1 files changed, 865 insertions, 0 deletions
diff --git a/vendor/kstring/src/string.rs b/vendor/kstring/src/string.rs
new file mode 100644
index 000000000..ed39d188a
--- /dev/null
+++ b/vendor/kstring/src/string.rs
@@ -0,0 +1,865 @@
+use std::{borrow::Cow, fmt};
+
+use crate::stack::StackString;
+use crate::KStringCowBase;
+use crate::KStringRef;
+
+pub(crate) type StdString = std::string::String;
+
+/// A UTF-8 encoded, immutable string.
+pub type KString = KStringBase<crate::backend::DefaultStr>;
+
+/// A UTF-8 encoded, immutable string.
+#[derive(Clone)]
+#[repr(transparent)]
+pub struct KStringBase<B> {
+    inner: KStringInner<B>,
+}
+
+impl<B> KStringBase<B> {
+    pub const EMPTY: Self = KStringBase::from_static("");
+
+    /// Create a new empty `KStringBase`.
+    #[inline]
+    #[must_use]
+    pub fn new() -> Self {
+        Self::EMPTY
+    }
+
+    /// Create a reference to a `'static` data.
+    #[inline]
+    #[must_use]
+    pub const fn from_static(other: &'static str) -> Self {
+        Self {
+            inner: KStringInner::from_static(other),
+        }
+    }
+
+    /// Create an inline string, if possible
+    #[inline]
+    #[must_use]
+    pub fn try_inline(other: &str) -> Option<Self> {
+        KStringInner::try_inline(other).map(|inner| Self { inner })
+    }
+}
+
+impl<B: crate::backend::HeapStr> KStringBase<B> {
+    /// Create an owned `KStringBase`.
+    #[inline]
+    #[must_use]
+    pub fn from_boxed(other: crate::backend::BoxedStr) -> Self {
+        Self {
+            inner: KStringInner::from_boxed(other),
+        }
+    }
+
+    /// Create an owned `KStringBase`.
+    #[inline]
+    #[must_use]
+    pub fn from_string(other: StdString) -> Self {
+        Self {
+            inner: KStringInner::from_string(other),
+        }
+    }
+
+    /// Create an owned `KStringBase` optimally from a reference.
+    #[inline]
+    #[must_use]
+    pub fn from_ref(other: &str) -> Self {
+        Self {
+            inner: KStringInner::from_ref(other),
+        }
+    }
+
+    /// Get a reference to the `KStringBase`.
+    #[inline]
+    #[must_use]
+    pub fn as_ref(&self) -> KStringRef<'_> {
+        self.inner.as_ref()
+    }
+
+    /// Extracts a string slice containing the entire `KStringBase`.
+    #[inline]
+    #[must_use]
+    pub fn as_str(&self) -> &str {
+        self.inner.as_str()
+    }
+
+    /// Convert to a mutable string type, cloning the data if necessary.
+    #[inline]
+    #[must_use]
+    pub fn into_string(self) -> StdString {
+        String::from(self.into_boxed_str())
+    }
+
+    /// Convert to a mutable string type, cloning the data if necessary.
