1 files changed, 751 insertions, 0 deletions
diff --git a/xpcom/rust/nsstring/src/conversions.rs b/xpcom/rust/nsstring/src/conversions.rs
new file mode 100644
index 0000000000..c72c195c08
--- /dev/null
+++ b/xpcom/rust/nsstring/src/conversions.rs
@@ -0,0 +1,751 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+use crate::{
+    nsACString, nsAString, nsCStringLike, BulkWriteOk, Gecko_FallibleAssignCString,
+    Latin1StringLike,
+};
+use encoding_rs::mem::*;
+use encoding_rs::Encoding;
+use std::slice;
+
+/// Required math stated in the docs of
+/// `convert_utf16_to_utf8()`.
+#[inline(always)]
+fn times_three(a: usize) -> Option<usize> {
+    a.checked_mul(3)
+}
+
+#[inline(always)]
+fn identity(a: usize) -> Option<usize> {
+    Some(a)
+}
+
+#[inline(always)]
+fn plus_one(a: usize) -> Option<usize> {
+    a.checked_add(1)
+}
+
+/// Typical cache line size per
+/// https://stackoverflow.com/questions/14707803/line-size-of-l1-and-l2-caches
+///
+/// For consistent behavior, not trying to use 128 on aarch64
+/// or other fanciness like that.
+const CACHE_LINE: usize = 64;
+
+const CACHE_LINE_MASK: usize = CACHE_LINE - 1;
+
+/// Returns true if the string is both longer than a cache line
+/// and the first cache line is ASCII.
+#[inline(always)]
+fn long_string_starts_with_ascii(buffer: &[u8]) -> bool {
+    // We examine data only up to the end of the cache line
+    // to make this check minimally disruptive.
+    if buffer.len() <= CACHE_LINE {
+        return false;
+    }
+    let bound = CACHE_LINE - ((buffer.as_ptr() as usize) & CACHE_LINE_MASK);
+    is_ascii(&buffer[..bound])
+}
+
+/// Returns true if the string is both longer than two cache lines
+/// and the first two cache lines are Basic Latin.
+#[inline(always)]
+fn long_string_stars_with_basic_latin(buffer: &[u16]) -> bool {
+    // We look at two cache lines with code unit size of two. There is need
+    // to look at more than one cache line in the UTF-16 case, because looking
+    // at just one cache line wouldn't catch non-ASCII Latin with high enough
+    // probability with Latin-script languages that have relatively infrequent
+    // non-ASCII characters.
+    if buffer.len() <= CACHE_LINE {
+        return false;
+    }
+    let bound = (CACHE_LINE * 2 - ((buffer.as_ptr() as usize) & CACHE_LINE_MASK)) / 2;
+    is_basic_latin(&buffer[..bound])
+}
+
+// Ignoring the copy avoidance complications of conversions between Latin1 and
+// UTF-8, a conversion function has the outward form of
+// `fn F(&mut self, other: &[T], old_len: usize) -> Result<BulkWriteOk, ()>`,
+// where `T` is either `u8` or `u16`. `other` is the slice whose converted
+// content are to be appended to `self` and `old_len` indicates how many
+// code unit of `self` are to be preserved (0 for the assignment case and
+// `self.len()` for the appending case).
+//
+// As implementation parameters a conversion function needs to know the
+// math for computing the worst case conversion length in code units given
+// the input length in code units. For a _constant conversion_ the number
+// of code units the conversion produces equals the number of code units
+// in the input. For a _shinking conversion_ the maximum number of code
+// units the conversion can produce equals the number of code units in
+// the input, but the conversion can produce fewer code units. Still, due
+// to implementation details, the function might want _one_ unit more of
+// output space. For an _expanding conversion_ (no need for macro), the
+// minimum number of code units produced by the conversion is the number
+// of code units in the input, but the conversion can produce more.
+//
+// Copy avoidance conversions avoid copying a refcounted buffer when it's
+// ASCII-only.
+//
+// Internally, a conversion function needs to know the underlying
+// encoding_rs conversion function, the math for computing the required
+// output buffer size and, depending on the case, the underlying
+// encoding_rs ASCII prefix handling function.
