diff --git a/src/autoload.cpp b/src/autoload.cpp
index 7ecd3cbe9..8550dae1b 100644
--- a/src/autoload.cpp
+++ b/src/autoload.cpp
@@ -1,7 +1,6 @@
 // The classes responsible for autoloading functions and completions.
 #include "config.h"  // IWYU pragma: keep
 
-#include <assert.h>
 #include <errno.h>
 #include <pthread.h>
 #include <stddef.h>
diff --git a/src/autoload.h b/src/autoload.h
index 6a395cf08..63886390e 100644
--- a/src/autoload.h
+++ b/src/autoload.h
@@ -4,6 +4,7 @@
 
 #include <pthread.h>
 #include <time.h>
+
 #include <set>
 
 #include "common.h"
diff --git a/src/builtin.cpp b/src/builtin.cpp
index e7470806c..6ee4ec986 100644
--- a/src/builtin.cpp
+++ b/src/builtin.cpp
@@ -17,18 +17,18 @@
 // 4). Use 'git add doc_src/NAME.txt' to start tracking changes to the documentation file.
 #include "config.h"  // IWYU pragma: keep
 
-#include <assert.h>
 #include <errno.h>
 #include <fcntl.h>
 #include <limits.h>
 #include <stdarg.h>
-#include <stdio.h>
 #include <stdint.h>
+#include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <sys/stat.h>
 #include <unistd.h>
 #include <wchar.h>
+
 #include <algorithm>
 #include <map>
 #include <memory>
diff --git a/src/builtin.h b/src/builtin.h
index f27086799..17f4671a0 100644
--- a/src/builtin.h
+++ b/src/builtin.h
@@ -3,6 +3,7 @@
 #define FISH_BUILTIN_H
 
 #include <stddef.h>
+
 #include <vector>
 
 #include "common.h"
diff --git a/src/builtin_commandline.cpp b/src/builtin_commandline.cpp
index 1a81226bb..dafe6c286 100644
--- a/src/builtin_commandline.cpp
+++ b/src/builtin_commandline.cpp
@@ -1,7 +1,6 @@
 // Functions used for implementing the commandline builtin.
 #include "config.h"  // IWYU pragma: keep
 
-#include <assert.h>
 #include <errno.h>
 #include <stddef.h>
 #include <stdlib.h>
diff --git a/src/builtin_complete.h b/src/builtin_complete.h
index 3ef45a32d..96d667cba 100644
--- a/src/builtin_complete.h
+++ b/src/builtin_complete.h
@@ -3,6 +3,7 @@
 #define FISH_BUILTIN_COMPLETE_H
 
 #include <wchar.h>
+
 #include <cstring>
 
 class parser_t;
diff --git a/src/builtin_jobs.h b/src/builtin_jobs.h
index caf059f88..5c37043ac 100644
--- a/src/builtin_jobs.h
+++ b/src/builtin_jobs.h
@@ -3,6 +3,7 @@
 #define FISH_BUILTIN_JOBS_H
 
 #include <wchar.h>
+
 #include <cstring>
 
 class parser_t;
diff --git a/src/builtin_printf.h b/src/builtin_printf.h
index f99a864fd..1f9d44d58 100644
--- a/src/builtin_printf.h
+++ b/src/builtin_printf.h
@@ -3,6 +3,7 @@
 #define FISH_BUILTIN_PRINTF_H
 
 #include <wchar.h>
+
 #include <cstring>
 
 class parser_t;
diff --git a/src/builtin_set.cpp b/src/builtin_set.cpp
index 9aa5519e2..810a76aa6 100644
--- a/src/builtin_set.cpp
+++ b/src/builtin_set.cpp
@@ -453,9 +453,8 @@ int builtin_set(parser_t &parser, io_streams_t &streams, wchar_t **argv) {
             wchar_t *arg = argv[i];
             int slice = 0;
 
-            if (!(dest = wcsdup(arg))) {
-                DIE_MEM();
-            }
+            dest = wcsdup(arg);
+            assert(dest);
 
             if (wcschr(dest, L'[')) {
                 slice = 1;
@@ -512,9 +511,8 @@ int builtin_set(parser_t &parser, io_streams_t &streams, wchar_t **argv) {
         return retcode;
     }
 
-    if (!(dest = wcsdup(argv[w.woptind]))) {
-        DIE_MEM();
-    }
+    dest = wcsdup(argv[w.woptind]);
+    assert(dest);
 
     if (wcschr(dest, L'[')) {
         slice = 1;
diff --git a/src/builtin_set.h b/src/builtin_set.h
index 765857336..134fea28b 100644
--- a/src/builtin_set.h
+++ b/src/builtin_set.h
@@ -3,6 +3,7 @@
 #define FISH_BUILTIN_SET_H
 
 #include <wchar.h>
+
 #include <cstring>
 
 class parser_t;
diff --git a/src/builtin_set_color.cpp b/src/builtin_set_color.cpp
index fcbf5b02d..a75989736 100644
--- a/src/builtin_set_color.cpp
+++ b/src/builtin_set_color.cpp
@@ -1,6 +1,10 @@
 // Functions used for implementing the set_color builtin.
 #include "config.h"
 
