index_type

A Rust library providing strongly typed indices for collections, designed for both std and no_std environments.

What are typed indices?

In standard Rust, collections use usize for indexing. This works well but provides no compile-time protection against using an index from one collection with another. Typed indices solve this by creating custom index types that are statically associated with specific collections.

In standard Rust, a raw usize can index any collection. This allows subtle bugs:

let nodes: Vec<Node> = vec![Node::default(); 10];  // 10 nodes
let edges: Vec<Edge> = vec![Edge::default(); 5];   // 5 edges
let node_index = 3;
nodes[node_index];
edges[node_index]; // compiles just fine!

With typed indices, cross-contamination becomes a compile error:

#[derive(IndexType, Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
struct NodeId(u32);

#[derive(IndexType, Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
struct EdgeId(u32);

let nodes: TypedVec<NodeId, Node> = typed_vec![Node::default(); 10];
let edges: TypedVec<EdgeId, Edge> = typed_vec![Edge::default(); 10];
let node_id = NodeId(3);
nodes[node_id]; // OK
// edges[node_id]; // COMPILE ERROR: expected EdgeId, found NodeId

Features

• Type Safety: Prevents accidental misuse of indices between different collections at compile time
• no_std Support: Works in embedded systems and other no_std environments
• Memory Efficiency: Use smaller integer types (u8, u16) for indices when collections are bounded
• Niche Optimization: Supports NonZero types so Option<Index> has the same size as Index
• Rich Collections: Provides TypedSlice, TypedVec, TypedArray, and TypedArrayVec
• Derive Macros: Easy to define custom index types with #[derive(IndexType)]
• Range Iterators: Iterate over ranges using custom index types

Quick Start

use index_type::IndexType;
use index_type::typed_vec::TypedVec;

#[derive(IndexType, Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
struct MyIndex(u32);

let mut vec: TypedVec<MyIndex, i32> = TypedVec::new();
let idx = vec.push(42);

assert_eq!(vec[idx], 42);
// vec[0usize]; // This won't compile - requires MyIndex type

Defining Index Types

Use the #[derive(IndexType)] macro on a newtype struct:

use index_type::IndexType;

#[derive(IndexType, Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
struct MyIndex(u32);

The macro automatically implements the [IndexType] trait for your custom type. By default, it generates an error type MyIndexTooBigError. You can specify a custom error type:

use index_type::IndexType;
use index_type::IndexTooBigError;

#[derive(Debug, IndexTooBigError)]
#[index_too_big_error(msg = "item id too big")]
struct ItemIdTooBigError;

#[derive(IndexType, Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
#[index_type(error = ItemIdTooBigError)]
struct ItemId(u32);

Typed Collections

TypedVec

A growable vector with typed indexing. See TypedVec for the full API.

use index_type::IndexType;
use index_type::typed_vec::TypedVec;

#[derive(IndexType, Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
struct NodeId(u32);

let mut nodes: TypedVec<NodeId, String> = TypedVec::new();
let id0 = nodes.push("Alice".to_string());
let id1 = nodes.push("Bob".to_string());

println!("Node 0: {}", nodes[id0]);

Operations that can fail due to index overflow have both panicking and fallible variants:

let mut vec: TypedVec<MyIndex, i32> = TypedVec::new();
vec.push(1);                              // Panics if index too big
let result = vec.try_push(2);             // Returns Result<(), Error>

TypedSlice

A slice wrapper with typed indexing. See TypedSlice for the full API.

use index_type::IndexType;
use index_type::typed_vec::TypedVec;
use index_type::typed_slice::TypedSlice;

#[derive(IndexType, Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
struct RowId(u16);

let vec: TypedVec<RowId, f64> = TypedVec::from_vec(vec![1.0, 2.0, 3.0]);
let slice: &TypedSlice<RowId, f64> = vec.as_slice();

// Safe indexing with custom type
let first = slice[RowId::ZERO];

TypedArray

A fixed-size array with typed indexing. The array length N is checked at compile time to ensure it fits within the index type's range. See TypedArray for the full API.

use index_type::IndexType;
use index_type::typed_array::TypedArray;