+
+/// A conversion where the number of code units in the output is potentially
+/// smaller than the number of code units in the input.
+///
+/// Takes the name of the method to be generated, the name of the conversion
+/// function and the type of the input slice.
+///
+/// `$name` is the name of the function to generate
+/// `$convert` is the underlying `encoding_rs::mem` function to use
+/// `$other_ty` is the type of the input slice
+/// `$math` is the worst-case length math that `$convert` expects
+macro_rules! shrinking_conversion {
+    (name = $name:ident,
+     convert = $convert:ident,
+     other_ty = $other_ty:ty,
+     math = $math:ident) => {
+        fn $name(&mut self, other: $other_ty, old_len: usize) -> Result<BulkWriteOk, ()> {
+            let needed = $math(other.len()).ok_or(())?;
+            let mut handle =
+                unsafe { self.bulk_write(old_len.checked_add(needed).ok_or(())?, old_len, false)? };
+            let written = $convert(other, &mut handle.as_mut_slice()[old_len..]);
+            let new_len = old_len + written;
+            Ok(handle.finish(new_len, new_len > CACHE_LINE))
+        }
+    };
+}
+
+/// A conversion where the number of code units in the output is always equal
+/// to the number of code units in the input.
+///
+/// Takes the name of the method to be generated, the name of the conversion
+/// function and the type of the input slice.
+///
+/// `$name` is the name of the function to generate
+/// `$convert` is the underlying `encoding_rs::mem` function to use
+/// `$other_ty` is the type of the input slice
+macro_rules! constant_conversion {
+    (name = $name:ident,
+     convert = $convert:ident,
+     other_ty = $other_ty:ty) => {
+        fn $name(
+            &mut self,
+            other: $other_ty,
+            old_len: usize,
+            allow_shrinking: bool,
+        ) -> Result<BulkWriteOk, ()> {
+            let new_len = old_len.checked_add(other.len()).ok_or(())?;
+            let mut handle = unsafe { self.bulk_write(new_len, old_len, allow_shrinking)? };
+            $convert(other, &mut handle.as_mut_slice()[old_len..]);
+            Ok(handle.finish(new_len, false))
+        }
+    };
+}
+
+/// An intermediate check for avoiding a copy and having an `nsStringBuffer`
+/// refcount increment instead when both `self` and `other` are `nsACString`s,
+/// `other` is entirely ASCII and all old data in `self` is discarded.
+///
+/// `$name` is the name of the function to generate
+/// `$impl` is the underlying conversion that takes a slice and that is used
+///         when we can't just adopt the incoming buffer as-is
+/// `$string_like` is the kind of input taken
+macro_rules! ascii_copy_avoidance {
+    (name = $name:ident,
+     implementation = $implementation:ident,
+     string_like = $string_like:ident) => {
+        fn $name<T: $string_like + ?Sized>(
+            &mut self,
+            other: &T,
+            old_len: usize,
+        ) -> Result<BulkWriteOk, ()> {
+            let adapter = other.adapt();
+            let other_slice = adapter.as_ref();
+            let num_ascii = if adapter.is_abstract() && old_len == 0 {
+                let up_to = Encoding::ascii_valid_up_to(other_slice);
+                if up_to == other_slice.len() {
+                    // Calling something whose argument can be obtained from
+                    // the adapter rather than an nsStringLike avoids a huge
+                    // lifetime mess by keeping nsStringLike and
+                    // Latin1StringLike free of lifetime interdependencies.
+                    if unsafe { Gecko_FallibleAssignCString(self, other.adapt().as_ptr()) } {
+                        return Ok(BulkWriteOk {});
+                    } else {
+                        return Err(());
+                    }
+                }
+                Some(up_to)
+            } else {
+                None
+            };
+            self.$implementation(other_slice, old_len, num_ascii)
+        }
+    };
+}
+
+impl nsAString {
+    // Valid UTF-8 to UTF-16
+
+    // Documentation says the destination buffer needs to have
+    // as many code units as the input.