+    #[inline]
+    #[must_use]
+    pub fn into_boxed_str(self) -> crate::backend::BoxedStr {
+        self.inner.into_boxed_str()
+    }
+
+    /// Convert to a Cow str
+    #[inline]
+    #[must_use]
+    pub fn into_cow_str(self) -> Cow<'static, str> {
+        self.inner.into_cow_str()
+    }
+}
+
+impl<B: crate::backend::HeapStr> std::ops::Deref for KStringBase<B> {
+    type Target = str;
+
+    #[inline]
+    fn deref(&self) -> &str {
+        self.as_str()
+    }
+}
+
+impl<B: crate::backend::HeapStr> Eq for KStringBase<B> {}
+
+impl<'s, B: crate::backend::HeapStr> PartialEq<KStringBase<B>> for KStringBase<B> {
+    #[inline]
+    fn eq(&self, other: &Self) -> bool {
+        PartialEq::eq(self.as_str(), other.as_str())
+    }
+}
+
+impl<'s, B: crate::backend::HeapStr> PartialEq<str> for KStringBase<B> {
+    #[inline]
+    fn eq(&self, other: &str) -> bool {
+        PartialEq::eq(self.as_str(), other)
+    }
+}
+
+impl<'s, B: crate::backend::HeapStr> PartialEq<&'s str> for KStringBase<B> {
+    #[inline]
+    fn eq(&self, other: &&str) -> bool {
+        PartialEq::eq(self.as_str(), *other)
+    }
+}
+
+impl<'s, B: crate::backend::HeapStr> PartialEq<String> for KStringBase<B> {
+    #[inline]
+    fn eq(&self, other: &StdString) -> bool {
+        PartialEq::eq(self.as_str(), other.as_str())
+    }
+}
+
+impl<B: crate::backend::HeapStr> Ord for KStringBase<B> {
+    #[inline]
+    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
+        self.as_str().cmp(other.as_str())
+    }
+}
+
+impl<B: crate::backend::HeapStr> PartialOrd for KStringBase<B> {
+    #[inline]
+    fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
+        self.as_str().partial_cmp(other.as_str())
+    }
+}
+
+impl<B: crate::backend::HeapStr> std::hash::Hash for KStringBase<B> {
+    #[inline]
+    fn hash<H: std::hash::Hasher>(&self, state: &mut H) {
+        self.as_str().hash(state);
+    }
+}
+
+impl<B: crate::backend::HeapStr> fmt::Debug for KStringBase<B> {
+    #[inline]
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.as_str().fmt(f)
+    }
+}
+
+impl<B: crate::backend::HeapStr> fmt::Display for KStringBase<B> {
+    #[inline]
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Display::fmt(self.as_str(), f)
+    }
+}
+
+impl<B: crate::backend::HeapStr> AsRef<str> for KStringBase<B> {
+    #[inline]
+    fn as_ref(&self) -> &str {
+        self.as_str()
+    }
+}
+
+impl<B: crate::backend::HeapStr> AsRef<[u8]> for KStringBase<B> {
+    #[inline]
+    fn as_ref(&self) -> &[u8] {
+        self.as_bytes()
+    }
+}
+
+impl<B: crate::backend::HeapStr> AsRef<std::ffi::OsStr> for KStringBase<B> {
+    #[inline]
+    fn as_ref(&self) -> &std::ffi::OsStr {
+        (&**self).as_ref()
+    }
+}
+
+impl<B: crate::backend::HeapStr> AsRef<std::path::Path> for KStringBase<B> {
+    #[inline]
+    fn as_ref(&self) -> &std::path::Path {
+        std::path::Path::new(self)
+    }
+}
+
+impl<B: crate::backend::HeapStr> std::borrow::Borrow<str> for KStringBase<B> {
+    #[inline]
+    fn borrow(&self) -> &str {
+        self.as_str()
+    }
+}
+
+impl<B: crate::backend::HeapStr> Default for KStringBase<B> {
+    #[inline]
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl<'s, B: crate::backend::HeapStr> From<KStringRef<'s>> for KStringBase<B> {
+    #[inline]
+    fn from(other: KStringRef<'s>) -> Self {
+        other.to_owned()
+    }
+}
+
+impl<'s, B: crate::backend::HeapStr> From<&'s KStringRef<'s>> for KStringBase<B> {
+    #[inline]
+    fn from(other: &'s KStringRef<'s>) -> Self {
+        other.to_owned()
+    }
+}
+
+impl<'s, B: crate::backend::HeapStr> From<KStringCowBase<'s, B>> for KStringBase<B> {
+    #[inline]
+    fn from(other: KStringCowBase<'s, B>) -> Self {