+#include <stddef.h>
+#include <stdlib.h>
+#include <unistd.h>
+
 #if HAVE_NCURSES_H
 #include <ncurses.h>
 #elif HAVE_NCURSES_CURSES_H
@@ -14,10 +18,6 @@
 #include <ncurses/term.h>
 #endif
 
-#include <assert.h>
-#include <stddef.h>
-#include <stdlib.h>
-#include <unistd.h>
 #include <memory>
 #include <string>
 #include <vector>
diff --git a/src/builtin_set_color.h b/src/builtin_set_color.h
index 7e41cd58b..65647018a 100644
--- a/src/builtin_set_color.h
+++ b/src/builtin_set_color.h
@@ -3,6 +3,7 @@
 #define FISH_BUILTIN_SET_COLOR_H
 
 #include <wchar.h>
+
 #include <cstring>
 
 class parser_t;
diff --git a/src/builtin_string.cpp b/src/builtin_string.cpp
index 4293b0d06..f2b565fda 100644
--- a/src/builtin_string.cpp
+++ b/src/builtin_string.cpp
@@ -5,7 +5,6 @@
 #ifdef _WIN32
 #define PCRE2_STATIC
 #endif
-#include <assert.h>
 #include <errno.h>
 #include <limits.h>
 #include <stdarg.h>
@@ -370,9 +369,7 @@ struct compiled_regex_t {
         }
 
         match = pcre2_match_data_create_from_pattern(code, 0);
-        if (match == 0) {
-            DIE_MEM();
-        }
+        assert(match);
     }
 
     ~compiled_regex_t() {
@@ -705,9 +702,8 @@ bool regex_replacer_t::replace_matches(const wchar_t *arg) {
 
     bool done = false;
     while (!done) {
-        if (output == NULL) {
-            DIE_MEM();
-        }
+        assert(output);
+
         PCRE2_SIZE outlen = bufsize;
         pcre2_rc = pcre2_substitute(regex.code, PCRE2_SPTR(arg), arglen,
                                     0,  // start offset
diff --git a/src/builtin_string.h b/src/builtin_string.h
index 49247d9c1..0f530c641 100644
--- a/src/builtin_string.h
+++ b/src/builtin_string.h
@@ -3,6 +3,7 @@
 #define FISH_BUILTIN_STRING_H
 
 #include <wchar.h>
+
 #include <cstring>
 
 class parser_t;
diff --git a/src/builtin_test.cpp b/src/builtin_test.cpp
index 2842806dd..648315e3f 100644
--- a/src/builtin_test.cpp
+++ b/src/builtin_test.cpp
@@ -3,13 +3,13 @@
 // Implemented from scratch (yes, really) by way of IEEE 1003.1 as reference.
 #include "config.h"  // IWYU pragma: keep
 
-#include <assert.h>
 #include <errno.h>
 #include <stdarg.h>
 #include <sys/stat.h>
 #include <sys/types.h>
 #include <unistd.h>
 #include <wchar.h>
+
 #include <memory>
 #include <string>
 #include <type_traits>
diff --git a/src/builtin_ulimit.h b/src/builtin_ulimit.h
index 1e694e07c..1676b5a35 100644
--- a/src/builtin_ulimit.h
+++ b/src/builtin_ulimit.h
@@ -3,6 +3,7 @@
 #define FISH_BUILTIN_ULIMIT_H
 
 #include <wchar.h>
+
 #include <cstring>
 
 class parser_t;
diff --git a/src/color.cpp b/src/color.cpp
index 235e70841..3f6ebac2a 100644
--- a/src/color.cpp
+++ b/src/color.cpp
@@ -1,7 +1,6 @@
 // Color class implementation.
 #include "config.h"  // IWYU pragma: keep
 
-#include <assert.h>
 #include <stddef.h>
 #include <stdint.h>
 #include <stdlib.h>
@@ -298,7 +297,7 @@ color24_t rgb_color_t::to_color24() const {
 }
 
 unsigned char rgb_color_t::to_name_index() const {
-    // XXX this should look for the nearest color
+    // TODO: This should look for the nearest color.
     assert(type == type_named || type == type_rgb);
     if (type == type_named) return data.name_idx;
     if (type == type_rgb) return term16_color_for_rgb(data.color.rgb);
@@ -342,8 +341,7 @@ wcstring rgb_color_t::description() const {
             return L"normal";
         }
         default: {
-            abort();
-            return L"";
+            DIE("unknown color type");
         }
     }
 }
diff --git a/src/color.h b/src/color.h
index 592fdc30b..ec6d29dbb 100644
--- a/src/color.h
+++ b/src/color.h
@@ -3,6 +3,7 @@
 #define FISH_COLOR_H
 
 #include <string.h>
+
 #include <string>
 
 #include "common.h"
diff --git a/src/common.cpp b/src/common.cpp
index db906422e..6daca1ac2 100644
--- a/src/common.cpp
+++ b/src/common.cpp
@@ -1,7 +1,6 @@
 // Various functions, mostly string utilities, that are used by most parts of fish.
 #include "config.h"
 
