Merging upstream version 1.67.1+dfsg1.

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

3 files changed, 894 insertions, 0 deletions

diff --git a/vendor/writeable/src/impls.rs b/vendor/writeable/src/impls.rs
new file mode 100644
index 000000000..649ede4d5
--- /dev/null
+++ b/vendor/writeable/src/impls.rs
@@ -0,0 +1,213 @@
+// This file is part of ICU4X. For terms of use, please see the file
+// called LICENSE at the top level of the ICU4X source tree
+// (online at: https://github.com/unicode-org/icu4x/blob/main/LICENSE ).
+
+use crate::*;
+use alloc::borrow::Cow;
+use core::fmt;
+
+macro_rules! impl_write_num {
+    ($u:ty, $i:ty, $test:ident, $log10:ident) => {
+        impl $crate::Writeable for $u {
+            fn write_to<W: core::fmt::Write + ?Sized>(&self, sink: &mut W) -> core::fmt::Result {
+                let mut buf = [b'0'; $log10(<$u>::MAX) as usize + 1];
+                let mut n = *self;
+                let mut i = buf.len();
+                #[allow(clippy::indexing_slicing)] // n < 10^i
+                while n != 0 {
+                    i -= 1;
+                    buf[i] = b'0' + (n % 10) as u8;
+                    n /= 10;
+                }
+                if i == buf.len() {
+                    debug_assert_eq!(*self, 0);
+                    i -= 1;
+                }
+                #[allow(clippy::indexing_slicing)] // buf is ASCII
+                let s = unsafe { core::str::from_utf8_unchecked(&buf[i..]) };
+                sink.write_str(s)
+            }
+
+            fn writeable_length_hint(&self) -> $crate::LengthHint {
+                $crate::LengthHint::exact(if *self == 0 {
+                    1
+                } else {
+                    $log10(*self) as usize + 1
+                })
+            }
+        }
+
+        // TODO: use the library functions once stabilized.
+        // https://github.com/unicode-org/icu4x/issues/1428
+        #[inline]
+        const fn $log10(s: $u) -> u32 {
+            let b = (<$u>::BITS - 1) - s.leading_zeros();
+            // s ∈ [2ᵇ, 2ᵇ⁺¹-1] => ⌊log₁₀(s)⌋ ∈ [⌊log₁₀(2ᵇ)⌋, ⌊log₁₀(2ᵇ⁺¹-1)⌋]
+            //                 <=> ⌊log₁₀(s)⌋ ∈ [⌊log₁₀(2ᵇ)⌋, ⌊log₁₀(2ᵇ⁺¹)⌋]
+            //                 <=> ⌊log₁₀(s)⌋ ∈ [⌊b log₁₀(2)⌋, ⌊(b+1) log₁₀(2)⌋]
+            // The second line holds because there is no integer in
+            // [log₁₀(2ᶜ-1), log₁₀(2ᶜ)], if there were, there'd be some 10ⁿ in
+            // [2ᶜ-1, 2ᶜ], but it can't be 2ᶜ-1 due to parity nor 2ᶜ due to prime
+            // factors.
+
+            const M: u32 = (core::f64::consts::LOG10_2 * (1 << 26) as f64) as u32;
+            let low = (b * M) >> 26;
+            let high = ((b + 1) * M) >> 26;
+
+            // If the bounds aren't tight (e.g. 87 ∈ [64, 127] ⟹ ⌊log₁₀(87)⌋ ∈ [1,2]),
+            // compare to 10ʰ (100). This shouldn't happen too often as there are more
+            // powers of 2 than 10 (it happens for 14% of u32s).