+        other.into_owned()
+    }
+}
+
+impl<'s, B: crate::backend::HeapStr> From<&'s KStringCowBase<'s, B>> for KStringBase<B> {
+    #[inline]
+    fn from(other: &'s KStringCowBase<'s, B>) -> Self {
+        other.clone().into_owned()
+    }
+}
+
+impl<B: crate::backend::HeapStr> From<StdString> for KStringBase<B> {
+    #[inline]
+    fn from(other: StdString) -> Self {
+        Self::from_string(other)
+    }
+}
+
+impl<'s, B: crate::backend::HeapStr> From<&'s StdString> for KStringBase<B> {
+    #[inline]
+    fn from(other: &'s StdString) -> Self {
+        Self::from_ref(other)
+    }
+}
+
+impl<B: crate::backend::HeapStr> From<crate::backend::BoxedStr> for KStringBase<B> {
+    #[inline]
+    fn from(other: crate::backend::BoxedStr) -> Self {
+        Self::from_boxed(other)
+    }
+}
+
+impl<'s, B: crate::backend::HeapStr> From<&'s crate::backend::BoxedStr> for KStringBase<B> {
+    #[inline]
+    fn from(other: &'s crate::backend::BoxedStr) -> Self {
+        Self::from_ref(other)
+    }
+}
+
+impl<B: crate::backend::HeapStr> From<&'static str> for KStringBase<B> {
+    #[inline]
+    fn from(other: &'static str) -> Self {
+        Self::from_static(other)
+    }
+}
+
+impl<B: crate::backend::HeapStr> std::str::FromStr for KStringBase<B> {
+    type Err = std::convert::Infallible;
+    #[inline]
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        Ok(Self::from_ref(s))
+    }
+}
+
+#[cfg(feature = "serde")]
+impl<B: crate::backend::HeapStr> serde::Serialize for KStringBase<B> {
+    #[inline]
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: serde::Serializer,
+    {
+        serializer.serialize_str(self.as_str())
+    }
+}
+
+#[cfg(feature = "serde")]
+impl<'de, B: crate::backend::HeapStr> serde::Deserialize<'de> for KStringBase<B> {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: serde::Deserializer<'de>,
+    {
+        deserializer.deserialize_string(StringVisitor(std::marker::PhantomData))
+    }
+}
+
+#[cfg(feature = "serde")]
+struct StringVisitor<B>(std::marker::PhantomData<B>);
+
+#[cfg(feature = "serde")]
+impl<'de, B: crate::backend::HeapStr> serde::de::Visitor<'de> for StringVisitor<B> {
+    type Value = KStringBase<B>;
+
+    fn expecting(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
+        formatter.write_str("a string")
+    }
+
+    fn visit_str<E>(self, v: &str) -> Result<Self::Value, E>
+    where
+        E: serde::de::Error,
+    {
+        Ok(Self::Value::from_ref(v))
+    }
+
+    fn visit_string<E>(self, v: String) -> Result<Self::Value, E>
+    where
+        E: serde::de::Error,
+    {
+        Ok(Self::Value::from_string(v))
+    }
+
+    fn visit_bytes<E>(self, v: &[u8]) -> Result<Self::Value, E>
+    where
+        E: serde::de::Error,
+    {
+        match std::str::from_utf8(v) {
+            Ok(s) => Ok(Self::Value::from_ref(s)),
+            Err(_) => Err(serde::de::Error::invalid_value(
+                serde::de::Unexpected::Bytes(v),
+                &self,
+            )),
+        }
+    }
+
+    fn visit_byte_buf<E>(self, v: Vec<u8>) -> Result<Self::Value, E>
+    where
+        E: serde::de::Error,
+    {
+        match String::from_utf8(v) {
+            Ok(s) => Ok(Self::Value::from_string(s)),
+            Err(e) => Err(serde::de::Error::invalid_value(
+                serde::de::Unexpected::Bytes(&e.into_bytes()),
+                &self,
+            )),
+        }
+    }
+}
+
+use inner::KStringInner;
+
+#[cfg(not(feature = "unsafe"))]
+mod inner {
+    use super::*;
+
+    pub(super) enum KStringInner<B> {
+        Singleton(&'static str),
+        Inline(StackString<CAPACITY>),
+        Owned(B),
+    }
+
+    impl<B> KStringInner<B> {
+        /// Create a reference to a `'static` data.