-#include <assert.h>
 #include <cxxabi.h>
 #include <dlfcn.h>
 #include <errno.h>
@@ -28,6 +27,7 @@
 #ifdef HAVE_SYS_IOCTL_H
 #include <sys/ioctl.h>
 #endif
+
 #include <algorithm>
 #include <memory>  // IWYU pragma: keep
 #include <type_traits>
@@ -266,14 +266,14 @@ char *wcs2str(const wchar_t *in) {
         if (result) {
             // It converted into the local buffer, so copy it.
             result = strdup(result);
-            if (!result) DIE_MEM();
+            assert(result);
         }
         return result;
     }
 
     // Here we probably allocate a buffer probably much larger than necessary.
     char *out = (char *)malloc(MAX_UTF8_BYTES * wcslen(in) + 1);
-    if (!out) DIE_MEM();
+    assert(out);
     return wcs2str_internal(in, out);
 }
 
@@ -400,9 +400,7 @@ void append_formatv(wcstring &target, const wchar_t *format, va_list va_orig) {
                 break;
             }
             buff = (wchar_t *)realloc((buff == static_buff ? NULL : buff), size);
-            if (buff == NULL) {
-                DIE_MEM();
-            }
+            assert(buff != NULL);
         }
 
         // Try printing.
@@ -2031,3 +2029,10 @@ void redirect_tty_output() {
     if (tcgetattr(STDERR_FILENO, &t) == -1 && errno == EIO) dup2(fd, STDERR_FILENO);
     close(fd);
 }
+
+/// Display a failed assertion message, dump a stack trace if possible, then die.
+[[noreturn]] void __assert(const char *msg, const char *file, size_t line) {
+    debug(0, L"%s:%zu: failed assertion: %s", file, line, msg);
+    show_stackframe(L'E', 99, 1);
+    abort();
+}
diff --git a/src/common.h b/src/common.h
index 16798a130..a8deba97e 100644
--- a/src/common.h
+++ b/src/common.h
@@ -12,6 +12,7 @@
 #include <string.h>
 #include <termios.h>
 #include <wchar.h>
+
 #include <memory>
 #include <sstream>
 #include <string>
@@ -125,25 +126,6 @@ enum selection_direction_t {
     direction_deselect
 };
 
-inline bool selection_direction_is_cardinal(selection_direction_t dir) {
-    switch (dir) {
-        case direction_north:
-        case direction_east:
-        case direction_south:
-        case direction_west:
-        case direction_page_north:
-        case direction_page_south: {
-            return true;
-        }
-        case direction_next:
-        case direction_prev:
-        case direction_deselect: {
-            return false;
-        }
-        default: { abort(); }
-    }
-}
-
 /// Issue a debug message with printf-style string formating and automatic line breaking. The string
 /// will begin with the string \c program_name, followed by a colon and a whitespace.
 ///
@@ -244,20 +226,17 @@ extern bool has_working_tty_timestamps;
         exit_without_destructors(1);        \
     }
 
-/// Exit program at once after emitting an error message.
-#define DIE(msg)                                                                                  \
-    {                                                                                             \
-        debug(0, "%s on line %ld of file %s, shutting down fish", msg, (long)__LINE__, __FILE__); \
-        FATAL_EXIT();                                                                             \
-    }
-
-/// Exit program at once, leaving an error message about running out of memory.
-#define DIE_MEM()                                                                             \
-    {                                                                                         \
-        fwprintf(stderr, L"fish: Out of memory on line %ld of file %s, shutting down fish\n", \
-                 (long)__LINE__, __FILE__);                                                   \
-        FATAL_EXIT();                                                                         \
-    }
+/// Exit the program at once after emitting an error message and stack trace if possible.
+/// We use our own private implementation of `assert()` for two reasons. First, some implementations
+/// are subtly broken. For example, using `printf()` which can cause problems when mixed with wide
+/// stdio functions and should be writing the message to stderr rather than stdout. Second, if
+/// possible it is useful to provide additional context such as a stack backtrace.
+#undef assert
+#undef __assert
+//#define assert(e)  do {(void)((e) ? ((void)0) : __assert(#e, __FILE__, __LINE__)); } while(false)
+#define assert(e) (e) ? ((void)0) : __assert(#e, __FILE__, __LINE__)
+#define DIE(msg) __assert(msg, __FILE__, __LINE__)
+[[noreturn]] void __assert(const char *msg, const char *file, size_t line);
 
 /// Check if signals are blocked. If so, print an error message and return from the function
 /// performing this check.
diff --git a/src/complete.cpp b/src/complete.cpp
index 50e890617..2e609bac9 100644
--- a/src/complete.cpp
+++ b/src/complete.cpp
@@ -5,12 +5,12 @@
 ///
 #include "config.h"  // IWYU pragma: keep
 
-#include <assert.h>
 #include <pthread.h>
 #include <pwd.h>
 #include <stddef.h>
 #include <wchar.h>
 #include <wctype.h>
+
 #include <algorithm>
 #include <functional>
 #include <list>
diff --git a/src/complete.h b/src/complete.h
index 0820f0dce..afa3c7885 100644
--- a/src/complete.h
+++ b/src/complete.h
@@ -6,6 +6,7 @@
 #define FISH_COMPLETE_H
 