+    shrinking_conversion!(
+        name = fallible_append_str_impl,
+        convert = convert_str_to_utf16,
+        other_ty = &str,
+        math = identity
+    );
+
+    /// Convert a valid UTF-8 string into valid UTF-16 and replace the content
+    /// of this string with the conversion result.
+    pub fn assign_str(&mut self, other: &str) {
+        self.fallible_append_str_impl(other, 0)
+            .expect("Out of memory");
+    }
+
+    /// Convert a valid UTF-8 string into valid UTF-16 and fallibly replace the
+    /// content of this string with the conversion result.
+    pub fn fallible_assign_str(&mut self, other: &str) -> Result<(), ()> {
+        self.fallible_append_str_impl(other, 0).map(|_| ())
+    }
+
+    /// Convert a valid UTF-8 string into valid UTF-16 and append the conversion
+    /// to this string.
+    pub fn append_str(&mut self, other: &str) {
+        let len = self.len();
+        self.fallible_append_str_impl(other, len)
+            .expect("Out of memory");
+    }
+
+    /// Convert a valid UTF-8 string into valid UTF-16 and fallibly append the
+    /// conversion to this string.
+    pub fn fallible_append_str(&mut self, other: &str) -> Result<(), ()> {
+        let len = self.len();
+        self.fallible_append_str_impl(other, len).map(|_| ())
+    }
+
+    // Potentially-invalid UTF-8 to UTF-16
+
+    // Documentation says the destination buffer needs to have
+    // one more code unit than the input.
+    shrinking_conversion!(
+        name = fallible_append_utf8_impl,
+        convert = convert_utf8_to_utf16,
+        other_ty = &[u8],
+        math = plus_one
+    );
+
+    /// Convert a potentially-invalid UTF-8 string into valid UTF-16
+    /// (replacing invalid sequences with the REPLACEMENT CHARACTER) and
+    /// replace the content of this string with the conversion result.
+    pub fn assign_utf8(&mut self, other: &[u8]) {
+        self.fallible_append_utf8_impl(other, 0)
+            .expect("Out of memory");
+    }
+
+    /// Convert a potentially-invalid UTF-8 string into valid UTF-16
+    /// (replacing invalid sequences with the REPLACEMENT CHARACTER) and
+    /// fallibly replace the content of this string with the conversion result.
+    pub fn fallible_assign_utf8(&mut self, other: &[u8]) -> Result<(), ()> {
+        self.fallible_append_utf8_impl(other, 0).map(|_| ())
+    }
+
+    /// Convert a potentially-invalid UTF-8 string into valid UTF-16
+    /// (replacing invalid sequences with the REPLACEMENT CHARACTER) and
+    /// append the conversion result to this string.
+    pub fn append_utf8(&mut self, other: &[u8]) {
+        let len = self.len();
+        self.fallible_append_utf8_impl(other, len)
+            .expect("Out of memory");
+    }
+
+    /// Convert a potentially-invalid UTF-8 string into valid UTF-16
+    /// (replacing invalid sequences with the REPLACEMENT CHARACTER) and
+    /// fallibly append the conversion result to this string.
+    pub fn fallible_append_utf8(&mut self, other: &[u8]) -> Result<(), ()> {
+        let len = self.len();
+        self.fallible_append_utf8_impl(other, len).map(|_| ())
+    }
+
+    // Latin1 to UTF-16
+
+    constant_conversion!(
+        name = fallible_append_latin1_impl,
+        convert = convert_latin1_to_utf16,
+        other_ty = &[u8]
+    );
+
+    /// Convert a Latin1 (i.e. byte value equals scalar value; not windows-1252!)
+    /// into UTF-16 and replace the content of this string with the conversion result.
+    pub fn assign_latin1(&mut self, other: &[u8]) {
+        self.fallible_append_latin1_impl(other, 0, true)
+            .expect("Out of memory");
+    }
+
+    /// Convert a Latin1 (i.e. byte value equals scalar value; not windows-1252!)
+    /// into UTF-16 and fallibly replace the content of this string with the
+    /// conversion result.