+            if high == low {
+                low
+            } else if s < (10 as $u).pow(high) {
+                low
+            } else {
+                high
+            }
+        }
+
+        impl $crate::Writeable for $i {
+            fn write_to<W: core::fmt::Write + ?Sized>(&self, sink: &mut W) -> core::fmt::Result {
+                if self.is_negative() {
+                    sink.write_str("-")?;
+                }
+                self.unsigned_abs().write_to(sink)
+            }
+
+            fn writeable_length_hint(&self) -> $crate::LengthHint {
+                $crate::LengthHint::exact(if self.is_negative() { 1 } else { 0 })
+                    + self.unsigned_abs().writeable_length_hint()
+            }
+        }
+
+        #[test]
+        fn $test() {
+            use $crate::assert_writeable_eq;
+            assert_writeable_eq!(&(0 as $u), "0");
+            assert_writeable_eq!(&(0 as $u), "0");
+            assert_writeable_eq!(&(-0 as $i), "0");
+            assert_writeable_eq!(&(1 as $u), "1");
+            assert_writeable_eq!(&(1 as $i), "1");
+            assert_writeable_eq!(&(-1 as $i), "-1");
+            assert_writeable_eq!(&(10 as $u), "10");
+            assert_writeable_eq!(&(10 as $i), "10");
+            assert_writeable_eq!(&(-10 as $i), "-10");
+            assert_writeable_eq!(&(99 as $u), "99");
+            assert_writeable_eq!(&(99 as $i), "99");
+            assert_writeable_eq!(&(-99 as $i), "-99");
+            assert_writeable_eq!(&(100 as $u), "100");
+            assert_writeable_eq!(&(-100 as $i), "-100");
+            assert_writeable_eq!(&<$u>::MAX, <$u>::MAX.to_string());
+            assert_writeable_eq!(&<$i>::MAX, <$i>::MAX.to_string());
+            assert_writeable_eq!(&<$i>::MIN, <$i>::MIN.to_string());
+
+            use rand::{rngs::SmallRng, Rng, SeedableRng};
+            let mut rng = SmallRng::seed_from_u64(4); // chosen by fair dice roll.
+                                                      // guaranteed to be random.
+            for _ in 0..1000 {
+                let rand = rng.gen::<$u>();
+                assert_writeable_eq!(rand, rand.to_string());
+            }
+        }
+    };
+}
+
+impl_write_num!(u8, i8, test_u8, log10_u8);
+impl_write_num!(u16, i16, test_u16, log10_u16);
+impl_write_num!(u32, i32, test_u32, log10_u32);
+impl_write_num!(u64, i64, test_u64, log10_u64);
+impl_write_num!(u128, i128, test_u128, log10_u128);
+
+#[test]
+fn assert_log10_approximation() {
+    for i in 1..u128::BITS {
+        assert_eq!(i * 59 / 196, 2f64.powf(i.into()).log10().floor() as u32);
+    }
+}
+
+impl Writeable for str {
+    #[inline]
+    fn write_to<W: fmt::Write + ?Sized>(&self, sink: &mut W) -> fmt::Result {
+        sink.write_str(self)
+    }
+
+    #[inline]
+    fn writeable_length_hint(&self) -> LengthHint {
+        LengthHint::exact(self.len())
+    }
+
+    /// Returns a borrowed `str`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::borrow::Cow;
+    /// use writeable::Writeable;
+    ///
+    /// let cow = "foo".write_to_string();
+    /// assert!(matches!(cow, Cow::Borrowed(_)));
+    /// ```
+    #[inline]
+    fn write_to_string(&self) -> Cow<str> {
+        Cow::Borrowed(self)
+    }
+}
+
+impl Writeable for String {
+    #[inline]
+    fn write_to<W: fmt::Write + ?Sized>(&self, sink: &mut W) -> fmt::Result {
+        sink.write_str(self)
+    }
+
+    #[inline]
+    fn writeable_length_hint(&self) -> LengthHint {
+        LengthHint::exact(self.len())
+    }
+
+    #[inline]
+    fn write_to_string(&self) -> Cow<str> {
+        Cow::Borrowed(self)
+    }
+}
+
+impl<'a, T: Writeable + ?Sized> Writeable for &T {
+    #[inline]
+    fn write_to<W: fmt::Write + ?Sized>(&self, sink: &mut W) -> fmt::Result {
+        (*self).write_to(sink)
+    }
+
+    #[inline]
+    fn write_to_parts<W: PartsWrite + ?Sized>(&self, sink: &mut W) -> fmt::Result {
+        (*self).write_to_parts(sink)
+    }
+
+    #[inline]
+    fn writeable_length_hint(&self) -> LengthHint {
+        (*self).writeable_length_hint()
+    }
+
+    #[inline]
+    fn write_to_string(&self) -> Cow<str> {
+        (*self).write_to_string()
+    }
+}
+
+#[test]
+fn test_string_impls() {
+    fn check_writeable_slice<W: Writeable>(writeables: &[W]) {
+        assert_writeable_eq!(&writeables[0], "");
+        assert_writeable_eq!(&writeables[1], "abc");
+    }
+
+    // test str impl
+    let arr: &[&str] = &["", "abc"];
+    check_writeable_slice(arr);
+
+    // test String impl
+    let arr: &[String] = &["".to_string(), "abc".to_string()];
+    check_writeable_slice(arr);
+
+    // test &T impl
+    let arr: &[&String] = &[&"".to_string(), &"abc".to_string()];
+    check_writeable_slice(arr);
+}
diff --git a/vendor/writeable/src/lib.rs b/vendor/writeable/src/lib.rs
new file mode 100644
index 000000000..66be7f33b
--- /dev/null
+++ b/vendor/writeable/src/lib.rs
@@ -0,0 +1,390 @@
+// This file is part of ICU4X. For terms of use, please see the file
+// called LICENSE at the top level of the ICU4X source tree
+// (online at: https://github.com/unicode-org/icu4x/blob/main/LICENSE ).