 #include <stdint.h>
+
 #include <vector>
 
 #include "common.h"
diff --git a/src/env.cpp b/src/env.cpp
index e1009196c..40f796746 100644
--- a/src/env.cpp
+++ b/src/env.cpp
@@ -1,7 +1,6 @@
 // Functions for setting and getting environment variables.
 #include "config.h"  // IWYU pragma: keep
 
-#include <assert.h>
 #include <errno.h>
 #include <locale.h>
 #include <pthread.h>
diff --git a/src/env.h b/src/env.h
index 29877fec0..ca8be6462 100644
--- a/src/env.h
+++ b/src/env.h
@@ -4,6 +4,7 @@
 
 #include <stddef.h>
 #include <stdint.h>
+
 #include <map>
 #include <memory>
 #include <string>
diff --git a/src/env_universal_common.cpp b/src/env_universal_common.cpp
index 47b54cd4c..43f919551 100644
--- a/src/env_universal_common.cpp
+++ b/src/env_universal_common.cpp
@@ -2,16 +2,15 @@
 #include "config.h"  // IWYU pragma: keep
 
 #include <arpa/inet.h>  // IWYU pragma: keep
-#include <assert.h>
 #include <errno.h>
 #include <fcntl.h>
 // We need the sys/file.h for the flock() declaration on Linux but not OS X.
 #include <sys/file.h>  // IWYU pragma: keep
 // We need the ioctl.h header so we can check if SIOCGIFHWADDR is defined by it so we know if we're
 // on a Linux system.
-#include <sys/ioctl.h>  // IWYU pragma: keep
 #include <limits.h>
 #include <netinet/in.h>  // IWYU pragma: keep
+#include <sys/ioctl.h>   // IWYU pragma: keep
 #if !defined(__APPLE__) && !defined(__CYGWIN__)
 #include <pwd.h>
 #endif
@@ -26,7 +25,7 @@
 #include <sys/select.h>  // IWYU pragma: keep
 #endif
 #include <sys/stat.h>
-#include <sys/time.h>  // IWYU pragma: keep
+#include <sys/time.h>   // IWYU pragma: keep
 #include <sys/types.h>  // IWYU pragma: keep
 #include <unistd.h>
 #include <wchar.h>
diff --git a/src/env_universal_common.h b/src/env_universal_common.h
index dcd7b0756..4eb63dadb 100644
--- a/src/env_universal_common.h
+++ b/src/env_universal_common.h
@@ -4,6 +4,7 @@
 
 #include <pthread.h>
 #include <stdio.h>
+
 #include <memory>
 #include <set>
 #include <vector>
diff --git a/src/event.h b/src/event.h
index 809489f44..aa05030d9 100644
--- a/src/event.h
+++ b/src/event.h
@@ -7,6 +7,7 @@
 #define FISH_EVENT_H
 
 #include <unistd.h>
+
 #include <memory>
 #include <vector>
 
diff --git a/src/exec.cpp b/src/exec.cpp
index 9a8d68a38..8b4d57528 100644
--- a/src/exec.cpp
+++ b/src/exec.cpp
@@ -4,7 +4,6 @@
 // performed have been massive.
 #include "config.h"
 
-#include <assert.h>
 #include <errno.h>
 #include <fcntl.h>
 #ifdef HAVE_SIGINFO_H
diff --git a/src/exec.h b/src/exec.h
index 5a7fa78cd..7ce80fb6f 100644
--- a/src/exec.h
+++ b/src/exec.h
@@ -3,6 +3,7 @@
 #define FISH_EXEC_H
 
 #include <stddef.h>
+
 #include <vector>
 
 #include "common.h"
diff --git a/src/expand.cpp b/src/expand.cpp
index 90f3305e6..e8da82563 100644
--- a/src/expand.cpp
+++ b/src/expand.cpp
@@ -2,7 +2,6 @@
 // IWYU pragma: no_include <cstddef>
 #include "config.h"
 
-#include <assert.h>
 #include <errno.h>
 #include <pwd.h>
 #include <stdarg.h>
diff --git a/src/expand.h b/src/expand.h
index bf4c1233a..cf984d1b1 100644
--- a/src/expand.h
+++ b/src/expand.h
@@ -8,6 +8,7 @@
 #include "config.h"
 
 #include <stddef.h>
+
 #include <string>
 #include <vector>
 
diff --git a/src/fallback.cpp b/src/fallback.cpp
index 20af4fd5f..0d58db0be 100644
--- a/src/fallback.cpp
+++ b/src/fallback.cpp
@@ -7,7 +7,6 @@
 