+    pub fn fallible_assign_latin1(&mut self, other: &[u8]) -> Result<(), ()> {
+        self.fallible_append_latin1_impl(other, 0, true).map(|_| ())
+    }
+
+    /// Convert a Latin1 (i.e. byte value equals scalar value; not windows-1252!)
+    /// into UTF-16 and append the conversion result to this string.
+    pub fn append_latin1(&mut self, other: &[u8]) {
+        let len = self.len();
+        self.fallible_append_latin1_impl(other, len, false)
+            .expect("Out of memory");
+    }
+
+    /// Convert a Latin1 (i.e. byte value equals scalar value; not windows-1252!)
+    /// into UTF-16 and fallibly append the conversion result to this string.
+    pub fn fallible_append_latin1(&mut self, other: &[u8]) -> Result<(), ()> {
+        let len = self.len();
+        self.fallible_append_latin1_impl(other, len, false)
+            .map(|_| ())
+    }
+}
+
+impl nsACString {
+    // UTF-16 to UTF-8
+
+    fn fallible_append_utf16_to_utf8_impl(
+        &mut self,
+        other: &[u16],
+        old_len: usize,
+    ) -> Result<BulkWriteOk, ()> {
+        // We first size the buffer for ASCII if the first two cache lines are ASCII. If that turns out
+        // not to be enough, we size for the worst case given the length of the remaining input at that
+        // point. BUT if the worst case fits inside the inline capacity of an autostring, we skip
+        // the ASCII stuff.
+        let worst_case_needed = if let Some(inline_capacity) = self.inline_capacity() {
+            let worst_case = times_three(other.len()).ok_or(())?;
+            if worst_case <= inline_capacity {
+                Some(worst_case)
+            } else {
+                None
+            }
+        } else {
+            None
+        };
+        let (filled, read, mut handle) =
+            if worst_case_needed.is_none() && long_string_stars_with_basic_latin(other) {
+                let new_len_with_ascii = old_len.checked_add(other.len()).ok_or(())?;
+                let mut handle = unsafe { self.bulk_write(new_len_with_ascii, old_len, false)? };
+                let (read, written) =
+                    convert_utf16_to_utf8_partial(other, &mut handle.as_mut_slice()[old_len..]);
+                let left = other.len() - read;
+                if left == 0 {
+                    return Ok(handle.finish(old_len + written, true));
+                }
+                let filled = old_len + written;
+                let needed = times_three(left).ok_or(())?;
+                let new_len = filled.checked_add(needed).ok_or(())?;
+                unsafe {
+                    handle.restart_bulk_write(new_len, filled, false)?;
+                }
+                (filled, read, handle)
+            } else {
+                // Started with non-ASCII. Compute worst case
+                let needed = if let Some(n) = worst_case_needed {
+                    n
+                } else {
+                    times_three(other.len()).ok_or(())?
+                };
+                let new_len = old_len.checked_add(needed).ok_or(())?;
+                let handle = unsafe { self.bulk_write(new_len, old_len, false)? };
+                (old_len, 0, handle)
+            };
+        let written = convert_utf16_to_utf8(&other[read..], &mut handle.as_mut_slice()[filled..]);
+        Ok(handle.finish(filled + written, true))
+    }
+
+    /// Convert a potentially-invalid UTF-16 string into valid UTF-8
+    /// (replacing invalid sequences with the REPLACEMENT CHARACTER) and
+    /// replace the content of this string with the conversion result.
+    pub fn assign_utf16_to_utf8(&mut self, other: &[u16]) {
+        self.fallible_append_utf16_to_utf8_impl(other, 0)
+            .expect("Out of memory");
+    }
+
+    /// Convert a potentially-invalid UTF-16 string into valid UTF-8
+    /// (replacing invalid sequences with the REPLACEMENT CHARACTER) and
+    /// fallibly replace the content of this string with the conversion result.
