diff --git a/Makefile b/Makefile
index 3781d06a6c..e1024009a3 100644
--- a/Makefile
+++ b/Makefile
@@ -143,14 +143,10 @@ iwyu: build/.ran-cmake
 	cmake --build build > build/iwyu.log
 	iwyu-fix-includes --only_re="src/nvim" --ignore_re="(src/nvim/eval/encode.c|src/nvim/auto/|src/nvim/os/lang.c|src/nvim/map.c\
 	|src/nvim/api/extmark.h\
-	|src/nvim/api/private/dispatch.h\
 	|src/nvim/api/private/helpers.h\
 	|src/nvim/api/private/validate.h\
-	|src/nvim/api/ui.h\
-	|src/nvim/ascii_defs.h\
 	|src/nvim/assert_defs.h\
 	|src/nvim/autocmd.h\
-	|src/nvim/autocmd_defs.h\
 	|src/nvim/buffer.h\
 	|src/nvim/buffer_defs.h\
 	|src/nvim/channel.h\
@@ -178,7 +174,6 @@ iwyu: build/.ran-cmake
 	|src/nvim/event/time.h\
 	|src/nvim/event/wstream.h\
 	|src/nvim/ex_cmds.h\
-	|src/nvim/ex_cmds_defs.h\
 	|src/nvim/ex_docmd.h\
 	|src/nvim/extmark.h\
 	|src/nvim/file_search.h\
@@ -217,10 +212,7 @@ iwyu: build/.ran-cmake
 	|src/nvim/syntax.h\
 	|src/nvim/textobject.h\
 	|src/nvim/tui/input.h\
-	|src/nvim/tui/tui.h\
 	|src/nvim/ui.h\
-	|src/nvim/ui_client.h\
-	|src/nvim/ui_compositor.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 1f588322f3..cb43ed59c7 100755
--- a/src/clint.py
+++ b/src/clint.py
@@ -898,14 +898,10 @@ def CheckIncludes(filename, lines, error):
     # the Makefile.
     check_includes_ignore = [
             "src/nvim/api/extmark.h",
-            "src/nvim/api/private/dispatch.h",
             "src/nvim/api/private/helpers.h",
             "src/nvim/api/private/validate.h",
-            "src/nvim/api/ui.h",
-            "src/nvim/ascii_defs.h",
             "src/nvim/assert_defs.h",
             "src/nvim/autocmd.h",
-            "src/nvim/autocmd_defs.h",
             "src/nvim/buffer.h",
             "src/nvim/buffer_defs.h",
             "src/nvim/channel.h",
@@ -933,7 +929,6 @@ def CheckIncludes(filename, lines, error):
             "src/nvim/event/time.h",
             "src/nvim/event/wstream.h",
             "src/nvim/ex_cmds.h",
-            "src/nvim/ex_cmds_defs.h",
             "src/nvim/ex_docmd.h",
             "src/nvim/extmark.h",
             "src/nvim/file_search.h",
@@ -972,10 +967,7 @@ def CheckIncludes(filename, lines, error):
             "src/nvim/syntax.h",
             "src/nvim/textobject.h",
             "src/nvim/tui/input.h",
-            "src/nvim/tui/tui.h",
             "src/nvim/ui.h",
-            "src/nvim/ui_client.h",
-            "src/nvim/ui_compositor.h",
             "src/nvim/viml/parser/expressions.h",
             "src/nvim/viml/parser/parser.h",
             "src/nvim/window.h",
@@ -999,8 +991,10 @@ def CheckIncludes(filename, lines, error):
             if name in skip_headers:
                 continue
             if (not name.endswith('.h.generated.h') and
+                    not name.endswith('/defs.h') and
                     not name.endswith('_defs.h') and
-                    not name.endswith('/defs.h')):
+                    not name.endswith('_defs.generated.h') and
+                    not name.endswith('_enum.generated.h')):
                 error(filename, i, 'build/include_defs', 5,
                       'Headers should not include non-"_defs" headers')
 