 // IWYU likes to recommend adding term.h when we want ncurses.h.
 // IWYU pragma: no_include term.h
-#include <assert.h>  // IWYU pragma: keep
 #include <dirent.h>  // IWYU pragma: keep
 #include <errno.h>   // IWYU pragma: keep
 #include <fcntl.h>   // IWYU pragma: keep
diff --git a/src/fish.cpp b/src/fish.cpp
index 5b74264a4..eeafa5591 100644
--- a/src/fish.cpp
+++ b/src/fish.cpp
@@ -30,6 +30,7 @@ Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA
 #include <sys/stat.h>
 #include <unistd.h>
 #include <wchar.h>
+
 #include <memory>
 #include <string>
 #include <vector>
@@ -454,9 +455,9 @@ int main(int argc, char **argv) {
                 res = reader_read(fd, empty_ios);
 
                 if (res) {
-                    debug(1, _(L"Error while reading file %ls\n"), reader_current_filename()
-                                                                       ? reader_current_filename()
-                                                                       : _(L"Standard input"));
+                    debug(1, _(L"Error while reading file %ls\n"),
+                          reader_current_filename() ? reader_current_filename()
+                                                    : _(L"Standard input"));
                 }
                 reader_pop_current_filename();
             }
diff --git a/src/fish_indent.cpp b/src/fish_indent.cpp
index 80235a2df..e4e6b867f 100644
--- a/src/fish_indent.cpp
+++ b/src/fish_indent.cpp
@@ -17,7 +17,6 @@ Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA
 */
 #include "config.h"  // IWYU pragma: keep
 
-#include <assert.h>
 #include <errno.h>
 #include <getopt.h>
 #include <locale.h>
@@ -27,6 +26,7 @@ Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA
 #include <string.h>
 #include <wchar.h>
 #include <wctype.h>
+
 #include <memory>
 #include <string>
 #include <vector>
diff --git a/src/fish_tests.cpp b/src/fish_tests.cpp
index 3a055ad13..b5705aea7 100644
--- a/src/fish_tests.cpp
+++ b/src/fish_tests.cpp
@@ -3,7 +3,6 @@
 
 // IWYU pragma: no_include <cstring>
 // IWYU pragma: no_include <cstddef>
-#include <assert.h>
 #include <errno.h>
 #include <libgen.h>
 #include <limits.h>
diff --git a/src/function.cpp b/src/function.cpp
index da5fc30b5..b3dc699ea 100644
--- a/src/function.cpp
+++ b/src/function.cpp
@@ -10,6 +10,7 @@
 #include <pthread.h>
 #include <stddef.h>
 #include <wchar.h>
+
 #include <map>
 #include <memory>
 #include <set>
diff --git a/src/highlight.h b/src/highlight.h
index a4fd9b6bd..e4e255f27 100644
--- a/src/highlight.h
+++ b/src/highlight.h
@@ -2,9 +2,9 @@
 #ifndef FISH_HIGHLIGHT_H
 #define FISH_HIGHLIGHT_H
 
-#include <assert.h>
 #include <stddef.h>
 #include <stdint.h>
+
 #include <vector>
 
 #include "color.h"
diff --git a/src/history.cpp b/src/history.cpp
index 31b04263d..595304f62 100644
--- a/src/history.cpp
+++ b/src/history.cpp
@@ -1,7 +1,6 @@
 // History functions, part of the user interface.
 #include "config.h"  // IWYU pragma: keep
 
-#include <assert.h>
 #include <ctype.h>
 #include <errno.h>
 #include <fcntl.h>
diff --git a/src/history.h b/src/history.h
index 6a29d20ce..d398fd9dd 100644
--- a/src/history.h
+++ b/src/history.h
@@ -9,6 +9,7 @@
 #include <stdio.h>
 #include <time.h>
 #include <wctype.h>
+
 #include <deque>
 #include <memory>
 #include <set>
diff --git a/src/input.cpp b/src/input.cpp
index 4bdd78544..17b5c6dcc 100644
--- a/src/input.cpp
+++ b/src/input.cpp
@@ -1,7 +1,6 @@
 // Functions for reading a character of input from stdin.
 #include "config.h"
 
-#include <assert.h>
 #include <errno.h>
 #include <stdio.h>
 #include <stdlib.h>
diff --git a/src/input.h b/src/input.h
index 13ed5bea9..403dabf4c 100644
--- a/src/input.h
+++ b/src/input.h
@@ -4,6 +4,7 @@
 #define FISH_INPUT_H
 
 #include <stddef.h>
+
 #include <vector>
 
 #include "common.h"
diff --git a/src/input_common.h b/src/input_common.h
index 3b8bb0535..e48b32d91 100644
--- a/src/input_common.h
+++ b/src/input_common.h
@@ -3,6 +3,7 @@
 #define INPUT_COMMON_H
 
 #include <stddef.h>
+
 #include <functional>
 
 #include "common.h"
diff --git a/src/intern.cpp b/src/intern.cpp
index 2ca0340ac..d7fb6fff6 100644
--- a/src/intern.cpp
+++ b/src/intern.cpp
@@ -3,6 +3,7 @@
 
 #include <stddef.h>
 #include <wchar.h>
+
 #include <algorithm>
 #include <memory>
 #include <vector>
diff --git a/src/io.cpp b/src/io.cpp
index 22af1195e..0dfb1f75f 100644
--- a/src/io.cpp
+++ b/src/io.cpp
@@ -1,7 +1,6 @@
 // Utilities for io redirection.
 #include "config.h"  // IWYU pragma: keep
 