+    pub fn fallible_assign_utf16_to_utf8(&mut self, other: &[u16]) -> Result<(), ()> {
+        self.fallible_append_utf16_to_utf8_impl(other, 0)
+            .map(|_| ())
+    }
+
+    /// Convert a potentially-invalid UTF-16 string into valid UTF-8
+    /// (replacing invalid sequences with the REPLACEMENT CHARACTER) and
+    /// append the conversion result to this string.
+    pub fn append_utf16_to_utf8(&mut self, other: &[u16]) {
+        let len = self.len();
+        self.fallible_append_utf16_to_utf8_impl(other, len)
+            .expect("Out of memory");
+    }
+
+    /// Convert a potentially-invalid UTF-16 string into valid UTF-8
+    /// (replacing invalid sequences with the REPLACEMENT CHARACTER) and
+    /// fallibly append the conversion result to this string.
+    pub fn fallible_append_utf16_to_utf8(&mut self, other: &[u16]) -> Result<(), ()> {
+        let len = self.len();
+        self.fallible_append_utf16_to_utf8_impl(other, len)
+            .map(|_| ())
+    }
+
+    // UTF-16 to Latin1
+
+    constant_conversion!(
+        name = fallible_append_utf16_to_latin1_lossy_impl,
+        convert = convert_utf16_to_latin1_lossy,
+        other_ty = &[u16]
+    );
+
+    /// Convert a UTF-16 string whose all code points are below U+0100 into
+    /// a Latin1 (scalar value is byte value; not windows-1252!) string and
+    /// replace the content of this string with the conversion result.
+    ///
+    /// # Panics
+    ///
+    /// If the input contains code points above U+00FF or is not valid UTF-16,
+    /// panics in debug mode and produces garbage in a memory-safe way in
+    /// release builds. The nature of the garbage may differ based on CPU
+    /// architecture and must not be relied upon.
+    pub fn assign_utf16_to_latin1_lossy(&mut self, other: &[u16]) {
+        self.fallible_append_utf16_to_latin1_lossy_impl(other, 0, true)
+            .expect("Out of memory");
+    }
+
+    /// Convert a UTF-16 string whose all code points are below U+0100 into
+    /// a Latin1 (scalar value is byte value; not windows-1252!) string and
+    /// fallibly replace the content of this string with the conversion result.
+    ///
+    /// # Panics
+    ///
+    /// If the input contains code points above U+00FF or is not valid UTF-16,
+    /// panics in debug mode and produces garbage in a memory-safe way in
+    /// release builds. The nature of the garbage may differ based on CPU
+    /// architecture and must not be relied upon.
+    pub fn fallible_assign_utf16_to_latin1_lossy(&mut self, other: &[u16]) -> Result<(), ()> {
+        self.fallible_append_utf16_to_latin1_lossy_impl(other, 0, true)
+            .map(|_| ())
+    }
+
+    /// Convert a UTF-16 string whose all code points are below U+0100 into
+    /// a Latin1 (scalar value is byte value; not windows-1252!) string and
+    /// append the conversion result to this string.
+    ///
+    /// # Panics
+    ///
+    /// If the input contains code points above U+00FF or is not valid UTF-16,
+    /// panics in debug mode and produces garbage in a memory-safe way in
+    /// release builds. The nature of the garbage may differ based on CPU
+    /// architecture and must not be relied upon.
+    pub fn append_utf16_to_latin1_lossy(&mut self, other: &[u16]) {
+        let len = self.len();
+        self.fallible_append_utf16_to_latin1_lossy_impl(other, len, false)
+            .expect("Out of memory");
+    }
+
+    /// Convert a UTF-16 string whose all code points are below U+0100 into
+    /// a Latin1 (scalar value is byte value; not windows-1252!) string and
+    /// fallibly append the conversion result to this string.
+    ///
+    /// # Panics
+    ///
+    /// If the input contains code points above U+00FF or is not valid UTF-16,
+    /// panics in debug mode and produces garbage in a memory-safe way in
+    /// release builds. The nature of the garbage may differ based on CPU
+    /// architecture and must not be relied upon.