+
+// https://github.com/unicode-org/icu4x/blob/main/docs/process/boilerplate.md#library-annotations
+#![cfg_attr(all(not(test), not(doc)), no_std)]
+#![cfg_attr(
+    not(test),
+    deny(
+        clippy::indexing_slicing,
+        clippy::unwrap_used,
+        clippy::expect_used,
+        clippy::panic,
+        clippy::exhaustive_structs,
+        clippy::exhaustive_enums,
+        missing_debug_implementations,
+    )
+)]
+
+//! `writeable` is a utility crate of the [`ICU4X`] project.
+//!
+//! It includes [`Writeable`], a core trait representing an object that can be written to a
+//! sink implementing `std::fmt::Write`. It is an alternative to `std::fmt::Display` with the
+//! addition of a function indicating the number of bytes to be written.
+//!
+//! `Writeable` improves upon `std::fmt::Display` in two ways:
+//!
+//! 1. More efficient, since the sink can pre-allocate bytes.
+//! 2. Smaller code, since the format machinery can be short-circuited.
+//!
+//! # Examples
+//!
+//! ```
+//! use std::fmt;
+//! use writeable::assert_writeable_eq;
+//! use writeable::LengthHint;
+//! use writeable::Writeable;
+//!
+//! struct WelcomeMessage<'s> {
+//!     pub name: &'s str,
+//! }
+//!
+//! impl<'s> Writeable for WelcomeMessage<'s> {
+//!     fn write_to<W: fmt::Write + ?Sized>(&self, sink: &mut W) -> fmt::Result {
+//!         sink.write_str("Hello, ")?;
+//!         sink.write_str(self.name)?;
+//!         sink.write_char('!')?;
+//!         Ok(())
+//!     }
+//!
+//!     fn writeable_length_hint(&self) -> LengthHint {
+//!         // "Hello, " + '!' + length of name
+//!         LengthHint::exact(8 + self.name.len())
+//!     }
+//! }
+//!
+//! let message = WelcomeMessage { name: "Alice" };
+//! assert_writeable_eq!(&message, "Hello, Alice!");
+//!
+//! // Types implementing `Writeable` are recommended to also implement `fmt::Display`.
+//! // This can be simply done by redirecting to the `Writeable` implementation:
+//! writeable::impl_display_with_writeable!(WelcomeMessage<'_>);
+//! ```
+//!
+//! [`ICU4X`]: ../icu/index.html
+
+extern crate alloc;
+
+mod impls;
+mod ops;
+
+use alloc::borrow::Cow;
+use alloc::string::String;
+use alloc::vec::Vec;
+use core::fmt;
+
+/// A hint to help consumers of `Writeable` pre-allocate bytes before they call
+/// [`write_to`](Writeable::write_to).
+///
+/// This behaves like `Iterator::size_hint`: it is a tuple where the first element is the
+/// lower bound, and the second element is the upper bound. If the upper bound is `None`
+/// either there is no known upper bound, or the upper bound is larger than `usize`.
+///
+/// `LengthHint` implements std`::ops::{Add, Mul}` and similar traits for easy composition.