diff --git a/src/nvim/api/private/dispatch.h b/src/nvim/api/private/dispatch.h
index 6a2c9eaf54..88f846f813 100644
--- a/src/nvim/api/private/dispatch.h
+++ b/src/nvim/api/private/dispatch.h
@@ -14,12 +14,12 @@ typedef Object (*ApiDispatchWrapper)(uint64_t channel_id, Array args, Arena *are
 struct MsgpackRpcRequestHandler {
   const char *name;
   ApiDispatchWrapper fn;
-  bool fast;  // Function is safe to be executed immediately while running the
-              // uv loop (the loop is run very frequently due to breakcheck).
-              // If "fast" is false, the function is deferred, i e the call will
-              // be put in the event queue, for safe handling later.
-  bool arena_return;  // return value is allocated in the arena (or statically)
-                      // and should not be freed as such.
+  bool fast;  ///< Function is safe to be executed immediately while running the
+              ///< uv loop (the loop is run very frequently due to breakcheck).
+              ///< If "fast" is false, the function is deferred, i e the call will
+              ///< be put in the event queue, for safe handling later.
+  bool arena_return;  ///< return value is allocated in the arena (or statically)
+                      ///< and should not be freed as such.
 };
 
 extern const MsgpackRpcRequestHandler method_handlers[];
diff --git a/src/nvim/api/ui.h b/src/nvim/api/ui.h
index 26a91d0dbc..b1f4ff97d9 100644
--- a/src/nvim/api/ui.h
+++ b/src/nvim/api/ui.h
@@ -4,9 +4,8 @@
 
 #include "nvim/api/private/defs.h"  // IWYU pragma: keep
 #include "nvim/highlight_defs.h"  // IWYU pragma: keep
-#include "nvim/map_defs.h"
 #include "nvim/types_defs.h"  // IWYU pragma: keep
-#include "nvim/ui.h"
+#include "nvim/ui_defs.h"  // IWYU pragma: keep
 
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "api/ui.h.generated.h"
diff --git a/src/nvim/decoration.h b/src/nvim/decoration.h
index f5448c051b..e5bac169dc 100644
--- a/src/nvim/decoration.h
+++ b/src/nvim/decoration.h
@@ -14,9 +14,11 @@
 
 // actual Decor* data is in decoration_defs.h
 
-EXTERN const char *const virt_text_pos_str[] INIT( = { "eol", "overlay", "win_col", "right_align",
-                                                       "inline" });
+/// Keep in sync with VirtTextPos in decoration_defs.h
+EXTERN const char *const virt_text_pos_str[]
+INIT( = { "eol", "overlay", "win_col", "right_align", "inline" });
 
+/// Keep in sync with HlMode in decoration_defs.h
 EXTERN const char *const hl_mode_str[] INIT( = { "", "replace", "combine", "blend" });
 
 typedef enum {
@@ -43,8 +45,8 @@ typedef struct {
       VirtTextPos pos;
     } ui;
   } data;
-  int attr_id;  // cached lookup of inl.hl_id if it was a highlight
-  bool owned;  // ephemeral decoration, free memory immediately
+  int attr_id;  ///< cached lookup of inl.hl_id if it was a highlight
+  bool owned;   ///< ephemeral decoration, free memory immediately
   DecorPriority priority;
   DecorRangeKind kind;
   /// Screen column to draw the virtual text.
diff --git a/src/nvim/decoration_defs.h b/src/nvim/decoration_defs.h
index dc5d7b9ae4..6e7dc08f80 100644
--- a/src/nvim/decoration_defs.h
+++ b/src/nvim/decoration_defs.h
@@ -15,6 +15,7 @@ typedef struct {
 typedef kvec_t(VirtTextChunk) VirtText;
 #define VIRTTEXT_EMPTY ((VirtText)KV_INITIAL_VALUE)
 
+/// Keep in sync with virt_text_pos_str[] in decoration.h
 typedef enum {
   kVPosEndOfLine,
   kVPosOverlay,
@@ -28,6 +29,7 @@ typedef kvec_t(struct virt_line { VirtText line; bool left_col; }) VirtLines;
 typedef uint16_t DecorPriority;
 #define DECOR_PRIORITY_BASE 0x1000
 
+/// Keep in sync with hl_mode_str[] in decoration.h
 typedef enum {
   kHlModeUnknown,
   kHlModeReplace,
diff --git a/src/nvim/highlight.h b/src/nvim/highlight.h
index e5d3f3d1ca..346c8aa864 100644
--- a/src/nvim/highlight.h
+++ b/src/nvim/highlight.h
@@ -7,7 +7,7 @@
 #include "nvim/buffer_defs.h"  // IWYU pragma: keep
 #include "nvim/highlight_defs.h"  // IWYU pragma: export
 #include "nvim/option_vars.h"
-#include "nvim/ui.h"
+#include "nvim/ui_defs.h"  // IWYU pragma: keep
 
