strict-typing

A Rust procedural macro that enforces strict typing on struct fields, enum variants, function signatures, trait definitions, and impl blocks. It prevents the use of bare primitive types, encouraging newtype wrappers for clarity and safety.

Rust is all about correctness in my humble opinion. And I want it to stay that way in my personal experience. I think this is how Rust should have been made. This should be a compiler built-in.

Motivation

Bare primitives like u32 or bool compile fine but carry no domain meaning. A u32 could be a user ID, a pixel count, or a port number — the type system won't stop you from mixing them up. Newtype wrappers (struct UserId(u32)) make intent explicit and catch misuse at compile time.

In my projects, I always create the transparent newtypes that precisely define what they carry. If I ever need to, I also create only the correct versions of conversions between such and other types.

Originally wrote for my own projects, I decided to share it with others. Enjoy.

#[strict_types] turns this from a convention into a compiler-enforced rule.

P.S. I decided to name the project strict-typing initially with the intention to extend its functionality.

Usage

Add the dependency:

[dependencies]
strict-typing = "0.1"

Basic — reject primitives in struct fields

use strict_typing::strict_types;

struct Percentage(u8);
struct Label(String);

#[strict_types]
struct Config {
    ratio: Percentage, // OK — newtype wrapper
    name: Label,       // OK
    // count: u32,     // compile error: disallowed type `u32`
}

Enums

use strict_typing::strict_types;

struct Velocity(f64);

#[strict_types]
enum Event {
    Click { x: Velocity, y: Velocity },
    Disconnect,
    // Resize(u32, u32), // compile error
}

Functions, traits, and impl blocks

use strict_typing::strict_types;

struct Distance(f64);
struct Speed(f64);
struct Duration(f64);

#[strict_types]
fn compute_speed(d: Distance, t: Duration) -> Speed {
    Speed(d.0 / t.0)
}

Extending the disallow list

By default, all numeric primitives plus bool and char are disallowed. Add more with disallow(...):

use strict_typing::strict_types;

/// # Strictness
///
/// - [String] disallowed because domain names should use a newtype.
#[strict_types(disallow(String))]
struct Name {
    // value: String, // compile error
}

Allowing specific primitives

Remove types from the default disallow list with allow(...):

use strict_typing::strict_types;

/// # Strictness
///
/// - [bool] allowed here because a simple flag is sufficient.
#[strict_types(allow(bool))]
struct Flags {
    enabled: bool, // OK
}

Documentation requirement

When using allow(...) or disallow(...), you must document each override in a /// # Strictness doc section. This forces authors to justify every exception, keeping the decision visible in code review.

Default disallowed types

u8 u16 u32 u64 u128 usize i8 i16 i32 i64 i128 isize f32 f64 bool char

License

MIT