Implement a generic length parameter for Vec<T, N>

CHANGELOG.md

@@ -34,6 +34,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - Added `QueueView`, the `!Sized` version of `Queue`.
 - Added `SortedLinkedListView`, the `!Sized` version of `SortedLinkedList`.
 - Added implementation of `Borrow` and `BorrowMut` for `String` and `Vec`.
+- Added `LenT` generic to `Vec<T, N>` and `VecView<T>` to save memory when using a sane capacity value.
 
 ### Changed
 

src/binary_heap.rs

@@ -60,7 +60,7 @@ impl private::Sealed for Min {}
 /// struct if you want to write code that's generic over both.
 pub struct BinaryHeapInner<T, K, S: Storage> {
     pub(crate) _kind: PhantomData<K>,
-    pub(crate) data: VecInner<T, S>,
+    pub(crate) data: VecInner<T, usize, S>,
 }
 
 /// A priority queue implemented with a binary heap.
@@ -184,7 +184,7 @@ impl<T, K, const N: usize> BinaryHeap<T, K, N> {
 
 impl<T, K, const N: usize> BinaryHeap<T, K, N> {
     /// Returns the underlying `Vec<T,N>`. Order is arbitrary and time is *O*(1).
-    pub fn into_vec(self) -> Vec<T, N> {
+    pub fn into_vec(self) -> Vec<T, N, usize> {
         self.data
     }
 

src/de.rs

@@ -1,6 +1,6 @@
 use crate::{
     binary_heap::Kind as BinaryHeapKind, BinaryHeap, Deque, HistoryBuffer, IndexMap, IndexSet,
-    LinearMap, String, Vec,
+    LenType, LinearMap, String, Vec,
 };
 use core::{
     fmt,
@@ -95,21 +95,22 @@ where
     }
 }
 
-impl<'de, T, const N: usize> Deserialize<'de> for Vec<T, N>
+impl<'de, T, LenT: LenType, const N: usize> Deserialize<'de> for Vec<T, N, LenT>
 where
     T: Deserialize<'de>,
 {
     fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
     where
         D: Deserializer<'de>,
     {
-        struct ValueVisitor<'de, T, const N: usize>(PhantomData<(&'de (), T)>);
+        struct ValueVisitor<'de, T, LenT: LenType, const N: usize>(PhantomData<(&'de (), T, LenT)>);
 
-        impl<'de, T, const N: usize> serde::de::Visitor<'de> for ValueVisitor<'de, T, N>
+        impl<'de, T, LenT, const N: usize> serde::de::Visitor<'de> for ValueVisitor<'de, T, LenT, N>
         where
             T: Deserialize<'de>,
+            LenT: LenType,
         {
-            type Value = Vec<T, N>;
+            type Value = Vec<T, N, LenT>;
 
             fn expecting(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
                 formatter.write_str("a sequence")

src/defmt.rs

@@ -1,9 +1,9 @@
 //! Defmt implementations for heapless types
 
-use crate::{storage::Storage, string::StringInner, vec::VecInner};
+use crate::{storage::Storage, string::StringInner, vec::VecInner, LenType};
 use defmt::Formatter;
 
-impl<T, S: Storage> defmt::Format for VecInner<T, S>
+impl<T, LenT: LenType, S: Storage> defmt::Format for VecInner<T, LenT, S>
 where
     T: defmt::Format,
 {

src/indexmap.rs

@@ -138,7 +138,7 @@ macro_rules! probe_loop {
 }
 
 struct CoreMap<K, V, const N: usize> {
-    entries: Vec<Bucket<K, V>, N>,
+    entries: Vec<Bucket<K, V>, N, usize>,
     indices: [Option<Pos>; N],
 }
 
@@ -1298,7 +1298,7 @@ where
 
 #[derive(Clone)]
 pub struct IntoIter<K, V, const N: usize> {
-    entries: Vec<Bucket<K, V>, N>,
+    entries: Vec<Bucket<K, V>, N, usize>,
 }
 