+        #[inline]
+        pub const fn from_static(other: &'static str) -> Self {
+            Self::Singleton(other)
+        }
+
+        #[inline]
+        pub fn try_inline(other: &str) -> Option<Self> {
+            StackString::try_new(other).map(Self::Inline)
+        }
+    }
+
+    impl<B: crate::backend::HeapStr> KStringInner<B> {
+        #[inline]
+        pub(super) fn from_boxed(other: crate::backend::BoxedStr) -> Self {
+            #[allow(clippy::useless_conversion)]
+            Self::Owned(B::from_boxed_str(other))
+        }
+
+        #[inline]
+        pub(super) fn from_string(other: StdString) -> Self {
+            if (0..=CAPACITY).contains(&other.len()) {
+                let inline = { StackString::new(other.as_str()) };
+                Self::Inline(inline)
+            } else {
+                Self::from_boxed(other.into_boxed_str())
+            }
+        }
+
+        #[inline]
+        pub(super) fn from_ref(other: &str) -> Self {
+            if (0..=CAPACITY).contains(&other.len()) {
+                let inline = { StackString::new(other) };
+                Self::Inline(inline)
+            } else {
+                Self::Owned(B::from_str(other))
+            }
+        }
+
+        #[inline]
+        pub(super) fn as_ref(&self) -> KStringRef<'_> {
+            match self {
+                Self::Singleton(s) => KStringRef::from_static(s),
+                Self::Inline(s) => KStringRef::from_ref(s.as_str()),
+                Self::Owned(s) => KStringRef::from_ref(s.as_str()),
+            }
+        }
+
+        #[inline]
+        pub(super) fn as_str(&self) -> &str {
+            match self {
+                Self::Singleton(s) => s,
+                Self::Inline(s) => s.as_str(),
+                Self::Owned(s) => s.as_str(),
+            }
+        }
+
+        #[inline]
+        pub(super) fn into_boxed_str(self) -> crate::backend::BoxedStr {
+            match self {
+                Self::Singleton(s) => crate::backend::BoxedStr::from(s),
+                Self::Inline(s) => crate::backend::BoxedStr::from(s.as_str()),
+                Self::Owned(s) => crate::backend::BoxedStr::from(s.as_str()),
+            }
+        }
+
+        /// Convert to a Cow str
+        #[inline]
+        pub(super) fn into_cow_str(self) -> Cow<'static, str> {
+            match self {
+                Self::Singleton(s) => Cow::Borrowed(s),
+                Self::Inline(s) => Cow::Owned(s.as_str().into()),
+                Self::Owned(s) => Cow::Owned(s.as_str().into()),
+            }
+        }
+    }
+
+    // Explicit to avoid inlining which cuts clone times in half.
+    //
+    // An automatically derived `clone()` has 10ns overhead while the explicit `Deref`/`as_str` has
+    // none of that.  Being explicit and removing the `#[inline]` attribute dropped the overhead to
+    // 5ns.
+    //
+    // My only guess is that the `clone()` calls we delegate to are just that much bigger than
+    // `as_str()` that, when combined with a jump table, is blowing the icache, slowing things down.
+    impl<B: Clone> Clone for KStringInner<B> {
+        fn clone(&self) -> Self {
+            match self {
+                Self::Singleton(s) => Self::Singleton(s),
+                Self::Inline(s) => Self::Inline(*s),
+                Self::Owned(s) => Self::Owned(s.clone()),
+            }
+        }
+    }
+
+    #[allow(unused)]
+    const LEN_SIZE: usize = std::mem::size_of::<crate::stack::Len>();
+
+    #[allow(unused)]
+    const TAG_SIZE: usize = std::mem::size_of::<u8>();
+
+    #[allow(unused)]
+    const MAX_CAPACITY: usize =
+        std::mem::size_of::<crate::string::StdString>() - TAG_SIZE - LEN_SIZE;
+
+    // Performance seems to slow down when trying to occupy all of the padding left by `String`'s
+    // discriminant.  The question is whether faster len=1-16 "allocations" outweighs going to the heap
+    // for len=17-22.