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "highlight.h.generated.h"
diff --git a/src/nvim/lua/spell.c b/src/nvim/lua/spell.c
index c261c5105e..e6c38ea9cb 100644
--- a/src/nvim/lua/spell.c
+++ b/src/nvim/lua/spell.c
@@ -15,10 +15,11 @@
 #include "nvim/spell.h"
 
 #ifdef INCLUDE_GENERATED_DECLARATIONS
-# include "lua/spell.c.generated.h"  // IWYU pragma: export
+# include "lua/spell.c.generated.h"
 #endif
 
 int nlua_spell_check(lua_State *lstate)
+  FUNC_ATTR_NONNULL_ALL
 {
   if (lua_gettop(lstate) < 1) {
     return luaL_error(lstate, "Expected 1 argument");
@@ -99,6 +100,7 @@ static const luaL_Reg spell_functions[] = {
 };
 
 int luaopen_spell(lua_State *L)
+  FUNC_ATTR_NONNULL_ALL
 {
   lua_newtable(L);
   luaL_register(L, NULL, spell_functions);
diff --git a/src/nvim/tui/tui.c b/src/nvim/tui/tui.c
index 78e6cdf32a..c71eb633e9 100644
--- a/src/nvim/tui/tui.c
+++ b/src/nvim/tui/tui.c
@@ -34,8 +34,8 @@
 #include "nvim/tui/tui.h"
 #include "nvim/types_defs.h"
 #include "nvim/ugrid.h"
-#include "nvim/ui.h"
 #include "nvim/ui_client.h"
+#include "nvim/ui_defs.h"
 
 #ifdef MSWIN
 # include "nvim/os/os_win_console.h"
diff --git a/src/nvim/tui/tui.h b/src/nvim/tui/tui.h
index 8eb4ac9bd8..34a98004f3 100644
--- a/src/nvim/tui/tui.h
+++ b/src/nvim/tui/tui.h
@@ -3,7 +3,7 @@
 #include "nvim/api/private/defs.h"  // IWYU pragma: keep
 #include "nvim/highlight_defs.h"  // IWYU pragma: keep
 #include "nvim/types_defs.h"  // IWYU pragma: keep
-#include "nvim/ui.h"
+#include "nvim/ui_defs.h"  // IWYU pragma: keep
 
 typedef struct TUIData TUIData;
 
diff --git a/src/nvim/ui.c b/src/nvim/ui.c
index 36f34bc75a..07166d229e 100644
--- a/src/nvim/ui.c
+++ b/src/nvim/ui.c
@@ -1,7 +1,6 @@
 #include <assert.h>
 #include <limits.h>
 #include <stdbool.h>
-#include <stddef.h>
 #include <stdint.h>
 #include <stdlib.h>
 #include <string.h>
@@ -37,6 +36,11 @@
 #include "nvim/window.h"
 #include "nvim/winfloat.h"
 
+typedef struct ui_event_callback {
+  LuaRef cb;
+  bool ext_widgets[kUIGlobalCount];
+} UIEventCallback;
+
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "ui.c.generated.h"
 #endif
@@ -692,7 +696,7 @@ void ui_call_event(char *name, Array args)
   ui_log(name);
 }
 
-void ui_cb_update_ext(void)
+static void ui_cb_update_ext(void)
 {
   memset(ui_cb_ext, 0, ARRAY_SIZE(ui_cb_ext));
 
@@ -708,7 +712,7 @@ void ui_cb_update_ext(void)
   }
 }
 
-void free_ui_event_callback(UIEventCallback *event_cb)
+static void free_ui_event_callback(UIEventCallback *event_cb)
 {
   api_free_luaref(event_cb->cb);
   xfree(event_cb);
diff --git a/src/nvim/ui.h b/src/nvim/ui.h
index 666a869c89..f61398a7a0 100644
--- a/src/nvim/ui.h
+++ b/src/nvim/ui.h
@@ -1,34 +1,16 @@
 #pragma once
 
-#include <stdbool.h>
-#include <stddef.h>
-#include <stdint.h>
+#include <stdint.h>  // IWYU pragma: keep
 
