diff --git a/Makefile b/Makefile
index c6a0c053ea..3781d06a6c 100644
--- a/Makefile
+++ b/Makefile
@@ -221,7 +221,6 @@ iwyu: build/.ran-cmake
 	|src/nvim/ui.h\
 	|src/nvim/ui_client.h\
 	|src/nvim/ui_compositor.h\
-	|src/nvim/vim_defs.h\
 	|src/nvim/viml/parser/expressions.h\
 	|src/nvim/viml/parser/parser.h\
 	|src/nvim/window.h\
diff --git a/src/clint.py b/src/clint.py
index e730b9276b..1f588322f3 100755
--- a/src/clint.py
+++ b/src/clint.py
@@ -976,7 +976,6 @@ def CheckIncludes(filename, lines, error):
             "src/nvim/ui.h",
             "src/nvim/ui_client.h",
             "src/nvim/ui_compositor.h",
-            "src/nvim/vim_defs.h",
             "src/nvim/viml/parser/expressions.h",
             "src/nvim/viml/parser/parser.h",
             "src/nvim/window.h",
diff --git a/src/nvim/api/ui.c b/src/nvim/api/ui.c
index bf554a8829..836a68546c 100644
--- a/src/nvim/api/ui.c
+++ b/src/nvim/api/ui.c
@@ -31,7 +31,6 @@
 #include "nvim/option.h"
 #include "nvim/types_defs.h"
 #include "nvim/ui.h"
-#include "nvim/vim_defs.h"
 
 #define BUF_POS(data) ((size_t)((data)->buf_wptr - (data)->buf))
 
diff --git a/src/nvim/api/win_config.c b/src/nvim/api/win_config.c
index 874be200ee..4e23717dc6 100644
--- a/src/nvim/api/win_config.c
+++ b/src/nvim/api/win_config.c
@@ -22,6 +22,7 @@
 #include "nvim/memory.h"
 #include "nvim/option.h"
 #include "nvim/pos_defs.h"
+#include "nvim/strings.h"
 #include "nvim/syntax.h"
 #include "nvim/ui.h"
 #include "nvim/window.h"
diff --git a/src/nvim/cursor_shape.c b/src/nvim/cursor_shape.c
index 745c0cc6ed..5aff3b5598 100644
--- a/src/nvim/cursor_shape.c
+++ b/src/nvim/cursor_shape.c
@@ -18,7 +18,6 @@
 #include "nvim/state_defs.h"
 #include "nvim/strings.h"
 #include "nvim/ui.h"
-#include "nvim/vim_defs.h"
 
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "cursor_shape.c.generated.h"
diff --git a/src/nvim/highlight.c b/src/nvim/highlight.c
index 6cd966c3b4..141761c52e 100644
--- a/src/nvim/highlight.c
+++ b/src/nvim/highlight.c
@@ -25,6 +25,7 @@
 #include "nvim/message.h"
 #include "nvim/option.h"
 #include "nvim/popupmenu.h"
+#include "nvim/strings.h"
 #include "nvim/types_defs.h"
 #include "nvim/ui.h"
 #include "nvim/vim_defs.h"
diff --git a/src/nvim/keycodes.c b/src/nvim/keycodes.c
index 00d2ad3ec9..745500fe39 100644
--- a/src/nvim/keycodes.c
+++ b/src/nvim/keycodes.c
@@ -19,7 +19,7 @@
 #include "nvim/memory.h"
 #include "nvim/message.h"
 #include "nvim/mouse.h"
-#include "nvim/vim_defs.h"
+#include "nvim/strings.h"
 
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "keycodes.c.generated.h"
diff --git a/src/nvim/lua/stdlib.c b/src/nvim/lua/stdlib.c
index 9c46179269..33770b2e62 100644
--- a/src/nvim/lua/stdlib.c
+++ b/src/nvim/lua/stdlib.c
@@ -36,8 +36,8 @@
 #include "nvim/pos_defs.h"
 #include "nvim/regexp.h"
 #include "nvim/runtime.h"
+#include "nvim/strings.h"
 #include "nvim/types_defs.h"
-#include "nvim/vim_defs.h"
 
