 1 : mb_bytelen_tab[b])
+#define MB_BYTE2LEN(b)         utf8len_tab[b]
+#define MB_BYTE2LEN_CHECK(b)   (((b) < 0 || (b) > 255) ? 1 : utf8len_tab[b])
 
 /* properties used in enc_canon_table[] (first three mutually exclusive) */
 #define ENC_8BIT       0x01
@@ -28,6 +28,18 @@
 #define ENC_LATIN9     0x400       /* Latin9 */
 #define ENC_MACROMAN   0x800       /* Mac Roman (not Macro Man! :-) */
 
+// TODO(bfredl): eventually we should keep only one of the namings
+#define mb_ptr2len utfc_ptr2len
+#define mb_ptr2len_len utfc_ptr2len_len
+#define mb_char2len utf_char2len
+#define mb_char2bytes utf_char2bytes
+#define mb_ptr2cells utf_ptr2cells
+#define mb_ptr2cells_len utf_ptr2cells_len
+#define mb_char2cells utf_char2cells
+#define mb_off2cells utf_off2cells
+#define mb_ptr2char utf_ptr2char
+#define mb_head_off utf_head_off
+
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "mbyte.h.generated.h"
 #endif
diff --git a/src/nvim/ops.c b/src/nvim/ops.c
index 388a72adce..0263bd15da 100644
--- a/src/nvim/ops.c
+++ b/src/nvim/ops.c
@@ -1936,8 +1936,7 @@ int swapchar(int op_type, pos_T *pos)
   if (c >= 0x80 && op_type == OP_ROT13)
     return FALSE;
 
-  if (op_type == OP_UPPER && c == 0xdf
-      && (enc_latin1like || STRCMP(p_enc, "iso-8859-2") == 0)) {
+  if (op_type == OP_UPPER && c == 0xdf) {
     pos_T sp = curwin->w_cursor;
 
     /* Special handling of German sharp s: change to "SS". */
diff --git a/src/nvim/option.c b/src/nvim/option.c
index cde740daf1..311982982b 100644
--- a/src/nvim/option.c
+++ b/src/nvim/option.c
@@ -780,8 +780,11 @@ void set_init_1(void)
   }
   fenc_default = p;
 
-  // Initialize multibyte (utf-8) handling
-  mb_init();
+#ifdef HAVE_WORKING_LIBINTL
+  // GNU gettext 0.10.37 supports this feature: set the codeset used for
+  // translated messages independently from the current locale.
+  (void)bind_textdomain_codeset(PROJECT_NAME, (char *)p_enc);
+#endif
 
   /* Set the default for 'helplang'. */
   set_helplang_default(get_mess_lang());
diff --git a/src/nvim/screen.c b/src/nvim/screen.c
index 3e4d016fe7..7fd08b6d7b 100644
--- a/src/nvim/screen.c
+++ b/src/nvim/screen.c
@@ -5292,7 +5292,7 @@ void screen_puts_len(char_u *text, int textlen, int row, int col, int attr)
   int force_redraw_next = FALSE;
   int need_redraw;
 
-  const int l_has_mbyte = has_mbyte;
+  const bool l_has_mbyte = has_mbyte;
   const bool l_enc_utf8 = enc_utf8;
   const int l_enc_dbcs = enc_dbcs;
 
diff --git a/src/nvim/spell.c b/src/nvim/spell.c
index ba7f31be25..d9cdce8ca4 100644
--- a/src/nvim/spell.c
+++ b/src/nvim/spell.c
@@ -9266,9 +9266,7 @@ static void allcap_copy(char_u *word, char_u *wcopy)
     else
       c = *s++;
 
-    // We only change 0xdf to SS when we are certain latin1 is used.  It
-    // would cause weird errors in other 8-bit encodings.
-    if (enc_latin1like && c == 0xdf) {
+    if (c == 0xdf) {
       c = 'S';
       if (d - wcopy >= MAXWLEN - 1)
         break;
@@ -12602,7 +12600,7 @@ static int spell_edit_score(slang_T *slang, char_u *badword, char_u *goodword)
   char_u      *p;
   int wbadword[MAXWLEN];
   int wgoodword[MAXWLEN];
-  const int l_has_mbyte = has_mbyte;
+  const bool l_has_mbyte = has_mbyte;
 
   if (l_has_mbyte) {
     // Get the characters from the multi-byte strings and put them in an
diff --git a/src/nvim/tui/input.c b/src/nvim/tui/input.c
index 740716f0ef..9dc66420b0 100644
--- a/src/nvim/tui/input.c
+++ b/src/nvim/tui/input.c
@@ -31,8 +31,8 @@ void term_input_init(TermInput *input, Loop *loop)
   if (!term) {
     term = "";  // termkey_new_abstract assumes non-null (#2745)
   }
-  int enc_flag = enc_utf8 ? TERMKEY_FLAG_UTF8 : TERMKEY_FLAG_RAW;
-  input->tk = termkey_new_abstract(term, enc_flag);
+
+  input->tk = termkey_new_abstract(term, TERMKEY_FLAG_UTF8);
 
   int curflags = termkey_get_canonflags(input->tk);
   termkey_set_canonflags(input->tk, curflags | TERMKEY_CANON_DELBS);