+    pub fn fallible_append_utf16_to_latin1_lossy(&mut self, other: &[u16]) -> Result<(), ()> {
+        let len = self.len();
+        self.fallible_append_utf16_to_latin1_lossy_impl(other, len, false)
+            .map(|_| ())
+    }
+
+    // UTF-8 to Latin1
+
+    ascii_copy_avoidance!(
+        name = fallible_append_utf8_to_latin1_lossy_check,
+        implementation = fallible_append_utf8_to_latin1_lossy_impl,
+        string_like = nsCStringLike
+    );
+
+    fn fallible_append_utf8_to_latin1_lossy_impl(
+        &mut self,
+        other: &[u8],
+        old_len: usize,
+        maybe_num_ascii: Option<usize>,
+    ) -> Result<BulkWriteOk, ()> {
+        let new_len = old_len.checked_add(other.len()).ok_or(())?;
+        let num_ascii = maybe_num_ascii.unwrap_or(0);
+        // Already checked for overflow above, so this can't overflow.
+        let old_len_plus_num_ascii = old_len + num_ascii;
+        let mut handle = unsafe { self.bulk_write(new_len, old_len, false)? };
+        let written = {
+            let buffer = handle.as_mut_slice();
+            if num_ascii != 0 {
+                (&mut buffer[old_len..old_len_plus_num_ascii]).copy_from_slice(&other[..num_ascii]);
+            }
+            convert_utf8_to_latin1_lossy(&other[num_ascii..], &mut buffer[old_len_plus_num_ascii..])
+        };
+        Ok(handle.finish(old_len_plus_num_ascii + written, true))
+    }
+
+    /// Convert a UTF-8 string whose all code points are below U+0100 into
+    /// a Latin1 (scalar value is byte value; not windows-1252!) string and
+    /// replace the content of this string with the conversion result.
+    ///
+    /// # Panics
+    ///
+    /// If the input contains code points above U+00FF or is not valid UTF-8,
+    /// panics in debug mode and produces garbage in a memory-safe way in
+    /// release builds. The nature of the garbage may differ based on CPU
+    /// architecture and must not be relied upon.
+    pub fn assign_utf8_to_latin1_lossy<T: nsCStringLike + ?Sized>(&mut self, other: &T) {
+        self.fallible_append_utf8_to_latin1_lossy_check(other, 0)
+            .expect("Out of memory");
+    }
+
+    /// Convert a UTF-8 string whose all code points are below U+0100 into
+    /// a Latin1 (scalar value is byte value; not windows-1252!) string and
+    /// fallibly replace the content of this string with the conversion result.
+    ///
+    /// # Panics
+    ///
+    /// If the input contains code points above U+00FF or is not valid UTF-8,
+    /// panics in debug mode and produces garbage in a memory-safe way in
+    /// release builds. The nature of the garbage may differ based on CPU
+    /// architecture and must not be relied upon.
+    pub fn fallible_assign_utf8_to_latin1_lossy<T: nsCStringLike + ?Sized>(
+        &mut self,
+        other: &T,
+    ) -> Result<(), ()> {
+        self.fallible_append_utf8_to_latin1_lossy_check(other, 0)
+            .map(|_| ())
+    }
+
+    /// Convert a UTF-8 string whose all code points are below U+0100 into
+    /// a Latin1 (scalar value is byte value; not windows-1252!) string and
+    /// append the conversion result to this string.
+    ///
+    /// # Panics
+    ///
+    /// If the input contains code points above U+00FF or is not valid UTF-8,
+    /// panics in debug mode and produces garbage in a memory-safe way in
+    /// release builds. The nature of the garbage may differ based on CPU
+    /// architecture and must not be relied upon.
+    pub fn append_utf8_to_latin1_lossy<T: nsCStringLike + ?Sized>(&mut self, other: &T) {
+        let len = self.len();
+        self.fallible_append_utf8_to_latin1_lossy_check(other, len)
+            .expect("Out of memory");
+    }
+
+    /// Convert a UTF-8 string whose all code points are below U+0100 into
+    /// a Latin1 (scalar value is byte value; not windows-1252!) string and
+    /// fallibly append the conversion result to this string.