+    #[allow(unused)]
+    const ALIGNED_CAPACITY: usize = std::mem::size_of::<crate::backend::DefaultStr>() - LEN_SIZE;
+
+    #[cfg(feature = "max_inline")]
+    const CAPACITY: usize = MAX_CAPACITY;
+    #[cfg(not(feature = "max_inline"))]
+    const CAPACITY: usize = ALIGNED_CAPACITY;
+}
+
+#[cfg(feature = "unsafe")]
+mod inner {
+    use super::*;
+
+    pub(super) union KStringInner<B> {
+        tag: TagVariant,
+        singleton: SingletonVariant,
+        owned: std::mem::ManuallyDrop<OwnedVariant<B>>,
+        inline: InlineVariant,
+    }
+
+    impl<B> KStringInner<B> {
+        /// Create a reference to a `'static` data.
+        #[inline]
+        pub const fn from_static(other: &'static str) -> Self {
+            Self {
+                singleton: SingletonVariant::new(other),
+            }
+        }
+
+        #[inline]
+        pub fn try_inline(other: &str) -> Option<Self> {
+            StackString::try_new(other).map(|inline| Self {
+                inline: InlineVariant::new(inline),
+            })
+        }
+
+        #[inline]
+        const fn tag(&self) -> Tag {
+            unsafe {
+                // SAFETY: `tag` is in the same spot in each variant
+                self.tag.tag
+            }
+        }
+    }
+
+    impl<B: crate::backend::HeapStr> KStringInner<B> {
+        #[inline]
+        pub(super) fn from_boxed(other: crate::backend::BoxedStr) -> Self {
+            #[allow(clippy::useless_conversion)]
+            let payload = B::from_boxed_str(other);
+            Self {
+                owned: std::mem::ManuallyDrop::new(OwnedVariant::new(payload)),
+            }
+        }
+
+        #[inline]
+        pub(super) fn from_string(other: StdString) -> Self {
+            if (0..=CAPACITY).contains(&other.len()) {
+                let payload = unsafe {
+                    // SAFETY: range check ensured this is always safe
+                    StackString::new_unchecked(other.as_str())
+                };
+                Self {
+                    inline: InlineVariant::new(payload),
+                }
+            } else {
+                Self::from_boxed(other.into_boxed_str())
+            }
+        }
+
+        #[inline]
+        pub(super) fn from_ref(other: &str) -> Self {
+            if (0..=CAPACITY).contains(&other.len()) {
+                let payload = unsafe {
+                    // SAFETY: range check ensured this is always safe
+                    StackString::new_unchecked(other)
+                };
+                Self {
+                    inline: InlineVariant::new(payload),
+                }
+            } else {
+                #[allow(clippy::useless_conversion)]
+                let payload = B::from_str(other);
+                Self {
+                    owned: std::mem::ManuallyDrop::new(OwnedVariant::new(payload)),
+                }
+            }
+        }
+
+        #[inline]
+        pub(super) fn as_ref(&self) -> KStringRef<'_> {
+            let tag = self.tag();
+            unsafe {
+                // SAFETY: `tag` ensures access to correct variant
+                if tag.is_singleton() {
+                    KStringRef::from_static(self.singleton.payload)
+                } else if tag.is_owned() {
+                    KStringRef::from_ref(self.owned.payload.as_str())
+                } else {
+                    debug_assert!(tag.is_inline());
+                    KStringRef::from_ref(self.inline.payload.as_str())
+                }
+            }
+        }
+
+        #[inline]
+        pub(super) fn as_str(&self) -> &str {
+            let tag = self.tag();
+            unsafe {
+                // SAFETY: `tag` ensures access to correct variant
+                if tag.is_singleton() {
+                    self.singleton.payload
+                } else if tag.is_owned() {
+                    self.owned.payload.as_str()
+                } else {
+                    debug_assert!(tag.is_inline());
+                    self.inline.payload.as_str()
+                }
+            }
+        }
+
+        #[inline]
+        pub(super) fn into_boxed_str(self) -> crate::backend::BoxedStr {
+            let tag = self.tag();
+            unsafe {
+                // SAFETY: `tag` ensures access to correct variant
+                if tag.is_singleton() {
+                    crate::backend::BoxedStr::from(self.singleton.payload)
+                } else if tag.is_owned() {
+                    crate::backend::BoxedStr::from(self.owned.payload.as_str())
+                } else {
+                    debug_assert!(tag.is_inline());
+                    crate::backend::BoxedStr::from(self.inline.payload.as_ref())
+                }
+            }
+        }
+
+        /// Convert to a Cow str
+        #[inline]
+        pub(super) fn into_cow_str(self) -> Cow<'static, str> {
+            let tag = self.tag();
+            unsafe {
+                // SAFETY: `tag` ensures access to correct variant
+                if tag.is_singleton() {
+                    Cow::Borrowed(self.singleton.payload)
+                } else if tag.is_owned() {
+                    Cow::Owned(self.owned.payload.as_str().into())
+                } else {
+                    debug_assert!(tag.is_inline());
+                    Cow::Owned(self.inline.payload.as_str().into())
+                }
+            }
+        }
+    }
+
+    // Explicit to avoid inlining which cuts clone times in half.