+/// During computation, the lower bound will saturate at `usize::MAX`, while the upper
+/// bound will become `None` if `usize::MAX` is exceeded.
+#[derive(Debug, PartialEq, Eq, Copy, Clone)]
+#[non_exhaustive]
+pub struct LengthHint(pub usize, pub Option<usize>);
+
+impl LengthHint {
+    pub fn undefined() -> Self {
+        Self(0, None)
+    }
+
+    /// `write_to` will use exactly n bytes.
+    pub fn exact(n: usize) -> Self {
+        Self(n, Some(n))
+    }
+
+    /// `write_to` will use at least n bytes.
+    pub fn at_least(n: usize) -> Self {
+        Self(n, None)
+    }
+
+    /// `write_to` will use at most n bytes.
+    pub fn at_most(n: usize) -> Self {
+        Self(0, Some(n))
+    }
+
+    /// `write_to` will use between `n` and `m` bytes.
+    pub fn between(n: usize, m: usize) -> Self {
+        Self(Ord::min(n, m), Some(Ord::max(n, m)))
+    }
+
+    /// Returns a recommendation for the number of bytes to pre-allocate.
+    /// If an upper bound exists, this is used, otherwise the lower bound
+    /// (which might be 0).
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use writeable::Writeable;
+    ///
+    /// fn pre_allocate_string(w: &impl Writeable) -> String {
+    ///     String::with_capacity(w.writeable_length_hint().capacity())
+    /// }
+    /// ```
+    pub fn capacity(&self) -> usize {
+        self.1.unwrap_or(self.0)
+    }
+
+    /// Returns whether the `LengthHint` indicates that the string is exactly 0 bytes long.
+    pub fn is_zero(&self) -> bool {
+        self.1 == Some(0)
+    }
+}
+
+#[derive(Clone, Copy, Debug, PartialEq)]
+#[allow(clippy::exhaustive_structs)] // stable
+pub struct Part {
+    pub category: &'static str,
+    pub value: &'static str,
+}
+
+/// A sink that supports annotating parts of the string with `Part`s.
+pub trait PartsWrite: fmt::Write {
+    type SubPartsWrite: PartsWrite + ?Sized;
+
+    fn with_part(
+        &mut self,
+        part: Part,
+        f: impl FnMut(&mut Self::SubPartsWrite) -> fmt::Result,
+    ) -> fmt::Result;
+}
+
+/// `Writeable` is an alternative to `std::fmt::Display` with the addition of a length function.
+pub trait Writeable {
+    /// Writes a string to the given sink. Errors from the sink are bubbled up.
+    /// The default implementation delegates to `write_to_parts`, and discards any
+    /// `Part` annotations.
+    fn write_to<W: fmt::Write + ?Sized>(&self, sink: &mut W) -> fmt::Result {
+        struct CoreWriteAsPartsWrite<W: fmt::Write + ?Sized>(W);
+        impl<W: fmt::Write + ?Sized> fmt::Write for CoreWriteAsPartsWrite<W> {
+            fn write_str(&mut self, s: &str) -> fmt::Result {
+                self.0.write_str(s)
+            }
+
+            fn write_char(&mut self, c: char) -> fmt::Result {
+                self.0.write_char(c)
+            }
+        }
+
+        impl<W: fmt::Write + ?Sized> PartsWrite for CoreWriteAsPartsWrite<W> {
+            type SubPartsWrite = CoreWriteAsPartsWrite<W>;
+
+            fn with_part(
+                &mut self,
+                _part: Part,
+                mut f: impl FnMut(&mut Self::SubPartsWrite) -> fmt::Result,
+            ) -> fmt::Result {
+                f(self)
+            }
+        }
+
+        self.write_to_parts(&mut CoreWriteAsPartsWrite(sink))
+    }
+
+    /// Write bytes and `Part` annotations to the given sink. Errors from the
+    /// sink are bubbled up. The default implementation delegates to `write_to`,
+    /// and doesn't produce any `Part` annotations.
+    fn write_to_parts<S: PartsWrite + ?Sized>(&self, sink: &mut S) -> fmt::Result {
+        self.write_to(sink)
+    }
+
+    /// Returns a hint for the number of UTF-8 bytes that will be written to the sink.
+    ///
+    /// Override this method if it can be computed quickly.