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "lua/stdlib.c.generated.h"
diff --git a/src/nvim/lua/xdiff.c b/src/nvim/lua/xdiff.c
index 93c8933649..16c3aa5e11 100644
--- a/src/nvim/lua/xdiff.c
+++ b/src/nvim/lua/xdiff.c
@@ -14,7 +14,6 @@
 #include "nvim/macros_defs.h"
 #include "nvim/memory.h"
 #include "nvim/pos_defs.h"
-#include "nvim/vim_defs.h"
 #include "xdiff/xdiff.h"
 
 #define COMPARED_BUFFER0 (1 << 0)
diff --git a/src/nvim/memory.c b/src/nvim/memory.c
index a13091793c..df6c81fe0d 100644
--- a/src/nvim/memory.c
+++ b/src/nvim/memory.c
@@ -213,6 +213,18 @@ void *xmemdupz(const void *data, size_t len)
   return memcpy(xmallocz(len), data, len);
 }
 
+#ifndef HAVE_STRNLEN
+size_t xstrnlen(const char *s, size_t n)
+  FUNC_ATTR_NONNULL_ALL FUNC_ATTR_PURE
+{
+  const char *end = memchr(s, '\0', n);
+  if (end == NULL) {
+    return n;
+  }
+  return (size_t)(end - s);
+}
+#endif
+
 /// A version of strchr() that returns a pointer to the terminating NUL if it
 /// doesn't find `c`.
 ///
@@ -496,13 +508,6 @@ bool strequal(const char *a, const char *b)
   return (a == NULL && b == NULL) || (a && b && strcmp(a, b) == 0);
 }
 
-/// Case-insensitive `strequal`.
-bool striequal(const char *a, const char *b)
-  FUNC_ATTR_PURE FUNC_ATTR_WARN_UNUSED_RESULT
-{
-  return (a == NULL && b == NULL) || (a && b && STRICMP(a, b) == 0);
-}
-
 // Avoid repeating the error message many times (they take 1 second each).
 // Did_outofmem_msg is reset when a character is read.
 void do_outofmem_msg(size_t size)
diff --git a/src/nvim/memory.h b/src/nvim/memory.h
index c37866ebc5..ffdc4c7366 100644
--- a/src/nvim/memory.h
+++ b/src/nvim/memory.h
@@ -4,6 +4,7 @@
 #include <stdint.h>  // IWYU pragma: keep
 #include <time.h>  // IWYU pragma: keep
 
+#include "auto/config.h"
 #include "nvim/macros_defs.h"
 #include "nvim/memory_defs.h"  // IWYU pragma: export
 
@@ -57,3 +58,17 @@ EXTERN size_t arena_alloc_count INIT( = 0);
     *ptr_ = NULL; \
     (void)(*ptr_); \
   } while (0)
+
+#define CLEAR_FIELD(field)  memset(&(field), 0, sizeof(field))
+#define CLEAR_POINTER(ptr)  memset((ptr), 0, sizeof(*(ptr)))
+
+#ifndef HAVE_STRNLEN
+# define strnlen xstrnlen  // Older versions of SunOS may not have strnlen
+#endif
+
+#define STRCPY(d, s)        strcpy((char *)(d), (char *)(s))  // NOLINT(runtime/printf)
+
+// Like strcpy() but allows overlapped source and destination.
+#define STRMOVE(d, s)       memmove((d), (s), strlen(s) + 1)
+
+#define STRCAT(d, s)        strcat((char *)(d), (char *)(s))  // NOLINT(runtime/printf)
diff --git a/src/nvim/mouse.c b/src/nvim/mouse.c
index b07163d1f8..8fe3864424 100644
--- a/src/nvim/mouse.c
+++ b/src/nvim/mouse.c
@@ -24,6 +24,7 @@
 #include "nvim/mark.h"
 #include "nvim/mbyte.h"
 #include "nvim/memline.h"
+#include "nvim/memory.h"
 #include "nvim/menu.h"
 #include "nvim/message.h"
 #include "nvim/mouse.h"
diff --git a/src/nvim/os/fs.c b/src/nvim/os/fs.c
index 3fc9e0ab69..8f939c3b40 100644
--- a/src/nvim/os/fs.c
+++ b/src/nvim/os/fs.c
@@ -53,6 +53,7 @@
 #ifdef MSWIN
 # include "nvim/mbyte.h"
 # include "nvim/option.h"
+# include "nvim/strings.h"
 #endif
 