+    //
+    // An automatically derived `clone()` has 10ns overhead while the explicit `Deref`/`as_str` has
+    // none of that.  Being explicit and removing the `#[inline]` attribute dropped the overhead to
+    // 5ns.
+    //
+    // My only guess is that the `clone()` calls we delegate to are just that much bigger than
+    // `as_str()` that, when combined with a jump table, is blowing the icache, slowing things down.
+    impl<B: Clone> Clone for KStringInner<B> {
+        fn clone(&self) -> Self {
+            let tag = self.tag();
+            if tag.is_owned() {
+                unsafe {
+                    // SAFETY: `tag` ensures access to correct variant
+                    Self {
+                        owned: std::mem::ManuallyDrop::new(OwnedVariant::new(
+                            self.owned.payload.clone(),
+                        )),
+                    }
+                }
+            } else {
+                unsafe {
+                    // SAFETY: `tag` ensures access to correct variant
+                    // SAFETY: non-owned types are copyable
+                    std::mem::transmute_copy(self)
+                }
+            }
+        }
+    }
+
+    impl<B> Drop for KStringInner<B> {
+        fn drop(&mut self) {
+            let tag = self.tag();
+            if tag.is_owned() {
+                unsafe {
+                    // SAFETY: `tag` ensures we are using the right variant
+                    std::mem::ManuallyDrop::drop(&mut self.owned)
+                }
+            }
+        }
+    }
+
+    #[allow(unused)]
+    const LEN_SIZE: usize = std::mem::size_of::<crate::stack::Len>();
+
+    #[allow(unused)]
+    const TAG_SIZE: usize = std::mem::size_of::<Tag>();
+
+    #[allow(unused)]
+    const PAYLOAD_SIZE: usize = std::mem::size_of::<crate::backend::DefaultStr>();
+    type Payload = Padding<PAYLOAD_SIZE>;
+
+    #[allow(unused)]
+    const TARGET_SIZE: usize = std::mem::size_of::<Target>();
+    type Target = crate::string::StdString;
+
+    #[allow(unused)]
+    const MAX_CAPACITY: usize = TARGET_SIZE - LEN_SIZE - TAG_SIZE;
+
+    // Performance seems to slow down when trying to occupy all of the padding left by `String`'s
+    // discriminant.  The question is whether faster len=1-16 "allocations" outweighs going to the heap
+    // for len=17-22.