+    fn writeable_length_hint(&self) -> LengthHint {
+        LengthHint::undefined()
+    }
+
+    /// Creates a new `String` with the data from this `Writeable`. Like `ToString`,
+    /// but smaller and faster.
+    ///
+    /// The default impl allocates an owned `String`. However, if it is possible to return a
+    /// borrowed string, overwrite this method to return a `Cow::Borrowed`.
+    ///
+    /// To remove the `Cow` wrapper, call `.into_owned()` or `.as_str()` as appropriate.
+    ///
+    /// # Examples
+    ///
+    /// Inspect a `Writeable` before writing it to the sink:
+    ///
+    /// ```
+    /// use core::fmt::{Result, Write};
+    /// use writeable::Writeable;
+    ///
+    /// fn write_if_ascii<W, S>(w: &W, sink: &mut S) -> Result
+    /// where
+    ///     W: Writeable + ?Sized,
+    ///     S: Write + ?Sized,
+    /// {
+    ///     let s = w.write_to_string();
+    ///     if s.is_ascii() {
+    ///         sink.write_str(&s)
+    ///     } else {
+    ///         Ok(())
+    ///     }
+    /// }
+    /// ```
+    ///
+    /// Convert the `Writeable` into a fully owned `String`:
+    ///
+    /// ```
+    /// use writeable::Writeable;
+    ///
+    /// fn make_string(w: &impl Writeable) -> String {
+    ///     w.write_to_string().into_owned()
+    /// }
+    /// ```
+    fn write_to_string(&self) -> Cow<str> {
+        let mut output = String::with_capacity(self.writeable_length_hint().capacity());
+        let _ = self.write_to(&mut output);
+        Cow::Owned(output)
+    }
+}
+
+/// Implements [`Display`](core::fmt::Display) for types that implement [`Writeable`].
+///
+/// It's recommended to do this for every [`Writeable`] type, as it will add
+/// support for `core::fmt` features like [`fmt!`](std::fmt),
+/// [`print!`](std::print), [`write!`](std::write), etc.
+#[macro_export]
+macro_rules! impl_display_with_writeable {
+    ($type:ty) => {
+        /// This trait is implemented for compatibility with [`fmt!`](alloc::fmt).
+        /// To create a string, [`Writeable::write_to_string`] is usually more efficient.
+        impl core::fmt::Display for $type {
+            #[inline]
+            fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
+                $crate::Writeable::write_to(&self, f)
+            }
+        }
+    };
+}
+
+/// Testing macros for types implementing Writeable. The first argument should be a
+/// `Writeable`, the second argument a string, and the third argument (*_parts_eq only)
+/// a list of parts (`[(usize, usize, Part)]`).
+///
+/// The macros tests for equality of string content, parts (*_parts_eq only), and
+/// verify the size hint.
+///
+/// # Examples
+///
+/// ```
+/// # use writeable::Writeable;
+/// # use writeable::LengthHint;
+/// # use writeable::Part;
+/// # use writeable::assert_writeable_eq;
+/// # use writeable::assert_writeable_parts_eq;
+/// # use std::fmt::{self, Write};
+///
+/// const WORD: Part = Part {
+///     category: "foo",
+///     value: "word",
+/// };
+///
+/// struct Demo;
+/// impl Writeable for Demo {
+///     fn write_to_parts<S: writeable::PartsWrite + ?Sized>(&self, sink: &mut S) -> fmt::Result {
+///         sink.with_part(WORD, |w| w.write_str("foo"))
+///     }
+///     fn writeable_length_hint(&self) -> LengthHint {
+///         LengthHint::exact(3)
+///     }
+/// }
+///
+/// writeable::impl_display_with_writeable!(Demo);
+///
+/// assert_writeable_eq!(&Demo, "foo");
+/// assert_writeable_eq!(&Demo, "foo", "Message: {}", "Hello World");
+///
+/// assert_writeable_parts_eq!(&Demo, "foo", [(0, 3, WORD)]);
+/// assert_writeable_parts_eq!(&Demo, "foo", [(0, 3, WORD)], "Message: {}", "Hello World");
+/// ```
+#[macro_export]
+macro_rules! assert_writeable_eq {
+    ($actual_writeable:expr, $expected_str:expr $(,)?) => {
+        $crate::assert_writeable_eq!($actual_writeable, $expected_str, "");
+    };
+    ($actual_writeable:expr, $expected_str:expr, $($arg:tt)+) => {{
+        let actual_writeable = &$actual_writeable;
+        let (actual_str, _) = $crate::writeable_to_parts_for_test(actual_writeable).unwrap();
+        assert_eq!(actual_str, $expected_str, $($arg)*);
+        assert_eq!(actual_str, $crate::Writeable::write_to_string(actual_writeable), $($arg)+);
+        let length_hint = $crate::Writeable::writeable_length_hint(actual_writeable);
+        assert!(length_hint.0 <= actual_str.len(), $($arg)*);
+        if let Some(upper) = length_hint.1 {
+            assert!(actual_str.len() <= upper, $($arg)*);
+        }
+    }};
+}
+
+/// See [`assert_writeable_eq`].
