# Library-level guarantees

Most libraries rely on internal invariants, e.g. about their data, resource ownership, or protocol states. In Rust, broken invariants cannot produce segfaults, but they can still lead to wrong answers.

### Provide library-level guarantees whenever practical. [FIXME: needs RFC]

Library-level invariants should be turned into guarantees whenever practical. They should hold no matter what the client does, modulo explicit opt-outs. Depending on the kind of invariant, this can be achieved through a combination of static and dynamic enforcement, as described below.

#### Static enforcement:

Guaranteeing invariants almost always requires hiding, i.e. preventing the client from directly accessing or modifying internal data.

For example, the representation of the `str` type is hidden, which means that any value of type `str` must have been produced through an API under the control of the `str` module, and these APIs in turn ensure valid utf-8 encoding.

Rust's type system makes it possible to provide guarantees even while revealing more of the representation than usual. For example, the `as_bytes()` method on `&str` gives a read-only view into the underlying buffer, which cannot be used to violate the utf-8 property.

#### Dynamic enforcement:

Malformed inputs from the client are hazards to library-level guarantees, so library APIs should validate their input.

For example, `std::str::from_utf8_owned` attempts to convert a `u8` slice into an owned string, but dynamically checks that the slice is valid utf-8 and returns `Err` if not.

See the discussion on input validation for more detail.

### Prefer static enforcement of guarantees. [FIXME: needs RFC]

Static enforcement provides two strong benefits over dynamic enforcement:

- Bugs are caught at compile time.
- There is no runtime cost.

Sometimes purely static enforcement is impossible or impractical. In these cases, a library should check as much as possible statically, but defer to dynamic checks where needed.

For example, the `std::string` module exports a `String` type with the guarantee that all instances are valid utf-8:

- Any consumer of a `String` is statically guaranteed utf-8 contents. For example, the `append` method can push a `&str` onto the end of a `String` without checking anything dynamically, since the existing `String` and `&str` are statically guaranteed to be in utf-8.

- Some producers of a `String` must perform dynamic checks. For example, the `from_utf8` function attempts to convert a `Vec<u8>` into a `String`, but dynamically checks that the contents are utf-8.

### Provide opt-outs with caution; make them explicit. [FIXME: needs RFC]

Providing library-level guarantees sometimes entails inconvenience (for static checks) or overhead (for dynamic checks). So it is sometimes desirable to allow clients to sidestep this checking, while promising to use the API in a way that still provides the guarantee. Such escape hatches should only be introduced when there is a demonstrated need for them.

It should be trivial for clients to audit their use of the library for escape hatches.

See the discussion on input validation for conventions on marking opt-out functions.