+    ///
+    /// # Panics
+    ///
+    /// If the input contains code points above U+00FF or is not valid UTF-8,
+    /// panics in debug mode and produces garbage in a memory-safe way in
+    /// release builds. The nature of the garbage may differ based on CPU
+    /// architecture and must not be relied upon.
+    pub fn fallible_append_utf8_to_latin1_lossy<T: nsCStringLike + ?Sized>(
+        &mut self,
+        other: &T,
+    ) -> Result<(), ()> {
+        let len = self.len();
+        self.fallible_append_utf8_to_latin1_lossy_check(other, len)
+            .map(|_| ())
+    }
+
+    // Latin1 to UTF-8 CString
+
+    ascii_copy_avoidance!(
+        name = fallible_append_latin1_to_utf8_check,
+        implementation = fallible_append_latin1_to_utf8_impl,
+        string_like = Latin1StringLike
+    );
+
+    fn fallible_append_latin1_to_utf8_impl(
+        &mut self,
+        other: &[u8],
+        old_len: usize,
+        maybe_num_ascii: Option<usize>,
+    ) -> Result<BulkWriteOk, ()> {
+        let (filled, read, mut handle) = if let Some(num_ascii) = maybe_num_ascii {
+            // Wrapper checked for ASCII
+            let left = other.len() - num_ascii;
+            let filled = old_len + num_ascii;
+            let needed = left.checked_mul(2).ok_or(())?;
+            let new_len = filled.checked_add(needed).ok_or(())?;
+            let mut handle = unsafe { self.bulk_write(new_len, old_len, false)? };
+            if num_ascii != 0 {
+                (&mut handle.as_mut_slice()[old_len..filled]).copy_from_slice(&other[..num_ascii]);
+            }
+            (filled, num_ascii, handle)
+        } else {
+            let worst_case_needed = if let Some(inline_capacity) = self.inline_capacity() {
+                let worst_case = other.len().checked_mul(2).ok_or(())?;
+                if worst_case <= inline_capacity {
+                    Some(worst_case)
+                } else {
+                    None
+                }
+            } else {
+                None
+            };
+            if worst_case_needed.is_none() && long_string_starts_with_ascii(other) {
+                // Wrapper didn't check for ASCII, so let's see if `other` starts with ASCII
+                // `other` starts with ASCII, so let's first size the buffer
+                // with optimism that it's ASCII-only.
+                let new_len_with_ascii = old_len.checked_add(other.len()).ok_or(())?;
+                let mut handle = unsafe { self.bulk_write(new_len_with_ascii, old_len, false)? };
+                let (read, written) =
+                    convert_latin1_to_utf8_partial(other, &mut handle.as_mut_slice()[old_len..]);
+                let left = other.len() - read;
+                let filled = old_len + written;
+                if left == 0 {
+                    // `other` fit in the initial allocation
+                    return Ok(handle.finish(filled, true));
+                }
+                let needed = left.checked_mul(2).ok_or(())?;
+                let new_len = filled.checked_add(needed).ok_or(())?;
+                unsafe {
+                    handle.restart_bulk_write(new_len, filled, false)?;
+                }
+                (filled, read, handle)
+            } else {
+                // Started with non-ASCII. Assume worst case.
+                let needed = if let Some(n) = worst_case_needed {
+                    n
+                } else {
+                    other.len().checked_mul(2).ok_or(())?
+                };
+                let new_len = old_len.checked_add(needed).ok_or(())?;
+                let handle = unsafe { self.bulk_write(new_len, old_len, false)? };
+                (old_len, 0, handle)
+            }
+        };
+        let written = convert_latin1_to_utf8(&other[read..], &mut handle.as_mut_slice()[filled..]);
+        Ok(handle.finish(filled + written, true))
+    }
+
+    /// Convert a Latin1 (i.e. byte value equals scalar value; not windows-1252!)
+    /// into UTF-8 and replace the content of this string with the conversion result.
+    pub fn assign_latin1_to_utf8<T: Latin1StringLike + ?Sized>(&mut self, other: &T) {
+        self.fallible_append_latin1_to_utf8_check(other, 0)
+            .expect("Out of memory");
+    }
+
+    /// Convert a Latin1 (i.e. byte value equals scalar value; not windows-1252!)