+#[macro_export]
+macro_rules! assert_writeable_parts_eq {
+    ($actual_writeable:expr, $expected_str:expr, $expected_parts:expr $(,)?) => {
+        $crate::assert_writeable_parts_eq!($actual_writeable, $expected_str, $expected_parts, "");
+    };
+    ($actual_writeable:expr, $expected_str:expr, $expected_parts:expr, $($arg:tt)+) => {{
+        let actual_writeable = &$actual_writeable;
+        let (actual_str, actual_parts) = $crate::writeable_to_parts_for_test(actual_writeable).unwrap();
+        assert_eq!(actual_str, $expected_str, $($arg)+);
+        assert_eq!(actual_str, $crate::Writeable::write_to_string(actual_writeable), $($arg)+);
+        assert_eq!(actual_parts, $expected_parts, $($arg)+);
+        let length_hint = $crate::Writeable::writeable_length_hint(actual_writeable);
+        assert!(length_hint.0 <= actual_str.len(), $($arg)+);
+        if let Some(upper) = length_hint.1 {
+            assert!(actual_str.len() <= upper, $($arg)+);
+        }
+    }};
+}
+
+#[doc(hidden)]
+#[allow(clippy::type_complexity)]
+pub fn writeable_to_parts_for_test<W: Writeable>(
+    writeable: &W,
+) -> Result<(String, Vec<(usize, usize, Part)>), fmt::Error> {
+    struct State {
+        string: alloc::string::String,
+        parts: Vec<(usize, usize, Part)>,
+    }
+
+    impl fmt::Write for State {
+        fn write_str(&mut self, s: &str) -> fmt::Result {
+            self.string.write_str(s)
+        }
+        fn write_char(&mut self, c: char) -> fmt::Result {
+            self.string.write_char(c)
+        }
+    }
+
+    impl PartsWrite for State {
+        type SubPartsWrite = Self;
+        fn with_part(
+            &mut self,
+            part: Part,
+            mut f: impl FnMut(&mut Self::SubPartsWrite) -> fmt::Result,
+        ) -> fmt::Result {
+            let start = self.string.len();
+            f(self)?;
+            self.parts.push((start, self.string.len(), part));
+            Ok(())
+        }
+    }
+
+    let mut state = State {
+        string: alloc::string::String::new(),
+        parts: Vec::new(),
+    };
+    writeable.write_to_parts(&mut state)?;
+
+    // Sort by first open and last closed
+    state
+        .parts
+        .sort_unstable_by_key(|(begin, end, _)| (*begin, end.wrapping_neg()));
+    Ok((state.string, state.parts))
+}
diff --git a/vendor/writeable/src/ops.rs b/vendor/writeable/src/ops.rs
new file mode 100644
index 000000000..3ed4406d7
--- /dev/null
+++ b/vendor/writeable/src/ops.rs
@@ -0,0 +1,291 @@
+// This file is part of ICU4X. For terms of use, please see the file
+// called LICENSE at the top level of the ICU4X source tree
+// (online at: https://github.com/unicode-org/icu4x/blob/main/LICENSE ).