 #ifdef INCLUDE_GENERATED_DECLARATIONS
diff --git a/src/nvim/os/os.h b/src/nvim/os/os.h
index cbc2be47e0..302d84d066 100644
--- a/src/nvim/os/os.h
+++ b/src/nvim/os/os.h
@@ -10,6 +10,16 @@
 #include "nvim/os/os_defs.h"  // IWYU pragma: export
 #include "nvim/os/stdpaths_defs.h"  // IWYU pragma: keep
 
+#define HAVE_PATHDEF
+
+// Some file names are stored in pathdef.c, which is generated from the
+// Makefile to make their value depend on the Makefile.
+#ifdef HAVE_PATHDEF
+extern char *default_vim_dir;
+extern char *default_vimruntime_dir;
+extern char *default_lib_dir;
+#endif
+
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 // IWYU pragma: begin_exports
 # include "os/env.h.generated.h"
diff --git a/src/nvim/os/os_defs.h b/src/nvim/os/os_defs.h
index 9db559e7a5..12de55a227 100644
--- a/src/nvim/os/os_defs.h
+++ b/src/nvim/os/os_defs.h
@@ -6,10 +6,12 @@
 #include <sys/stat.h>
 #include <sys/types.h>
 
+#include "auto/config.h"
+
 // Note: Some systems need both string.h and strings.h (Savage).
 #include <string.h>
 #ifdef HAVE_STRINGS_H
-# include <strings.h>
+# include <strings.h>  // IWYU pragma: export
 #endif
 
 #ifdef MSWIN
@@ -105,3 +107,9 @@
 #  define S_ISLNK(m)    0
 # endif
 #endif
+
+// BSD is supposed to cover FreeBSD and similar systems.
+#if (defined(BSD) || defined(__FreeBSD_kernel__)) \
+  && (defined(S_ISCHR) || defined(S_IFCHR))
+# define OPEN_CHR_FILES
+#endif
diff --git a/src/nvim/sha256.c b/src/nvim/sha256.c
index 1a07c0856d..d49224a987 100644
--- a/src/nvim/sha256.c
+++ b/src/nvim/sha256.c
@@ -15,8 +15,8 @@
 #include <stdio.h>
 #include <string.h>
 
+#include "nvim/memory.h"
 #include "nvim/sha256.h"
-#include "nvim/vim_defs.h"
 
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "sha256.c.generated.h"
diff --git a/src/nvim/state.c b/src/nvim/state.c
index 91258a5c8b..900eac0826 100644
--- a/src/nvim/state.c
+++ b/src/nvim/state.c
@@ -16,6 +16,7 @@
 #include "nvim/log.h"
 #include "nvim/macros_defs.h"
 #include "nvim/main.h"
+#include "nvim/memory.h"
 #include "nvim/option.h"
 #include "nvim/option_vars.h"
 #include "nvim/os/input.h"
@@ -23,7 +24,6 @@
 #include "nvim/strings.h"
 #include "nvim/types_defs.h"
 #include "nvim/ui.h"
-#include "nvim/vim_defs.h"
 
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "state.c.generated.h"  // IWYU pragma: export
diff --git a/src/nvim/statusline.c b/src/nvim/statusline.c
index 604ec3dfae..4dac1b1451 100644
--- a/src/nvim/statusline.c
+++ b/src/nvim/statusline.c
@@ -42,7 +42,6 @@
 #include "nvim/strings.h"
 #include "nvim/ui.h"
 #include "nvim/undo.h"
-#include "nvim/vim_defs.h"
 #include "nvim/window.h"
 