 impl<K, V, const N: usize> Iterator for IntoIter<K, V, N> {

src/len_type.rs

@@ -0,0 +1,112 @@
+use core::{
+    fmt::{Debug, Display},
+    ops::{Add, AddAssign, Sub, SubAssign},
+};
+
+mod private {
+    pub trait Sealed {}
+
+    impl Sealed for u8 {}
+    impl Sealed for u16 {}
+    #[cfg(any(target_pointer_width = "32", target_pointer_width = "64"))]
+    impl Sealed for u32 {}
+
+    impl Sealed for usize {}
+}
+
+macro_rules! impl_lentype {
+    ($($(#[$meta:meta])* $LenT:ty),*) => {$(
+        $(#[$meta])*
+        impl LenType for $LenT {
+            const ZERO: Self = 0;
+            const ONE: Self = 1;
+            const MAX: usize = Self::MAX as _;
+        }
+    )*}
+}
+
+/// A sealed trait representing a valid type to use as a length for a container.
+///
+/// This cannot be implemented in user code, and is restricted to `u8`, `u16`, `u32`, and `usize`.
+pub trait LenType:
+    private::Sealed
+    + Send
+    + Sync
+    + Copy
+    + Display
+    + Debug
+    + PartialEq
+    + Add<Output = Self>
+    + AddAssign
+    + Sub<Output = Self>
+    + SubAssign
+    + PartialOrd
+    + TryFrom<usize, Error: Debug>
+    + TryInto<usize, Error: Debug>
+{
+    /// The zero value of the integer type.
+    const ZERO: Self;
+    /// The one value of the integer type.
+    const ONE: Self;
+    /// The maximum value of this type, as a `usize`.
+    const MAX: usize;
+
+    /// An infallible conversion from `usize` to `LenT`.
+    fn from_usize(val: usize) -> Self {
+        val.try_into().unwrap()
+    }
+
+    /// An infallible conversion from `LenT` to `usize`.
+    fn into_usize(self) -> usize {
+        self.try_into().unwrap()
+    }
+}
+
+impl_lentype!(
+    u8,
+    u16,
+    #[cfg(any(target_pointer_width = "32", target_pointer_width = "64"))]
+    u32,
+    usize
+);
+
+macro_rules! impl_lentodefault {
+    ($LenT:ty: $($len:literal),*) => {$(
+        impl SmallestLenType for Const<$len> {
+            type Type = $LenT;
+        }
+    )*};
+}
+
+/// A struct to create individual types for mapping with [`SmallestLenType`].
+///
+/// See the documentation of [`DefaultLenType`] for a detailed explanation.
+pub struct Const<const N: usize>;
+
+/// A trait to map [`Const`] to it's respective [`LenType`].
+///
+/// See the documentation of [`DefaultLenType`] for a detailed explanation.
+#[diagnostic::on_unimplemented(
+    message = "Length `N` does not have a default `LenType` mapping",
+    note = "Provide the `LenType` explicitly, such as `usize`"
+)]
+pub trait SmallestLenType {
+    type Type: LenType;
+}
+
+/// A type alias to perform the `const N: usize` -> `LenType` mapping.
+///
+/// This is impossible to perform directly, but it is possible to write a `const N: usize` -> related `Type` mapping via a const generic argument,
+/// then map from that to an unrelated type via a trait with associated types.
+///
+/// [`Const`] is the "related type" in the above explaination, [`SmallestLenType`] is the mapping trait.
+pub type DefaultLenType<const N: usize> = <Const<N> as SmallestLenType>::Type;
+
+impl_lentodefault!(u8: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 163, 164, 165, 166, 167, 168, 169, 170, 171, 172, 173, 174, 175, 176, 177, 178, 179, 180, 181, 182, 183, 184, 185, 186, 187, 188, 189, 190, 191, 192, 193, 194, 195, 196, 197, 198, 199, 200, 201, 202, 203, 204, 205, 206, 207, 208, 209, 210, 211, 212, 213, 214, 215, 216, 217, 218, 219, 220, 221, 222, 223, 224, 225, 226, 227, 228, 229, 230, 231, 232, 233, 234, 235, 236, 237, 238, 239, 240, 241, 242, 243, 244, 245, 246, 247, 248, 249, 250, 251, 252, 253, 254, 255);
+impl_lentodefault!(u16: 256, 300, 400, 500, 512, 600, 700, 800, 900, 1000, 1024, 2000, 2048, 4000, 4096, 8000, 8192, 16000, 16384, 32000, 32768, 65000, 65535);
+#[cfg(any(target_pointer_width = "32", target_pointer_width = "64"))]
+impl_lentodefault!(u32: 65536, 131072, 262144, 524288, 1048576, 2097152, 4194304, 8388608, 16777216, 33554432, 67108864, 134217728, 268435456, 536870912, 1073741824, 2147483648);
+
+pub const fn check_capacity_fits<LenT: LenType, const N: usize>() {
+    assert!(LenT::MAX >= N, "The capacity is larger than `LenT` can hold, increase the size of `LenT` or reduce the capacity")
+}

src/lib.rs

@@ -94,6 +94,7 @@ pub use indexmap::{
     ValuesMut as IndexMapValuesMut,
 };
 pub use indexset::{FnvIndexSet, IndexSet, Iter as IndexSetIter};
+pub use len_type::LenType;
 pub use linear_map::LinearMap;
 pub use string::String;
 
@@ -107,6 +108,7 @@ pub mod deque;
 pub mod histbuf;
 mod indexmap;
 mod indexset;
+mod len_type;
 pub mod linear_map;
 mod slice;
 pub mod storage;

src/linear_map.rs

@@ -12,7 +12,7 @@ use crate::{
 
 /// Base struct for [`LinearMap`] and [`LinearMapView`]
 pub struct LinearMapInner<K, V, S: Storage> {
-    pub(crate) buffer: VecInner<(K, V), S>,
+    pub(crate) buffer: VecInner<(K, V), usize, S>,
 }
 