-#include <assert.h>
 #include <errno.h>
 #include <stddef.h>
 #include <stdio.h>
diff --git a/src/io.h b/src/io.h
index 25a277cd7..a2340362e 100644
--- a/src/io.h
+++ b/src/io.h
@@ -4,6 +4,7 @@
 #include <stdarg.h>
 #include <stddef.h>
 #include <stdlib.h>
+
 #include <vector>
 // Note that we have to include something to get any _LIBCPP_VERSION defined so we can detect libc++
 // So it's key that vector go above. If we didn't need vector for other reasons, we might include
diff --git a/src/iothread.cpp b/src/iothread.cpp
index 81dd8537d..f4807dd21 100644
--- a/src/iothread.cpp
+++ b/src/iothread.cpp
@@ -1,6 +1,5 @@
 #include "config.h"  // IWYU pragma: keep
 
-#include <assert.h>
 #include <errno.h>
 #include <limits.h>
 #include <pthread.h>
@@ -9,6 +8,7 @@
 #include <sys/time.h>
 #include <sys/types.h>
 #include <unistd.h>
+
 #include <queue>
 
 #include "common.h"
@@ -40,8 +40,7 @@ struct spawn_request_t {
 
     spawn_request_t() {}
 
-    spawn_request_t(void_function_t &&f, void_function_t &&comp)
-        : handler(f), completion(comp) {}
+    spawn_request_t(void_function_t &&f, void_function_t &&comp) : handler(f), completion(comp) {}
 
     // Move-only
     spawn_request_t &operator=(const spawn_request_t &) = delete;
diff --git a/src/lru.h b/src/lru.h
index c3159c8f3..51d7c5e5f 100644
--- a/src/lru.h
+++ b/src/lru.h
@@ -2,8 +2,8 @@
 #ifndef FISH_LRU_H
 #define FISH_LRU_H
 
-#include <assert.h>
 #include <wchar.h>
+
 #include <map>
 
 #include "common.h"
@@ -303,19 +303,19 @@ class lru_cache_t {
         size_t count = 0;
         while (cursor != &mouth) {
             if (cursor->prev != prev) {
-                assert(0 && "Node busted previous link");
+                DIE("node busted previous link");
             }
             prev = cursor;
             cursor = cursor->next;
             if (count++ > max) {
-                assert(0 && "LRU cache unable to re-reach the mouth - not circularly linked?");
+                DIE("LRU cache unable to re-reach the mouth - not circularly linked?");
             }
         }
         if (mouth.prev != prev) {
-            assert(0 && "mouth.prev does not connect to last node");
+            DIE("mouth.prev does not connect to last node");
         }
         if (count != expected_count) {
-            assert(0 && "Linked list count mismatch from map count");
+            DIE("linked list count mismatch from map count");
         }
 
         // Count iterators
@@ -325,7 +325,7 @@ class lru_cache_t {
             iter_dist++;
         }
         if (iter_dist != count) {
-            assert(0 && "Linked list iterator mismatch from map count");
+            DIE("linked list iterator mismatch from map count");
         }
     }
 };
diff --git a/src/output.cpp b/src/output.cpp
index 061b8dff9..92b2355bb 100644
--- a/src/output.cpp
+++ b/src/output.cpp
@@ -18,6 +18,7 @@
 #endif
 #include <limits.h>
 #include <wchar.h>
+
 #include <memory>
 #include <string>
 #include <vector>
diff --git a/src/output.h b/src/output.h
index 6ca31d92c..26fbdc260 100644
--- a/src/output.h
+++ b/src/output.h
@@ -6,6 +6,7 @@
 #define FISH_OUTPUT_H
 
 #include <stddef.h>
+
 #include <vector>
 
 #include "color.h"
diff --git a/src/pager.cpp b/src/pager.cpp
index d84f7913d..e8b0c4967 100644
--- a/src/pager.cpp
+++ b/src/pager.cpp
@@ -1,7 +1,6 @@
 #include "config.h"  // IWYU pragma: keep
 
 // IWYU pragma: no_include <cstddef>
-#include <assert.h>
 #include <stddef.h>
 #include <wchar.h>
 #include <wctype.h>
@@ -40,6 +39,24 @@ typedef std::vector<comp_t> comp_info_list_t;
 /// Text we use for the search field.
 #define SEARCH_FIELD_PROMPT _(L"search: ")
 
+inline bool selection_direction_is_cardinal(selection_direction_t dir) {
+    switch (dir) {
+        case direction_north:
+        case direction_east:
+        case direction_south:
+        case direction_west:
+        case direction_page_north:
+        case direction_page_south: {
+            return true;
+        }
+        case direction_next:
+        case direction_prev:
+        case direction_deselect: {
+            return false;
+        }
+    }
+}
+
 /// Returns numer / denom, rounding up. As a "courtesy" 0/0 is 0.
 static size_t divide_round_up(size_t numer, size_t denom) {
     if (numer == 0) return 0;
diff --git a/src/pager.h b/src/pager.h
index 20c5bcf50..1d1571fb3 100644
--- a/src/pager.h
+++ b/src/pager.h
@@ -3,6 +3,7 @@
 #define FISH_PAGER_H
 