 // Determines how deeply nested %{} blocks will be evaluated in statusline.
diff --git a/src/nvim/strings.c b/src/nvim/strings.c
index 1533a486bc..a439d11818 100644
--- a/src/nvim/strings.c
+++ b/src/nvim/strings.c
@@ -380,18 +380,6 @@ void del_trailing_spaces(char *ptr)
   }
 }
 
-#if !defined(HAVE_STRNLEN)
-size_t xstrnlen(const char *s, size_t n)
-  FUNC_ATTR_NONNULL_ALL FUNC_ATTR_PURE
-{
-  const char *end = memchr(s, '\0', n);
-  if (end == NULL) {
-    return n;
-  }
-  return (size_t)(end - s);
-}
-#endif
-
 #if (!defined(HAVE_STRCASECMP) && !defined(HAVE_STRICMP))
 // Compare two strings, ignoring case, using current locale.
 // Doesn't work for multi-byte characters.
@@ -441,6 +429,13 @@ int vim_strnicmp(const char *s1, const char *s2, size_t len)
 }
 #endif
 
+/// Case-insensitive `strequal`.
+bool striequal(const char *a, const char *b)
+  FUNC_ATTR_PURE FUNC_ATTR_WARN_UNUSED_RESULT
+{
+  return (a == NULL && b == NULL) || (a && b && STRICMP(a, b) == 0);
+}
+
 /// strchr() version which handles multibyte strings
 ///
 /// @param[in]  string  String to search in.
diff --git a/src/nvim/strings.h b/src/nvim/strings.h
index f71396817d..d717362f87 100644
--- a/src/nvim/strings.h
+++ b/src/nvim/strings.h
@@ -3,9 +3,11 @@
 #include <stdarg.h>  // IWYU pragma: keep
 #include <string.h>
 
+#include "auto/config.h"
 #include "klib/kvec.h"
 #include "nvim/eval/typval_defs.h"  // IWYU pragma: keep
 #include "nvim/func_attr.h"
+#include "nvim/os/os_defs.h"
 #include "nvim/types_defs.h"  // IWYU pragma: keep
 
 /// Append string to string and return pointer to the next byte
@@ -30,3 +32,23 @@ typedef kvec_t(char) StringBuilder;
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "strings.h.generated.h"
 #endif
+
+#ifdef HAVE_STRCASECMP
+# define STRICMP(d, s)      strcasecmp((char *)(d), (char *)(s))
+#else
+# ifdef HAVE_STRICMP
+#  define STRICMP(d, s)     stricmp((char *)(d), (char *)(s))
+# else
+#  define STRICMP(d, s)     vim_stricmp((char *)(d), (char *)(s))
+# endif
+#endif
+
+#ifdef HAVE_STRNCASECMP
+# define STRNICMP(d, s, n)  strncasecmp((char *)(d), (char *)(s), (size_t)(n))
+#else
+# ifdef HAVE_STRNICMP
+#  define STRNICMP(d, s, n) strnicmp((char *)(d), (char *)(s), (size_t)(n))
+# else
+#  define STRNICMP(d, s, n) vim_strnicmp((char *)(d), (char *)(s), (size_t)(n))
+# endif
+#endif
diff --git a/src/nvim/version.c b/src/nvim/version.c
index 227ecf7b3c..cb9088afae 100644
--- a/src/nvim/version.c
+++ b/src/nvim/version.c
@@ -30,9 +30,9 @@
 #include "nvim/memory.h"
 #include "nvim/message.h"
 #include "nvim/option_vars.h"
+#include "nvim/os/os.h"
 #include "nvim/strings.h"
 #include "nvim/version.h"
-#include "nvim/vim_defs.h"
 
