Add FixedLengthVarULE and user in icu_plurals

components/plurals/src/provider.rs

@@ -27,11 +27,11 @@ use yoke::Yokeable;
 use zerofrom::ZeroFrom;
 use zerovec::ule::vartuple::VarTuple;
 use zerovec::ule::vartuple::VarTupleULE;
-use zerovec::ule::AsULE;
 use zerovec::ule::EncodeAsVarULE;
 use zerovec::ule::UleError;
 use zerovec::ule::VarULE;
 use zerovec::ule::ULE;
+use zerovec::ule::{AsULE, FixedLengthVarULE};
 use zerovec::VarZeroSlice;
 
 pub mod rules;
@@ -482,6 +482,69 @@ where
         core::mem::transmute(bytes)
     }
 
+    /// Creates a singleton [`PluralElementsPackedULE`] in a const context.
+    ///
+    /// Const parameters:
+    ///
+    /// - `M`: the length of `input`
+    /// - `N`: the length of the return value which is `M + 1`
+    ///
+    /// When [generic_const_exprs] is stabilized, we will be able to add a new
+    /// function signature without both const parameters.
+    ///
+    /// # Panics
+    ///
+    /// Panics if N != M + 1.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use icu::plurals::provider::PluralElementsPackedULE;
+    /// use icu::plurals::PluralRules;
+    /// use icu::locale::locale;
+    /// use zerovec::ule::FixedLengthVarULE;
+    ///
+    /// let value = "hello, world!"; // 13 bytes long
+    /// let inner_ule = FixedLengthVarULE::<13, str>::try_from_encodeable(value).unwrap();
+    /// let plural_ule = PluralElementsPackedULE::new_singleton_mn::<13, 14>(inner_ule);
+    /// let rules = PluralRules::try_new(locale!("en").into(), Default::default()).unwrap();
+    ///
+    /// assert_eq!(plural_ule.as_varule().get(0.into(), &rules).1, "hello, world!");
+    /// assert_eq!(plural_ule.as_varule().get(1.into(), &rules).1, "hello, world!");
+    /// assert_eq!(plural_ule.as_varule().get(2.into(), &rules).1, "hello, world!");
+    /// ```
+    ///
+    /// [generic_const_exprs]: https://doc.rust-lang.org/beta/unstable-book/language-features/generic-const-exprs.html#generic_const_exprs
+    pub const fn new_singleton_mn<const M: usize, const N: usize>(
+        input: FixedLengthVarULE<M, V>,
+    ) -> FixedLengthVarULE<N, PluralElementsPackedULE<V>> {
+        #[allow(clippy::panic)] // for safety, and documented
+        if N != M + 1 {
+            panic!(concat!(
+                "new_singleton_mn: N (",
+                stringify!(N),
+                ") != 1 + M (",
+                stringify!(M),
+                ")"
+            ));
+        }
+        let mut bytes = [0u8; N];
+        #[allow(clippy::unwrap_used)] // the bytes are nonempty because N > 0
+        let (start, remainder) = bytes.split_first_mut().unwrap();
+        // TODO(1.87): use copy_from_slice
+        let mut i = 0;
+        #[allow(clippy::indexing_slicing)] // both remainder and input are length M
+        while i < M {
+            remainder[i] = input.as_bytes()[i];
+            i += 1;
+        }
+        *start = 0;
+        // Safety: bytes are a valid representation of this type:
+        // 1. The first byte is 0 which indicates a singleton
+        // 2. The remainder is a valid V by invariant of the input parameter
+        unsafe { FixedLengthVarULE::new_unchecked(bytes) }
+    }
+
     /// Returns a tuple with:
     /// 1. The lead byte
     /// 2. Bytes corresponding to the default V