#[derive(IndexType, Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
struct PixelIdx(u8);

let mut pixels: TypedArray<PixelIdx, [u8; 3], 4> = TypedArray::default();
pixels[PixelIdx::ZERO] = [255, 0, 0];  // Red
pixels[PixelIdx(1)] = [0, 255, 0];     // Green

TypedArrayVec

A fixed-capacity vector ideal for embedded systems. It never allocates after creation. See TypedArrayVec for the full API.

use index_type::IndexType;
use index_type::typed_array_vec::TypedArrayVec;

#[derive(IndexType, Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
struct BufferIndex(u8);

let mut buffer: TypedArrayVec<BufferIndex, u8, 16> = TypedArrayVec::new();
buffer.push(42);
assert_eq!(buffer.len().to_raw_index(), 1);

A TypedArrayVec<u8, u8, 3> is only 4 bytes (3 bytes for data + 1 byte for length).

Memory-Efficient Indices

Using smaller integer types reduces memory when storing many indices:

#[derive(IndexType, Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
struct SmallIndex(u8);  // Only 1 byte per index!

println!("SmallIndex: {} bytes", std::mem::size_of::<SmallIndex>());
println!("u32 index: {} bytes", std::mem::size_of::<u32>());

For collections with at most 255 elements, u8 saves 75% memory compared to u32.

NonZero Indices and Niche Optimization

Using NonZero types enables niche optimization, where Option<Index> has the same size as Index:

use index_type::IndexType;
use core::num::NonZeroU32;

#[derive(IndexType, Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
struct SafeId(NonZeroU32);

// Option<SafeId> takes only 4 bytes, not 8!
assert_eq!(std::mem::size_of::<SafeId>(), 4);
assert_eq!(std::mem::size_of::<Option<SafeId>>(), 4);

Range Iterators

Standard Rust ranges require the unstable Step trait. This crate provides TypedRangeIterExt for iterating over ranges with custom index types:

use index_type::IndexType;
use index_type::typed_range::TypedRangeIterExt;

#[derive(IndexType, Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
struct MyIdx(u32);

let start = MyIdx(5);
let end = MyIdx(10);

for idx in (start..end).iter() {
    println!("{:?}", idx);
}

Typed Enumerate

Use TypedIteratorExt to enumerate any iterator with typed indices:

use index_type::IndexType;
use index_type::typed_enumerate::TypedIteratorExt;

#[derive(IndexType, Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
struct RowIdx(u32);

let pairs: Vec<_> = ["a", "b", "c"]
    .into_iter()
    .typed_enumerate::<RowIdx>()
    .collect();

assert_eq!(pairs[1].0, RowIdx(1));
assert_eq!(pairs[1].1, "b");

Macros

Convenience macros for creating typed collections:

use index_type::{typed_vec, typed_array, typed_array_vec, typed_slice, typed_slice_mut, IndexType};
use index_type::typed_vec::TypedVec;
use index_type::typed_array::TypedArray;
use index_type::typed_array_vec::TypedArrayVec;
use index_type::typed_slice::TypedSlice;

#[derive(IndexType, Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
struct MyIndex(u32);

// Create a TypedVec
let v: TypedVec<MyIndex, i32> = typed_vec![1, 2, 3];

// Create a TypedArray
let a: TypedArray<MyIndex, i32, 3> = typed_array![1, 2, 3];

// Create a TypedArrayVec
let av: TypedArrayVec<MyIndex, u8, 4> = typed_array_vec![1, 2, 3, 4];

// Create a TypedSlice reference
let s: &TypedSlice<MyIndex, i32> = typed_slice![1, 2, 3];

Error Handling

Operations that can fail due to index overflow return Result types:

use index_type::IndexType;
use index_type::typed_vec::TypedVec;

#[derive(IndexType, Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
struct MyIndex(u8);  // MAX_RAW_INDEX = 255

let mut vec: TypedVec<MyIndex, i32> = TypedVec::new();

// Fill up to capacity
for i in 0..255 {
    vec.try_push(i).unwrap();
}

// This fails gracefully
assert!(vec.try_push(255).is_err());

no_std Compatibility

This crate is no_std compatible. The alloc feature (enabled by default) enables heap-allocated collections (TypedVec and related macros).

For pure no_std environments without heap allocation, disable the alloc feature:

[dependencies]
index_type = { version = "...", default-features = false }

License: MIT OR Apache-2.0