Skill v1.0.1

version: "1.0.1" name: data-structure description: Mandatory use when editing or defining data structures in the compiler.

Design Principles

Prefer Indices Over References

When referencing elements within arenas/IndexVecs, store typed indices rather than references. This avoids lifetime complexity and enables mutation.

Use Spans for Contiguous Ranges

When storing a variable number of children/elements, prefer storing a Span<I> pointing into a shared arena over Vec<T> per-parent.

Never Manually Construct Indices

Never manually construct typed indices (e.g., MyIdx::new(5)). Instead, obtain indices from collection operations:

• .push() returns the new element's index
• .enumerate_idx() yields (I, &T) pairs
• .iter_idx() yields indices only

This ensures indices always correspond to valid elements and prevents off-by-one errors or stale index usage.

Anti-Patterns

• ❌ Using Vec<T> when elements are referenced by index
• ❌ Using usize indices across different collections
• ❌ Storing &T references instead of typed indices
• ❌ Using Range<I> when Span<I> would allow Copy
• ❌ Manually constructing indices instead of obtaining from iterators/push

Useful Libraries and Patterns

The plank_core crate houses some foundational data structures & helpers to be used when defining and structuring data.

New Typed Indices

Define new indices by importing and using the newtype_index macro:

use plank_core::{newtype_index};
 
newtype_index! {
    pub struct ExampleId;
    pub struct StorageIdx;
}

Newtyped indices are compact u32 indices to prevent accidental mixup of different IDs/indices. They are niche optimized such that Option<NewtypedId> & NewtypedId fit into 4-bytes.

plank_core::IndexVec<I, T>

Similar to Rust's native Vec just that element access and iteration is restricted to the associated index type for type safety. Favor IndexVec over Vec unless using for stack/queue type purposes where you're not referencing specific elements directly.

Use .enumerate_idx() to iterate over elements plus indices in a type-safe manner. .push returns the new element's index. Other useful methods:

• .enumerate_mut_idx
• .get
• .get_mut
• .len_idx
• .iter_idx (iterate over indices only)

plank_core::Span<T>

More convenient version of std::ops::Range. Implements Copy if T is copy, allowing containing structs to be Copy, but requires .iter() to iterate.

Instantiate with Span::new

plank_core::DenseIndexSet<I>

Type safe bit set that's indexed via I. Instantiate with with_capacity, new or with_capacity_in_bits.

Core methods:

• set.contains(i: I) -> bool
• set.add(i: I) -> bool (returns true if added, false if already present)
• set.remove(i: I) -> bool (returns true if removed, false if not within set)
• set.clear() (clears all elements, retains capacity)