+    #[allow(unused)]
+    const ALIGNED_CAPACITY: usize = PAYLOAD_SIZE - LEN_SIZE;
+
+    #[cfg(feature = "max_inline")]
+    const CAPACITY: usize = MAX_CAPACITY;
+    #[cfg(not(feature = "max_inline"))]
+    const CAPACITY: usize = ALIGNED_CAPACITY;
+
+    const PAYLOAD_PAD_SIZE: usize = TARGET_SIZE - PAYLOAD_SIZE - TAG_SIZE;
+    const INLINE_PAD_SIZE: usize = TARGET_SIZE - CAPACITY - LEN_SIZE - TAG_SIZE;
+
+    #[derive(Copy, Clone)]
+    #[repr(C)]
+    struct TagVariant {
+        payload: Payload,
+        pad: Padding<PAYLOAD_PAD_SIZE>,
+        tag: Tag,
+    }
+    static_assertions::assert_eq_size!(Target, TagVariant);
+
+    #[derive(Copy, Clone)]
+    #[repr(C)]
+    struct SingletonVariant {
+        payload: &'static str,
+        pad: Padding<PAYLOAD_PAD_SIZE>,
+        tag: Tag,
+    }
+    static_assertions::assert_eq_size!(Payload, &'static str);
+    static_assertions::assert_eq_size!(Target, SingletonVariant);
+
+    impl SingletonVariant {
+        #[inline]
+        const fn new(payload: &'static str) -> Self {
+            Self {
+                payload,
+                pad: Padding::new(),
+                tag: Tag::SINGLETON,
+            }
+        }
+    }
+
+    impl std::fmt::Debug for SingletonVariant {
+        #[inline]
+        fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+            self.payload.fmt(f)
+        }
+    }
+
+    #[derive(Clone)]
+    #[repr(C)]
+    struct OwnedVariant<B> {
+        payload: B,
+        pad: Padding<PAYLOAD_PAD_SIZE>,
+        tag: Tag,
+    }
+    static_assertions::assert_eq_size!(Payload, crate::backend::DefaultStr);
+    static_assertions::assert_eq_size!(Target, OwnedVariant<crate::backend::DefaultStr>);
+
+    impl<B> OwnedVariant<B> {
+        #[inline]
+        const fn new(payload: B) -> Self {
+            Self {
+                payload,
+                pad: Padding::new(),
+                tag: Tag::OWNED,
+            }
+        }
+    }
+
+    impl<B: crate::backend::HeapStr> std::fmt::Debug for OwnedVariant<B> {
+        #[inline]
+        fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+            self.payload.fmt(f)
+        }
+    }
+
+    #[derive(Copy, Clone)]
+    #[repr(C)]
+    struct InlineVariant {
+        payload: StackString<CAPACITY>,
+        pad: Padding<INLINE_PAD_SIZE>,
+        tag: Tag,
+    }
+    static_assertions::assert_eq_size!(Target, InlineVariant);
+
+    impl InlineVariant {
+        #[inline]
+        const fn new(payload: StackString<CAPACITY>) -> Self {
+            Self {
+                payload,
+                pad: Padding::new(),
+                tag: Tag::INLINE,
+            }
+        }
+    }
+
+    impl std::fmt::Debug for InlineVariant {
+        #[inline]
+        fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+            self.payload.fmt(f)
+        }
+    }
+
+    #[derive(Copy, Clone, PartialEq, Eq)]
+    #[repr(transparent)]
+    struct Tag(u8);
+
+    impl Tag {
+        const SINGLETON: Tag = Tag(0);
+        const OWNED: Tag = Tag(u8::MAX);
+        const INLINE: Tag = Tag(1);
+
+        #[inline]
+        const fn is_singleton(self) -> bool {
+            self.0 == Self::SINGLETON.0
+        }
+
+        #[inline]
+        const fn is_owned(self) -> bool {
+            self.0 == Self::OWNED.0
+        }
+
+        #[inline]
+        const fn is_inline(self) -> bool {
+            !self.is_singleton() && !self.is_owned()
+        }
+    }
+
+    #[derive(Copy, Clone)]
+    #[repr(transparent)]
+    struct Padding<const L: usize>([std::mem::MaybeUninit<u8>; L]);
+
+    impl<const L: usize> Padding<L> {
+        const fn new() -> Self {
+            let padding = unsafe {
+                // SAFETY: Padding, never actually used
+                std::mem::MaybeUninit::uninit().assume_init()
+            };
+            Self(padding)
+        }
+    }
+
+    impl<const L: usize> Default for Padding<L> {
+        fn default() -> Self {
+            Self::new()
+        }
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[test]
+    fn test_size() {
+        println!("KString: {}", std::mem::size_of::<KString>());
+    }
+}