 /// A fixed capacity map/dictionary that performs lookups via linear search.
@@ -445,7 +445,7 @@ pub struct IntoIter<K, V, const N: usize>
 where
     K: Eq,
 {
-    inner: <Vec<(K, V), N> as IntoIterator>::IntoIter,
+    inner: <Vec<(K, V), N, usize> as IntoIterator>::IntoIter,
 }
 
 impl<K, V, const N: usize> Iterator for IntoIter<K, V, N>

src/ser.rs

@@ -8,7 +8,7 @@ use crate::{
     storage::Storage,
     string::StringInner,
     vec::VecInner,
-    IndexMap, IndexSet,
+    IndexMap, IndexSet, LenType,
 };
 use serde::ser::{Serialize, SerializeMap, SerializeSeq, Serializer};
 
@@ -49,7 +49,7 @@ where
     }
 }
 
-impl<T, St: Storage> Serialize for VecInner<T, St>
+impl<T, LenT: LenType, St: Storage> Serialize for VecInner<T, LenT, St>
 where
     T: Serialize,
 {

src/string/mod.rs

@@ -12,6 +12,7 @@ use core::{
 };
 
 use crate::{
+    len_type::LenType,
     storage::{OwnedStorage, Storage, ViewStorage},
     vec::VecInner,
     Vec,
@@ -47,7 +48,7 @@ impl fmt::Display for FromUtf16Error {
 /// In most cases you should use [`String`] or [`StringView`] directly. Only use this
 /// struct if you want to write code that's generic over both.
 pub struct StringInner<S: Storage> {
-    vec: VecInner<u8, S>,
+    vec: VecInner<u8, usize, S>,
 }
 
 /// A fixed capacity [`String`](https://doc.rust-lang.org/std/string/struct.String.html).
@@ -210,9 +211,11 @@ impl<const N: usize> String<N> {
     /// # Ok::<(), core::str::Utf8Error>(())
     /// ```
     #[inline]
-    pub fn from_utf8(vec: Vec<u8, N>) -> Result<Self, Utf8Error> {
+    pub fn from_utf8<LenT: LenType>(vec: Vec<u8, N, LenT>) -> Result<Self, Utf8Error> {
         core::str::from_utf8(&vec)?;
-        Ok(Self { vec })
+
+        // SAFETY: UTF-8 invariant has just been checked by `str::from_utf8`.
+        Ok(unsafe { Self::from_utf8_unchecked(vec) })
     }
 
     /// Convert UTF-8 bytes into a `String`, without checking that the string
@@ -237,8 +240,10 @@ impl<const N: usize> String<N> {
     /// assert_eq!("💖", sparkle_heart);
     /// ```
     #[inline]
-    pub unsafe fn from_utf8_unchecked(vec: Vec<u8, N>) -> Self {
-        Self { vec }
+    pub unsafe fn from_utf8_unchecked<LenT: LenType>(vec: Vec<u8, N, LenT>) -> Self {
+        Self {
+            vec: vec.cast_len_type(),
+        }
     }
 
     /// Converts a `String` into a byte vector.
@@ -260,7 +265,7 @@ impl<const N: usize> String<N> {
     /// # Ok::<(), ()>(())
     /// ```
     #[inline]
-    pub fn into_bytes(self) -> Vec<u8, N> {
+    pub fn into_bytes(self) -> Vec<u8, N, usize> {
         self.vec
     }
 
@@ -417,7 +422,7 @@ impl<S: Storage> StringInner<S> {
     /// assert_eq!(s, "olleh");
     /// # Ok::<(), ()>(())
     /// ```
-    pub unsafe fn as_mut_vec(&mut self) -> &mut VecInner<u8, S> {
+    pub unsafe fn as_mut_vec(&mut self) -> &mut VecInner<u8, usize, S> {
         &mut self.vec
     }
 
@@ -1025,7 +1030,7 @@ mod tests {
     #[test]
     fn into_bytes() {
         let s: String<4> = String::try_from("ab").unwrap();
-        let b: Vec<u8, 4> = s.into_bytes();
+        let b: Vec<u8, 4, usize> = s.into_bytes();
         assert_eq!(b.len(), 2);
         assert_eq!(&[b'a', b'b'], &b[..]);
     }

src/ufmt.rs

@@ -1,4 +1,4 @@
-use crate::{storage::Storage, string::StringInner, vec::VecInner};
+use crate::{storage::Storage, string::StringInner, vec::VecInner, LenType};
 use ufmt_write::uWrite;
 
 impl<S: Storage> uWrite for StringInner<S> {
@@ -8,7 +8,7 @@ impl<S: Storage> uWrite for StringInner<S> {
     }
 }
 
-impl<S: Storage> uWrite for VecInner<u8, S> {
+impl<LenT: LenType, S: Storage> uWrite for VecInner<u8, LenT, S> {
     type Error = ();
     fn write_str(&mut self, s: &str) -> Result<(), Self::Error> {
         self.extend_from_slice(s.as_bytes())