+    /// into UTF-8 and fallibly replace the content of this string with the
+    /// conversion result.
+    pub fn fallible_assign_latin1_to_utf8<T: Latin1StringLike + ?Sized>(
+        &mut self,
+        other: &T,
+    ) -> Result<(), ()> {
+        self.fallible_append_latin1_to_utf8_check(other, 0)
+            .map(|_| ())
+    }
+
+    /// Convert a Latin1 (i.e. byte value equals scalar value; not windows-1252!)
+    /// into UTF-8 and append the conversion result to this string.
+    pub fn append_latin1_to_utf8<T: Latin1StringLike + ?Sized>(&mut self, other: &T) {
+        let len = self.len();
+        self.fallible_append_latin1_to_utf8_check(other, len)
+            .expect("Out of memory");
+    }
+
+    /// Convert a Latin1 (i.e. byte value equals scalar value; not windows-1252!)
+    /// into UTF-8 and fallibly append the conversion result to this string.
+    pub fn fallible_append_latin1_to_utf8<T: Latin1StringLike + ?Sized>(
+        &mut self,
+        other: &T,
+    ) -> Result<(), ()> {
+        let len = self.len();
+        self.fallible_append_latin1_to_utf8_check(other, len)
+            .map(|_| ())
+    }
+}
+
+#[no_mangle]
+pub unsafe extern "C" fn nsstring_fallible_append_utf8_impl(
+    this: *mut nsAString,
+    other: *const u8,
+    other_len: usize,
+    old_len: usize,
+) -> bool {
+    let other_slice = slice::from_raw_parts(other, other_len);
+    (*this)
+        .fallible_append_utf8_impl(other_slice, old_len)
+        .is_ok()
+}
+
+#[no_mangle]
+pub unsafe extern "C" fn nsstring_fallible_append_latin1_impl(
+    this: *mut nsAString,
+    other: *const u8,
+    other_len: usize,
+    old_len: usize,
+    allow_shrinking: bool,
+) -> bool {
+    let other_slice = slice::from_raw_parts(other, other_len);
+    (*this)
+        .fallible_append_latin1_impl(other_slice, old_len, allow_shrinking)
+        .is_ok()
+}
+
+#[no_mangle]
+pub unsafe extern "C" fn nscstring_fallible_append_utf16_to_utf8_impl(
+    this: *mut nsACString,
+    other: *const u16,
+    other_len: usize,
+    old_len: usize,
+) -> bool {
+    let other_slice = slice::from_raw_parts(other, other_len);
+    (*this)
+        .fallible_append_utf16_to_utf8_impl(other_slice, old_len)
+        .is_ok()
+}
+
+#[no_mangle]
+pub unsafe extern "C" fn nscstring_fallible_append_utf16_to_latin1_lossy_impl(
+    this: *mut nsACString,
+    other: *const u16,
+    other_len: usize,
+    old_len: usize,
+    allow_shrinking: bool,
+) -> bool {
+    let other_slice = slice::from_raw_parts(other, other_len);
+    (*this)
+        .fallible_append_utf16_to_latin1_lossy_impl(other_slice, old_len, allow_shrinking)
+        .is_ok()
+}
+
+#[no_mangle]
+pub unsafe extern "C" fn nscstring_fallible_append_utf8_to_latin1_lossy_check(
+    this: *mut nsACString,
+    other: *const nsACString,
+    old_len: usize,
+) -> bool {
+    (*this)
+        .fallible_append_utf8_to_latin1_lossy_check(&*other, old_len)
+        .is_ok()
+}
+
+#[no_mangle]
+pub unsafe extern "C" fn nscstring_fallible_append_latin1_to_utf8_check(
+    this: *mut nsACString,
+    other: *const nsACString,
+    old_len: usize,
+) -> bool {
+    (*this)
+        .fallible_append_latin1_to_utf8_check(&*other, old_len)
+        .is_ok()
+}