 // for ":version", ":intro", and "nvim --version"
 #ifndef NVIM_VERSION_MEDIUM
diff --git a/src/nvim/vim_defs.h b/src/nvim/vim_defs.h
index 84c81e0960..faf79b1c79 100644
--- a/src/nvim/vim_defs.h
+++ b/src/nvim/vim_defs.h
@@ -1,8 +1,5 @@
 #pragma once
 
-#include "nvim/pos_defs.h"
-#include "nvim/types_defs.h"
-
 // Some defines from the old feature.h
 #define SESSION_FILE "Session.vim"
 #define MAX_MSG_HIST_LEN 200
@@ -10,15 +7,6 @@
 #define RUNTIME_DIRNAME "runtime"
 
 #include "auto/config.h"
-#define HAVE_PATHDEF
-
-// Some file names are stored in pathdef.c, which is generated from the
-// Makefile to make their value depend on the Makefile.
-#ifdef HAVE_PATHDEF
-extern char *default_vim_dir;
-extern char *default_vimruntime_dir;
-extern char *default_lib_dir;
-#endif
 
 // Check if configure correctly managed to find sizeof(int).  If this failed,
 // it becomes zero.  This is likely a problem of not being able to run the
@@ -27,17 +15,14 @@ extern char *default_lib_dir;
 # error Configure did not run properly.
 #endif
 
-#include "nvim/os/os_defs.h"       // bring lots of system header files
+// bring lots of system header files
+#include "nvim/os/os_defs.h"  // IWYU pragma: keep
 
 /// length of a buffer to store a number in ASCII (64 bits binary + NUL)
 enum { NUMBUFLEN = 65, };
 
 #define MAX_TYPENR 65535
 
-#include "nvim/gettext.h"
-#include "nvim/keycodes.h"
-#include "nvim/macros_defs.h"
-
 /// Directions.
 typedef enum {
   kDirectionNotSet = 0,
@@ -54,44 +39,3 @@ typedef enum {
 #endif
 #define FAIL                    0
 #define NOTDONE                 2   // not OK or FAIL but skipped
-
-#define CLEAR_FIELD(field)  memset(&(field), 0, sizeof(field))
-#define CLEAR_POINTER(ptr)  memset((ptr), 0, sizeof(*(ptr)))
-
-// (vim_strchr() is now in strings.c)
-
-#ifndef HAVE_STRNLEN
-# define strnlen xstrnlen  // Older versions of SunOS may not have strnlen
-#endif
-
-#define STRCPY(d, s)        strcpy((char *)(d), (char *)(s))  // NOLINT(runtime/printf)
-#ifdef HAVE_STRCASECMP
-# define STRICMP(d, s)      strcasecmp((char *)(d), (char *)(s))
-#else
-# ifdef HAVE_STRICMP
-#  define STRICMP(d, s)     stricmp((char *)(d), (char *)(s))
-# else
-#  define STRICMP(d, s)     vim_stricmp((char *)(d), (char *)(s))
-# endif
-#endif
-
-// Like strcpy() but allows overlapped source and destination.
-#define STRMOVE(d, s)       memmove((d), (s), strlen(s) + 1)
-
-#ifdef HAVE_STRNCASECMP
-# define STRNICMP(d, s, n)  strncasecmp((char *)(d), (char *)(s), (size_t)(n))
-#else
-# ifdef HAVE_STRNICMP
-#  define STRNICMP(d, s, n) strnicmp((char *)(d), (char *)(s), (size_t)(n))
-# else
-#  define STRNICMP(d, s, n) vim_strnicmp((char *)(d), (char *)(s), (size_t)(n))
-# endif
-#endif
-
-#define STRCAT(d, s)        strcat((char *)(d), (char *)(s))  // NOLINT(runtime/printf)
-
-// BSD is supposed to cover FreeBSD and similar systems.
-#if (defined(BSD) || defined(__FreeBSD_kernel__)) \
-  && (defined(S_ISCHR) || defined(S_IFCHR))
-# define OPEN_CHR_FILES
-#endif