Smart String Library

This library is a collection of string types and traits designed for enhanced string manipulation. It's born out of a need to centralize and avoid code repetition, particularly unsafe operations, from the author's previous projects. While the tools and methods here reflect certain patterns frequently observed in those projects, it's worth noting that the library itself is in its early stages of development.

Status

Currently, Smart String is in active development, and its API might undergo changes. Although it encapsulates tried-and-true patterns from earlier works, the library as a standalone entity is relatively new. Hence, it's advised to use it with caution and feel free to provide feedback, report issues, or suggest improvements.

Some core behavior is covered by unit tests, but coverage is incomplete.

Features

• serde - Enables serde support.

MSRV (Minimum Supported Rust Version)

MSRV is Rust 1.59.0 (with default features).

Motivation:

• This crate uses the 2021 edition and relies on language features needed by the public API (notably a default const generic parameter in SmartString<const N: usize = DEFAULT_CAPACITY>).
• Keeping MSRV relatively low matters for library users; if we ever need to bump it, we should document it in release notes.

What this guarantees (and what it doesn't):

• Guaranteed: smart-string itself builds and tests with rustc 1.59.0.
• Not guaranteed: that cargo +1.59.0 will always be able to resolve and build the latest dependency graph from crates.io in the future without pinning, because transitive dependencies may raise their own MSRV over time.

In CI we run an MSRV job that compiles with rustc 1.59.0. If MSRV breaks due to dependency drift, we either:

• pin direct dependency versions / add upper bounds (policy decision), or
• bump MSRV (and document it).

What's in the box

• PascalString<N>: A string with a fixed capacity, either stored on the stack or in-place within larger structures and arrays.
• DisplayExt: A suite of methods to streamline string formatting.
• SmartString: A string that dynamically decides its storage location (stack or heap) based on its length.

Roadmap

Primary Goals

• StringsStack: A dedicated storage solution for multiple strings, allowing them to be housed within a single allocation.
• StringsSet: A storage medium designed for strings, facilitating both consolidated allocation and utilization as a hash set.

Additional Goals

• PascalStringLong<N>: An enhanced variant of PascalString<N> offering support for capacities up to 2^32-1 bytes, catering to scenarios where a 255-byte limit falls short.
• Compatibility with no_std environments.
• Integration support for ufmt.

Open to more suggestions!

SmartString storage semantics (explicit conversions)

SmartString may promote from stack to heap during mutating operations (e.g. push_str, reserve) when the stack capacity is exceeded.

It does not automatically demote from heap to stack when the content becomes shorter (including during in-place deserialization). This is intentional: implicit demotion can cause surprising realloc/dealloc churn in real workloads (e.g. shorten → re-grow).

If you want to attempt a demotion, call try_into_stack. If you want to force heap storage, call into_heap.

Safety & invariants (unsafe code)

This crate uses unsafe in a few carefully-scoped places to avoid repeated UTF‑8 validation and bounds checks when projecting internal byte buffers as &str / &mut str.

The key invariants are:

• PascalString: len <= CAPACITY and data[..len] is always valid UTF‑8.
• StrStack: data is always valid UTF‑8 and ends entries are valid segment boundaries within data.

Policy: every unsafe { ... } block must have a local // SAFETY: comment explaining what invariant makes it sound, and tests must cover UTF‑8 boundary and capacity edge cases.

Compatibility with std::String

SmartString aims to be a pragmatic, mostly drop-in alternative to String:

• It supports common String-like APIs and traits.
• Some mutation APIs currently promote to heap and delegate (trading stack retention for simpler, correct semantics).

For a living checklist of what’s implemented vs planned, see API-PARITY.md.

Note on PascalString: it is fixed-capacity; for String-like “infallible” ergonomics it provides push / push_str which panic on overflow (use try_push / try_push_str for fallible behavior).

License

Licensed under either of

• Apache License, Version 2.0 (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
• MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT) at your option.

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.

Development

Recommended (enable repo hooks once per clone):

git config core.hooksPath .githooks

Quality gates:

cargo +nightly fmt --all -- --check
cargo check --all-targets
cargo test
cargo +stable clippy --all-targets -- -D warnings

See also: CONTRIBUTING.md.