 #include <stddef.h>
+
 #include <memory>
 #include <string>
 #include <vector>
diff --git a/src/parse_execution.cpp b/src/parse_execution.cpp
index 7d65622d5..689c9a16c 100644
--- a/src/parse_execution.cpp
+++ b/src/parse_execution.cpp
@@ -8,7 +8,6 @@
 // for the execution to finish to see them.
 #include "config.h"  // IWYU pragma: keep
 
-#include <assert.h>
 #include <errno.h>
 #include <stdarg.h>
 #include <stdio.h>
diff --git a/src/parse_tree.cpp b/src/parse_tree.cpp
index e6372f3a7..41dc50896 100644
--- a/src/parse_tree.cpp
+++ b/src/parse_tree.cpp
@@ -1,7 +1,6 @@
 // Programmatic representation of fish code.
 #include "config.h"  // IWYU pragma: keep
 
-#include <assert.h>
 #include <stdarg.h>
 #include <stddef.h>
 #include <stdio.h>
diff --git a/src/parse_tree.h b/src/parse_tree.h
index 4f78fd4b0..3d686ce58 100644
--- a/src/parse_tree.h
+++ b/src/parse_tree.h
@@ -2,10 +2,10 @@
 #ifndef FISH_PARSE_PRODUCTIONS_H
 #define FISH_PARSE_PRODUCTIONS_H
 
-#include <assert.h>
 #include <stddef.h>
 #include <stdint.h>
 #include <sys/types.h>
+
 #include <memory>
 #include <vector>
 
diff --git a/src/parse_util.cpp b/src/parse_util.cpp
index eea3680a8..bf30dad32 100644
--- a/src/parse_util.cpp
+++ b/src/parse_util.cpp
@@ -4,7 +4,6 @@
 // that are somehow related to parsing the code.
 #include "config.h"  // IWYU pragma: keep
 
-#include <assert.h>
 #include <stdarg.h>
 #include <stdlib.h>
 #include <wchar.h>
@@ -300,9 +299,7 @@ static void job_or_process_extent(const wchar_t *buff, size_t cursor_pos, const
     if (a) *a = begin;
     if (b) *b = end;
     buffcpy = wcsndup(begin, end - begin);
-    if (!buffcpy) {
-        DIE_MEM();
-    }
+    assert(buffcpy != NULL);
 
     tokenizer_t tok(buffcpy, TOK_ACCEPT_UNFINISHED);
     tok_t token;
@@ -659,8 +656,9 @@ std::vector<int> parse_util_compute_indents(const wcstring &src) {
     // foo ; cas', we get an invalid parse tree (since 'cas' is not valid) but we indent it as if it
     // were a case item list.
     parse_node_tree_t tree;
-    parse_tree_from_string(src, parse_flag_continue_after_error | parse_flag_include_comments |
-                                    parse_flag_accept_incomplete_tokens,
+    parse_tree_from_string(src,
+                           parse_flag_continue_after_error | parse_flag_include_comments |
+                               parse_flag_accept_incomplete_tokens,
                            &tree, NULL /* errors */);
 
     // Start indenting at the first node. If we have a parse error, we'll have to start indenting
diff --git a/src/parse_util.h b/src/parse_util.h
index a4d4e29ef..19a85a8ee 100644
--- a/src/parse_util.h
+++ b/src/parse_util.h
@@ -3,6 +3,7 @@
 #define FISH_PARSE_UTIL_H
 
 #include <stddef.h>
+
 #include <vector>
 
 #include "common.h"
diff --git a/src/parser.cpp b/src/parser.cpp
index 0a3194cc2..27da941a8 100644
--- a/src/parser.cpp
+++ b/src/parser.cpp
@@ -1,9 +1,9 @@
 // The fish parser. Contains functions for parsing and evaluating code.
 #include "config.h"  // IWYU pragma: keep
 
-#include <assert.h>
 #include <stdio.h>
 #include <wchar.h>
+
 #include <algorithm>
 #include <memory>
 
diff --git a/src/path.cpp b/src/path.cpp
index ae4be9618..6ae5c11bd 100644
--- a/src/path.cpp
+++ b/src/path.cpp
@@ -3,7 +3,6 @@
 // issues.
 #include "config.h"  // IWYU pragma: keep
 
-#include <assert.h>
 #include <errno.h>
 #include <stdio.h>
 #include <string.h>
diff --git a/src/proc.cpp b/src/proc.cpp
index 2641aff6c..2a57e5ab1 100644
--- a/src/proc.cpp
+++ b/src/proc.cpp
@@ -14,8 +14,7 @@
 #include <unistd.h>
 #include <wchar.h>
 #include <wctype.h>
-#include <memory>
-#include <vector>
+
 #if HAVE_TERM_H
 #include <term.h>
 #elif HAVE_NCURSES_TERM_H
@@ -29,7 +28,10 @@
 #endif
 #include <sys/time.h>  // IWYU pragma: keep
 #include <sys/types.h>
+
 #include <algorithm>  // IWYU pragma: keep
+#include <memory>
+#include <vector>
 
