cadd

cadd: painless checked arithmetics and conversions

Features:

• [ops]: checked arithmetics with Result, informative errors, and backtrace.
• Cinto: TryInto alternative for type conversions with better error messages and backtrace. Works for integer types, other primitives, arrays, and string-like types.
• SaturatingInto: infallible number conversion that returns the closest valid value.
• non_zero and to_non_zero(): conversion to NonZero with Result, informative errors, and backtrace.
• .into_type::<T>() as an alternative to into() and try_into() without type inference errors.
• no_std support (alloc is still required).

Introduction

Rust generally offers amazing capability to create safe, correct programs. However, since it is still a systems programming language, there are still a few aspects where the "default" or the easiest way to do something is the cheapest, but not always the correct one. One of the most ubiquitous example is the behavior of arithmetic operations on integer types and casts between them:

• Many arithmetic operators (+, -, *, ...) and some functions (like .pow()) are unchecked in Release mode. This means that they can silently overflow and wrap, producing a value that is not the true output of the operation.
• Some of the operations (like a / b and a.ilog(b)) will panic if their preconditions are unmet.
• as conversion between integer types will silently truncate values as necessary to fit the output type. It's also capable of reinterpreting signed values as unsigned and vice versa, producing a value that is not equal to the input.

Sometimes this behavior is fine. However, there are many cases where performance is not as important as correctness. For most high level applications, calculating the values precisely is much more important, as these values may represent things like amount of money or amount of physical goods. Arguably, checked math and conversions should be the default. A faster, but potentially wrapping or truncating alternative should only be chosen in specific cases, after ensuring that it will not cause a logic bug.

Rust's core library provides very good APIs for checked operations. The only problem with them is that they require much more verbose code. Compare a simple a + b with a properly done checked addition: a.checked_add(b).ok_or(Error::Overflow)?. Any non-trivial code quickly becomes a wall of checked calls and numerous error conversions.

This is where cadd comes in. This crate aims to provide a set of APIs that are (almost) as easy to use as the basic operations, while being a safe, correct default. These functions never wrap, truncate, reinterpret, or panic.

Any errors returned by cadd will contain as much information as possible (to a reasonable limit) and will also capture a backtrace if it's enabled. This makes it easy to integrate it into any error handling approach, be it custom error types or "catch-all" errors like anyhow.

Example

The easiest, but error-prone approach:

let amount = price - discount_rate * price / 100;
let output = amount as u32;

A safe approach that uses std checked functions and returns a non-informative error on overflow:

let amount = discount_rate
    .checked_mul(price)
    .and_then(|v| v.checked_div(100))
    .and_then(|v| price.checked_sub(v))
    .ok_or_else(|| Error::Overflow)?;
let output: u32 = amount.try_into().map_err(|_| Error::Overflow)?;

A safe and concise approach with traits and functions from cadd::ops, returning an informative error:

use cadd::{ops::{csub, Cmul, Cdiv}, convert::IntoType};

let amount = csub(price, discount_rate.cmul(price)?.cdiv(100)?)?;
let output = amount.cinto_type::<u32>()?;

Errors may look like this:

failed to compute 20 - 40: u64 overflow
stack backtrace:
   0: std::backtrace_rs::backtrace::libunwind::trace
...

Recommended clippy lints

In order to eliminate unintended side effects, wraps, truncations, and signed-vs-unsigned reinterpretation, it's recommended to set the following clippy lints to warn level:

• arithmetic_side_effects
• cast_possible_wrap
• cast_precision_loss
• cast_sign_loss

Updating generated code in ops.rs

cargo run -p cadd_generator -- . && cargo fmt && cargo clippy && cargo doc

Updating README.md

cargo install cargo-readme
cargo readme --output README.md

License: MIT OR Apache-2.0