-#include "nvim/api/private/defs.h"
+#include "nvim/api/private/defs.h"  // IWYU pragma: keep
 #include "nvim/event/multiqueue.h"
-#include "nvim/globals.h"
-#include "nvim/highlight_defs.h"
+#include "nvim/grid_defs.h"  // IWYU pragma: keep
+#include "nvim/highlight_defs.h"  // IWYU pragma: keep
 #include "nvim/macros_defs.h"
-#include "nvim/memory.h"
-#include "nvim/types_defs.h"
-
-struct ui_t;
-
-typedef enum {
-  kUICmdline = 0,
-  kUIPopupmenu,
-  kUITabline,
-  kUIWildmenu,
-  kUIMessages,
-#define kUIGlobalCount kUILinegrid
-  kUILinegrid,
-  kUIMultigrid,
-  kUIHlState,
-  kUITermColors,
-  kUIFloatDebug,
-  kUIExtCount,
-} UIExtension;
+#include "nvim/types_defs.h"  // IWYU pragma: keep
+#include "nvim/ui_defs.h"  // IWYU pragma: export
 
+/// Keep in sync with UIExtension in ui_defs.h
 EXTERN const char *ui_ext_names[] INIT( = {
   "ext_cmdline",
   "ext_popupmenu",
@@ -42,83 +24,6 @@ EXTERN const char *ui_ext_names[] INIT( = {
   "_debug_float",
 });
 
-typedef struct ui_t UI;
-
-enum {
-  kLineFlagWrap = 1,
-  kLineFlagInvalid = 2,
-};
-
-typedef int LineFlags;
-
-typedef struct {
-  uint64_t channel_id;
-
-#define UI_BUF_SIZE 4096  ///< total buffer size for pending msgpack data.
-  /// guaranteed size available for each new event (so packing of simple events
-  /// and the header of grid_line will never fail)
-#define EVENT_BUF_SIZE 256
-  char buf[UI_BUF_SIZE];  ///< buffer of packed but not yet sent msgpack data
-  char *buf_wptr;  ///< write head of buffer
-  const char *cur_event;  ///< name of current event (might get multiple arglists)
-  Array call_buf;  ///< buffer for constructing a single arg list (max 16 elements!)
-
-  // state for write_cb, while packing a single arglist to msgpack. This
-  // might fail due to buffer overflow.
-  size_t pack_totlen;
-  bool buf_overflow;
-  char *temp_buf;
-
-  // We start packing the two outermost msgpack arrays before knowing the total
-  // number of elements. Thus track the location where array size will need
-  // to be written in the msgpack buffer, once the specific array is finished.
-  char *nevents_pos;
-  char *ncalls_pos;
-  uint32_t nevents;  ///< number of distinct events (top-level args to "redraw"
-  uint32_t ncalls;  ///< number of calls made to the current event (plus one for the name!)
-  bool flushed_events;  ///< events where sent to client without "flush" event
-
-  size_t ncells_pending;  ///< total number of cells since last buffer flush
-
-  int hl_id;  // Current highlight for legacy put event.
-  Integer cursor_row, cursor_col;  // Intended visible cursor position.
-
-  // Position of legacy cursor, used both for drawing and visible user cursor.
-  Integer client_row, client_col;
-  bool wildmenu_active;
-} UIData;
-
-struct ui_t {
-  bool rgb;
-  bool override;  ///< Force highest-requested UI capabilities.
-  bool composed;
-  bool ui_ext[kUIExtCount];  ///< Externalized UI capabilities.
-  int width;
-  int height;
-  int pum_nlines;  /// actual nr. lines shown in PUM
-  bool pum_pos;  /// UI reports back pum position?
-  double pum_row;
-  double pum_col;
-  double pum_height;
-  double pum_width;
-
-  // TUI fields.
-  char *term_name;
-  char *term_background;  ///< Deprecated. No longer needed since background color detection happens
-                          ///< in Lua. To be removed in a future release.
-  int term_colors;
-  bool stdin_tty;
-  bool stdout_tty;
-
-  // TODO(bfredl): integrate into struct!
-  UIData data[1];
-};
-
-typedef struct ui_event_callback {
-  LuaRef cb;
-  bool ext_widgets[kUIGlobalCount];
-} UIEventCallback;
-
 // uncrustify:off
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "ui.h.generated.h"
diff --git a/src/nvim/ui_compositor.h b/src/nvim/ui_compositor.h
index f3f5981680..b0dd8e126b 100644
--- a/src/nvim/ui_compositor.h
+++ b/src/nvim/ui_compositor.h
@@ -1,10 +1,9 @@
 #pragma once
 