+
+use crate::LengthHint;
+
+impl core::ops::Add<LengthHint> for LengthHint {
+    type Output = Self;
+
+    fn add(self, other: LengthHint) -> Self {
+        LengthHint(
+            self.0.saturating_add(other.0),
+            match (self.1, other.1) {
+                (Some(c), Some(d)) => c.checked_add(d),
+                _ => None,
+            },
+        )
+    }
+}
+
+impl core::ops::AddAssign<LengthHint> for LengthHint {
+    fn add_assign(&mut self, other: Self) {
+        *self = *self + other;
+    }
+}
+
+impl core::iter::Sum<LengthHint> for LengthHint {
+    fn sum<I>(iter: I) -> Self
+    where
+        I: Iterator<Item = LengthHint>,
+    {
+        iter.fold(LengthHint::exact(0), core::ops::Add::add)
+    }
+}
+
+impl core::ops::Add<usize> for LengthHint {
+    type Output = Self;
+
+    fn add(self, other: usize) -> Self {
+        Self(
+            self.0.saturating_add(other),
+            self.1.and_then(|upper| upper.checked_add(other)),
+        )
+    }
+}
+
+impl core::ops::AddAssign<usize> for LengthHint {
+    fn add_assign(&mut self, other: usize) {
+        *self = *self + other;
+    }
+}
+
+impl core::ops::Mul<usize> for LengthHint {
+    type Output = Self;
+
+    fn mul(self, other: usize) -> Self {
+        Self(
+            self.0.saturating_mul(other),
+            self.1.and_then(|upper| upper.checked_mul(other)),
+        )
+    }
+}
+
+impl core::ops::MulAssign<usize> for LengthHint {
+    fn mul_assign(&mut self, other: usize) {
+        *self = *self * other;
+    }
+}
+
+impl core::ops::BitOr<LengthHint> for LengthHint {
+    type Output = Self;
+
+    /// Returns a new hint that is correct wherever `self` is correct, and wherever
+    /// `other` is correct.
+    ///
+    /// Example:
+    /// ```
+    /// # use writeable::{LengthHint, Writeable};
+    /// # use core::fmt;
+    /// # fn coin_flip() -> bool { true }
+    ///
+    /// struct NonDeterministicWriteable(String, String);
+    ///
+    /// impl Writeable for NonDeterministicWriteable {
+    ///     fn write_to<W: fmt::Write + ?Sized>(&self, sink: &mut W) -> fmt::Result {
+    ///         sink.write_str(if coin_flip() { &self.0 } else { &self.1 })
+    ///     }
+    ///
+    ///     fn writeable_length_hint(&self) -> LengthHint {
+    ///         LengthHint::exact(self.0.len()) | LengthHint::exact(self.1.len())
+    ///     }
+    /// }
+    ///
+    /// writeable::impl_display_with_writeable!(NonDeterministicWriteable);
+    /// ```
+    fn bitor(self, other: LengthHint) -> Self {
+        LengthHint(
+            Ord::min(self.0, other.0),
+            match (self.1, other.1) {
+                (Some(c), Some(d)) => Some(Ord::max(c, d)),
+                _ => None,
+            },
+        )
+    }
+}
+
+impl core::ops::BitOrAssign<LengthHint> for LengthHint {
+    fn bitor_assign(&mut self, other: Self) {
+        *self = *self | other;
+    }
+}
+
+impl core::iter::Sum<usize> for LengthHint {
+    fn sum<I>(iter: I) -> Self
+    where
+        I: Iterator<Item = usize>,
+    {
+        LengthHint::exact(iter.sum::<usize>())
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_add() {
+        assert_eq!(LengthHint::exact(3) + 2, LengthHint::exact(5));
+        assert_eq!(
+            LengthHint::exact(3) + LengthHint::exact(2),
+            LengthHint::exact(5)
+        );
+        assert_eq!(
+            LengthHint::exact(3) + LengthHint::undefined(),
+            LengthHint::at_least(3)
+        );
+
+        assert_eq!(LengthHint::undefined() + 2, LengthHint::at_least(2));
+        assert_eq!(
+            LengthHint::undefined() + LengthHint::exact(2),
+            LengthHint::at_least(2)
+        );
+        assert_eq!(
+            LengthHint::undefined() + LengthHint::undefined(),
+            LengthHint::undefined()
+        );
+
+        assert_eq!(
+            LengthHint::at_least(15) + LengthHint::exact(3),
+            LengthHint::at_least(18)
+        );