 #include "common.h"
 #include "event.h"
diff --git a/src/proc.h b/src/proc.h
index ef2aaa759..928e6a8c2 100644
--- a/src/proc.h
+++ b/src/proc.h
@@ -5,7 +5,6 @@
 #define FISH_PROC_H
 #include "config.h"  // IWYU pragma: keep
 
-#include <assert.h>
 #include <signal.h>
 #include <stddef.h>
 #include <sys/time.h>  // IWYU pragma: keep
diff --git a/src/reader.cpp b/src/reader.cpp
index 1aac55357..5c4fe0be6 100644
--- a/src/reader.cpp
+++ b/src/reader.cpp
@@ -14,7 +14,6 @@
 #include "config.h"
 
 // IWYU pragma: no_include <type_traits>
-#include <assert.h>
 #include <errno.h>
 #include <fcntl.h>
 #include <pthread.h>
diff --git a/src/reader.h b/src/reader.h
index da675fc68..72680bd87 100644
--- a/src/reader.h
+++ b/src/reader.h
@@ -5,6 +5,7 @@
 #define FISH_READER_H
 
 #include <stddef.h>
+
 #include <string>
 #include <vector>
 
diff --git a/src/screen.cpp b/src/screen.cpp
index 3bccc2411..cad973d4b 100644
--- a/src/screen.cpp
+++ b/src/screen.cpp
@@ -10,7 +10,10 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <time.h>
 #include <unistd.h>
+#include <wchar.h>
+
 #if HAVE_NCURSES_H
 #include <ncurses.h>
 #elif HAVE_NCURSES_CURSES_H
@@ -23,9 +26,7 @@
 #elif HAVE_NCURSES_TERM_H
 #include <ncurses/term.h>
 #endif
-#include <assert.h>
-#include <time.h>
-#include <wchar.h>
+
 #include <algorithm>
 #include <string>
 #include <vector>
diff --git a/src/screen.h b/src/screen.h
index 6ae6f0cb7..2c4212fe6 100644
--- a/src/screen.h
+++ b/src/screen.h
@@ -10,12 +10,10 @@
 #define FISH_SCREEN_H
 #include "config.h"  // IWYU pragma: keep
 
-#include <assert.h>
 #include <stddef.h>
 #include <sys/stat.h>
 
 #include <algorithm>
-#include <cstddef>
 #include <map>
 #include <memory>
 #include <set>
diff --git a/src/tokenizer.cpp b/src/tokenizer.cpp
index 5fecf7f2f..cb99b98e7 100644
--- a/src/tokenizer.cpp
+++ b/src/tokenizer.cpp
@@ -2,7 +2,6 @@
 // extended to support marks, tokenizing multiple strings and disposing of unused string segments.
 #include "config.h"  // IWYU pragma: keep
 
-#include <assert.h>
 #include <fcntl.h>
 #include <limits.h>
 #include <unistd.h>
diff --git a/src/utf8.cpp b/src/utf8.cpp
index 55d404fe8..738d82168 100644
--- a/src/utf8.cpp
+++ b/src/utf8.cpp
@@ -17,6 +17,7 @@
 
 #include <stdint.h>  // IWYU pragma: keep
 #include <sys/types.h>
+
 #include <limits>
 #include <string>
 
diff --git a/src/utf8.h b/src/utf8.h
index 3879459b4..49d2b62b8 100644
--- a/src/utf8.h
+++ b/src/utf8.h
@@ -19,6 +19,7 @@
 #define _UTF8_H_
 
 #include <stddef.h>
+
 #include <string>
 
 #define UTF8_IGNORE_ERROR 0x01
diff --git a/src/wildcard.cpp b/src/wildcard.cpp
index 8cc6ca3de..f871db448 100644
--- a/src/wildcard.cpp
+++ b/src/wildcard.cpp
@@ -2,7 +2,6 @@
 // provides recursive wildcards using **.
 #include "config.h"  // IWYU pragma: keep
 
-#include <assert.h>
 #include <dirent.h>
 #include <errno.h>
 #include <stddef.h>
@@ -294,7 +293,7 @@ static bool wildcard_complete_internal(const wchar_t *str, const wchar_t *wc,
             return false;
         }
         default: {
-            DIE("Unreachable code reached");
+            DIE("unreachable code reached");
             break;
         }
     }
diff --git a/src/wutil.cpp b/src/wutil.cpp
index 432fd597a..f771696d5 100644
--- a/src/wutil.cpp
+++ b/src/wutil.cpp
@@ -2,7 +2,6 @@
 #define FISH_NO_ISW_WRAPPERS
 #include "config.h"
 
-#include <assert.h>
 #include <dirent.h>
 #include <errno.h>
 #include <fcntl.h>
@@ -353,7 +352,7 @@ wchar_t *wrealpath(const wcstring &pathname, wchar_t *resolved_path) {
             if (pathsep_idx == cstring::npos) {
                 // No pathsep means a single path component relative to pwd.
                 narrow_res = realpath(".", NULL);
-                if (!narrow_res) DIE("unexpected realpath(\".\") failure");
+                assert(narrow_res != NULL);
                 pathsep_idx = 0;
             } else {
                 // Only call realpath() on the portion up to the last component.