diff --git a/fish-rust/src/fallback.rs b/fish-rust/src/fallback.rs
new file mode 100644
index 000000000..3a1ba2e10
--- /dev/null
+++ b/fish-rust/src/fallback.rs
@@ -0,0 +1,123 @@
+//! This file only contains fallback implementations of functions which have been found to be missing
+//! or broken by the configuration scripts.
+//!
+//! Many of these functions are more or less broken and incomplete.
+
+use crate::widecharwidth::{WcLookupTable, WcWidth};
+use crate::{common::is_console_session, wchar::wstr};
+use once_cell::sync::Lazy;
+use std::sync::atomic::{AtomicI32, Ordering};
+use std::{ffi::CString, mem, os::fd::RawFd};
+
+// Width of ambiguous characters. 1 is typical default.
+static mut FISH_AMBIGUOUS_WIDTH: AtomicI32 = AtomicI32::new(1);
+
+// Width of emoji characters.
+// 1 is the typical emoji width in Unicode 8.
+static mut FISH_EMOJI_WIDTH: AtomicI32 = AtomicI32::new(1);
+
+fn fish_get_emoji_width() -> i32 {
+    unsafe { FISH_EMOJI_WIDTH.load(Ordering::Relaxed) }
+}
+
+extern "C" {
+    pub fn wcwidth(c: libc::wchar_t) -> libc::c_int;
+}
+fn system_wcwidth(c: char) -> i32 {
+    const _: () = assert!(mem::size_of::<libc::wchar_t>() >= mem::size_of::<char>());
+    unsafe { wcwidth(c as libc::wchar_t) }
+}
+
+static WC_LOOKUP_TABLE: Lazy<WcLookupTable> = Lazy::new(WcLookupTable::new);
+
+// Big hack to use our versions of wcswidth where we know them to be broken, which is
+// EVERYWHERE (https://github.com/fish-shell/fish-shell/issues/2199)
+pub fn fish_wcwidth(c: char) -> i32 {
+    // The system version of wcwidth should accurately reflect the ability to represent characters
+    // in the console session, but knows nothing about the capabilities of other terminal emulators
+    // or ttys. Use it from the start only if we are logged in to the physical console.
+    if is_console_session() {
+        return system_wcwidth(c);
+    }
+
+    // Check for VS16 which selects emoji presentation. This "promotes" a character like U+2764
+    // (width 1) to an emoji (probably width 2). So treat it as width 1 so the sums work. See #2652.
+    // VS15 selects text presentation.
+    let variation_selector_16 = '\u{FE0F}';
+    let variation_selector_15 = '\u{FE0E}';
+    if c == variation_selector_16 {
+        return 1;
+    } else if c == variation_selector_15 {
+        return 0;
+    }
+
+    // Check for Emoji_Modifier property. Only the Fitzpatrick modifiers have this, in range
+    // 1F3FB..1F3FF. This is a hack because such an emoji appearing on its own would be drawn as
+    // width 2, but that's unlikely to be useful. See #8275.
+    if ('\u{F3FB}'..='\u{1F3FF}').contains(&c) {
+        return 0;
+    }
+
+    let width = WC_LOOKUP_TABLE.classify(c);
+    match width {
+        WcWidth::NonCharacter | WcWidth::NonPrint | WcWidth::Combining | WcWidth::Unassigned => {
+            // Fall back to system wcwidth in this case.
+            system_wcwidth(c)
+        }
+        WcWidth::Ambiguous | WcWidth::PrivateUse => {
+            // TR11: "All private-use characters are by default classified as Ambiguous".
+            unsafe { FISH_AMBIGUOUS_WIDTH.load(Ordering::Relaxed) }
+        }
+        WcWidth::One => 1,
+        WcWidth::Two => 2,
+        WcWidth::WidenedIn9 => fish_get_emoji_width(),
+    }
+}
+
+/// fish's internal versions of wcwidth and wcswidth, which can use an internal implementation if
+/// the system one is busted.
+pub fn fish_wcswidth(s: &wstr) -> i32 {
+    let mut result = 0;
+    for c in s.chars() {
+        let w = fish_wcwidth(c);
+        if w < 0 {
+            return -1;
+        }
+        result += w;
+    }
+    result
+}
+
+// Replacement for mkostemp(str, O_CLOEXEC)
+// This uses mkostemp if available,
+// otherwise it uses mkstemp followed by fcntl
+pub fn fish_mkstemp_cloexec(name_template: CString) -> (RawFd, CString) {
+    let name = name_template.into_raw();
+    #[cfg(not(target_os = "macos"))]
+    let fd = {
+        use libc::O_CLOEXEC;
+        unsafe { libc::mkostemp(name, O_CLOEXEC) }
+    };
+    #[cfg(target_os = "macos")]
+    let fd = {
+        use libc::{FD_CLOEXEC, F_SETFD};
+        let fd = unsafe { libc::mkstemp(name) };
+        if fd != -1 {
+            unsafe { libc::fcntl(fd, F_SETFD, FD_CLOEXEC) };
+        }
+        fd
+    };
+    (fd, unsafe { CString::from_raw(name) })
+}
+
+pub fn fish_tparm() {
+    todo!()
+}
+
+pub fn wcscasecmp(_s1: &wstr, _s2: &wstr) {
+    todo!()
+}
+
+pub fn wcsncasecmp(_s1: &wstr, _s2: &wstr) {
+    todo!()
+}
diff --git a/fish-rust/src/lib.rs b/fish-rust/src/lib.rs
index 8773c736b..38b038f43 100644
--- a/fish-rust/src/lib.rs
+++ b/fish-rust/src/lib.rs
@@ -17,6 +17,7 @@ mod compat;
 mod env;
 mod event;
 mod expand;
+mod fallback;
 mod fd_monitor;
 mod fd_readable_set;
 mod fds;