+
+        assert_eq!(
+            LengthHint::at_least(15) + LengthHint::at_most(3),
+            LengthHint::at_least(15)
+        );
+
+        assert_eq!(LengthHint::between(48, 92) + 5, LengthHint::between(53, 97));
+
+        let mut len = LengthHint::exact(5);
+        len += LengthHint::exact(3);
+        assert_eq!(len, LengthHint::exact(8));
+        len += 2;
+        assert_eq!(len, LengthHint::exact(10));
+        len += LengthHint::undefined();
+        assert_eq!(len, LengthHint::at_least(10));
+
+        len += LengthHint::exact(3);
+        assert_eq!(len, LengthHint::at_least(13));
+        len += 2;
+        assert_eq!(len, LengthHint::at_least(15));
+        len += LengthHint::undefined();
+        assert_eq!(len, LengthHint::at_least(15));
+
+        assert_eq!(
+            LengthHint::between(usize::MAX - 10, usize::MAX - 5) + LengthHint::exact(20),
+            LengthHint::at_least(usize::MAX)
+        );
+    }
+
+    #[test]
+    fn test_sum() {
+        let lens = vec![
+            LengthHint::exact(4),
+            LengthHint::exact(1),
+            LengthHint::exact(1),
+        ];
+        assert_eq!(
+            lens.iter().copied().sum::<LengthHint>(),
+            LengthHint::exact(6)
+        );
+
+        let lens = vec![
+            LengthHint::exact(4),
+            LengthHint::undefined(),
+            LengthHint::at_least(1),
+        ];
+        assert_eq!(
+            lens.iter().copied().sum::<LengthHint>(),
+            LengthHint::at_least(5)
+        );
+
+        let lens = vec![
+            LengthHint::exact(4),
+            LengthHint::undefined(),
+            LengthHint::at_most(1),
+        ];
+        assert_eq!(
+            lens.iter().copied().sum::<LengthHint>(),
+            LengthHint::at_least(4)
+        );
+
+        let lens = vec![4, 1, 1];
+        assert_eq!(
+            lens.iter().copied().sum::<LengthHint>(),
+            LengthHint::exact(6)
+        );
+    }
+
+    #[test]
+    fn test_mul() {
+        assert_eq!(LengthHint::exact(3) * 2, LengthHint::exact(6));
+
+        assert_eq!(LengthHint::undefined() * 2, LengthHint::undefined());
+
+        assert_eq!(
+            LengthHint::between(48, 92) * 2,
+            LengthHint::between(96, 184)
+        );
+
+        let mut len = LengthHint::exact(5);
+        len *= 2;
+        assert_eq!(len, LengthHint::exact(10));
+
+        assert_eq!(
+            LengthHint::between(usize::MAX - 10, usize::MAX - 5) * 2,
+            LengthHint::at_least(usize::MAX)
+        );
+    }
+
+    #[test]
+    fn test_bitor() {
+        assert_eq!(
+            LengthHint::exact(3) | LengthHint::exact(2),
+            LengthHint::between(2, 3)
+        );
+        assert_eq!(
+            LengthHint::exact(3) | LengthHint::undefined(),
+            LengthHint::undefined()
+        );
+
+        assert_eq!(
+            LengthHint::undefined() | LengthHint::undefined(),
+            LengthHint::undefined()
+        );
+
+        assert_eq!(
+            LengthHint::exact(10) | LengthHint::exact(10),
+            LengthHint::exact(10)
+        );
+
+        assert_eq!(
+            LengthHint::at_least(15) | LengthHint::exact(3),
+            LengthHint::at_least(3)
+        );
+
+        assert_eq!(
+            LengthHint::at_least(15) | LengthHint::at_most(18),
+            LengthHint::undefined()
+        );
+
+        assert_eq!(
+            LengthHint::at_least(15) | LengthHint::at_least(18),
+            LengthHint::at_least(15)
+        );
+
+        assert_eq!(
+            LengthHint::at_most(15) | LengthHint::at_most(18),
+            LengthHint::at_most(18)
+        );
+
+        assert_eq!(
+            LengthHint::between(5, 10) | LengthHint::at_most(3),
+            LengthHint::at_most(10)
+        );
+
+        let mut len = LengthHint::exact(5);
+        len |= LengthHint::exact(3);
+        assert_eq!(len, LengthHint::between(5, 3));
+    }
+}