 #include "nvim/api/private/defs.h"  // IWYU pragma: keep
-#include "nvim/event/defs.h"
 #include "nvim/grid_defs.h"  // IWYU pragma: keep
 #include "nvim/types_defs.h"  // IWYU pragma: keep
-#include "nvim/ui.h"
+#include "nvim/ui_defs.h"  // IWYU pragma: keep
 
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "ui_compositor.h.generated.h"
diff --git a/src/nvim/ui_defs.h b/src/nvim/ui_defs.h
new file mode 100644
index 0000000000..c4707d4d7c
--- /dev/null
+++ b/src/nvim/ui_defs.h
@@ -0,0 +1,95 @@
+#pragma once
+
+#include <stdbool.h>
+#include <stddef.h>
+#include <stdint.h>
+
+#include "nvim/api/private/defs.h"
+
+/// Keep in sync with ui_ext_names[] in ui.h
+typedef enum {
+  kUICmdline = 0,
+  kUIPopupmenu,
+  kUITabline,
+  kUIWildmenu,
+  kUIMessages,
+#define kUIGlobalCount kUILinegrid
+  kUILinegrid,
+  kUIMultigrid,
+  kUIHlState,
+  kUITermColors,
+  kUIFloatDebug,
+  kUIExtCount,
+} UIExtension;
+
+enum {
+  kLineFlagWrap = 1,
+  kLineFlagInvalid = 2,
+};
+
+typedef int LineFlags;
+
+typedef struct ui_t UI;
+
+typedef struct {
+  uint64_t channel_id;
+
+#define UI_BUF_SIZE 4096  ///< total buffer size for pending msgpack data.
+  /// guaranteed size available for each new event (so packing of simple events
+  /// and the header of grid_line will never fail)
+#define EVENT_BUF_SIZE 256
+  char buf[UI_BUF_SIZE];  ///< buffer of packed but not yet sent msgpack data
+  char *buf_wptr;  ///< write head of buffer
+  const char *cur_event;  ///< name of current event (might get multiple arglists)
+  Array call_buf;  ///< buffer for constructing a single arg list (max 16 elements!)
+
+  // state for write_cb, while packing a single arglist to msgpack. This
+  // might fail due to buffer overflow.
+  size_t pack_totlen;
+  bool buf_overflow;
+  char *temp_buf;
+
+  // We start packing the two outermost msgpack arrays before knowing the total
+  // number of elements. Thus track the location where array size will need
+  // to be written in the msgpack buffer, once the specific array is finished.
+  char *nevents_pos;
+  char *ncalls_pos;
+  uint32_t nevents;  ///< number of distinct events (top-level args to "redraw"
+  uint32_t ncalls;  ///< number of calls made to the current event (plus one for the name!)
+  bool flushed_events;  ///< events where sent to client without "flush" event
+
+  size_t ncells_pending;  ///< total number of cells since last buffer flush
+
+  int hl_id;  // Current highlight for legacy put event.
+  Integer cursor_row, cursor_col;  // Intended visible cursor position.
+
+  // Position of legacy cursor, used both for drawing and visible user cursor.
+  Integer client_row, client_col;
+  bool wildmenu_active;
+} UIData;
+
+struct ui_t {
+  bool rgb;
+  bool override;  ///< Force highest-requested UI capabilities.
+  bool composed;
+  bool ui_ext[kUIExtCount];  ///< Externalized UI capabilities.
+  int width;
+  int height;
+  int pum_nlines;  /// actual nr. lines shown in PUM
+  bool pum_pos;  /// UI reports back pum position?
+  double pum_row;
+  double pum_col;
+  double pum_height;
+  double pum_width;
+
+  // TUI fields.
+  char *term_name;
+  char *term_background;  ///< Deprecated. No longer needed since background color detection happens
+                          ///< in Lua. To be removed in a future release.
+  int term_colors;
+  bool stdin_tty;
+  bool stdout_tty;
+
+  // TODO(bfredl): integrate into struct!
+  UIData data[1];
+};