utils/zerovec/src/ule/fixed_length.rs

@@ -0,0 +1,95 @@
+// 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::ule::{EncodeAsVarULE, UleError, VarULE};
+use core::fmt;
+use core::marker::PhantomData;
+use core::ops::Deref;
+
+/// A container for a [`VarULE`] with a fixed byte length.
+///
+/// This container may be useful if the length of your VarULE is known at compile-time.
+///
+/// # Examples
+///
+/// ```
+/// use zerovec::ule::FixedLengthVarULE;
+///
+/// let container = FixedLengthVarULE::<13, str>::try_from_encodeable("hello, world!").unwrap();
+///
+/// assert_eq!(&*container, "hello, world!");
+///
+/// // Returns an error if the container is not the correct size:
+/// FixedLengthVarULE::<20, str>::try_from_encodeable("hello, world!").unwrap_err();
+/// ```
+#[derive(Copy, Clone, PartialEq, Eq)]
+pub struct FixedLengthVarULE<const N: usize, V: VarULE + ?Sized> {
+    /// Invariant: The bytes MUST be a valid VarULE representation of `V`.
+    bytes: [u8; N],
+    _marker: PhantomData<V>,
+}
+
+impl<const N: usize, V: VarULE + ?Sized> FixedLengthVarULE<N, V> {
+    /// Creates one of these from an [`EncodeAsVarULE`].
+    ///
+    /// Returns an error if the byte length in the container is not the correct length
+    /// for the encodeable object.
+    pub fn try_from_encodeable(input: impl EncodeAsVarULE<V>) -> Result<Self, UleError> {
+        let len = input.encode_var_ule_len();
+        if len != N {
+            return Err(UleError::length::<V>(len));
+        }
+        let mut bytes = [0u8; N];
+        input.encode_var_ule_write(&mut bytes);
+        // Safety: the bytes were just written from an EncodeAsVarULE impl
+        unsafe { Ok(Self::new_unchecked(bytes)) }
+    }
+
+    /// Creates one of these directly from bytes.
+    ///
+    /// # Safety
+    ///
+    /// The bytes MUST be a valid VarULE representation of `V`.
+    pub const unsafe fn new_unchecked(bytes: [u8; N]) -> Self {
+        Self {
+            bytes,
+            _marker: PhantomData,
+        }
+    }
+
+    /// Returns the bytes backing this [`FixedLengthVarULE`], which are
+    /// guaranteed to be a valid VarULE representation of `V`.
+    pub const fn as_bytes(&self) -> &[u8; N] {
+        &self.bytes
+    }
+
+    /// Returns the container as an instance of `V`.
+    pub fn as_varule(&self) -> &V {
+        debug_assert!(V::validate_bytes(&self.bytes).is_ok());
+        // Safety: self.bytes are a valid VarULE representation of `V`.
+        unsafe { V::from_bytes_unchecked(&self.bytes) }
+    }
+}
+
+impl<const N: usize, V: VarULE + ?Sized> fmt::Debug for FixedLengthVarULE<N, V>
+where
+    V: fmt::Debug,
+{
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.as_varule().fmt(f)
+    }
+}
+
+impl<const N: usize, V: VarULE + ?Sized> AsRef<V> for FixedLengthVarULE<N, V> {
+    fn as_ref(&self) -> &V {
+        self.as_varule()
+    }
+}
+
+impl<const N: usize, V: VarULE + ?Sized> Deref for FixedLengthVarULE<N, V> {
+    type Target = V;
+    fn deref(&self) -> &Self::Target {
+        self.as_varule()
+    }
+}

utils/zerovec/src/ule/mod.rs

@@ -14,6 +14,7 @@ mod chars;
 #[cfg(doc)]
 pub mod custom;
 mod encode;
+mod fixed_length;
 mod macros;
 mod multi;
 mod niche;
@@ -30,6 +31,7 @@ pub use chars::CharULE;
 #[cfg(feature = "alloc")]
 pub use encode::encode_varule_to_box;
 pub use encode::EncodeAsVarULE;
+pub use fixed_length::FixedLengthVarULE;
 pub use multi::MultiFieldsULE;
 pub use niche::{NicheBytes, NichedOption, NichedOptionULE};
 pub use option::{OptionULE, OptionVarULE};
@@ -441,6 +443,8 @@ impl UleError {
     }
 
     /// Construct an "invalid length" error for the given type and length
+    ///
+    /// The length is of the input bytes, not the expected length.
     pub fn length<T: ?Sized + 'static>(len: usize) -> UleError {
         UleError::InvalidLength {
             ty: any::type_name::<T>(),