rust-lang/rust-clippy

Have a lint against usize-to-u64 casts (or, against *all* integer casts)

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#9,231 opened on Jul 23, 2022

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A-lintgood first issue

Description

What it does

I would like clippy to lint against all integer casts. So I have set:

#![warn(
    clippy::cast_possible_wrap, // unsigned -> signed
    clippy::cast_sign_loss, // signed -> unsigned
    clippy::cast_lossless,
    clippy::cast_possible_truncation,
)]

However, I just by accident noticed that this does not lint against usize-to-u64 casts. I guess cast_possible_truncation says "this cannot truncate because we don't have more than 64bit pointer size", and "cast_lossless" says "ah this might be lossy on platforms with pointers larger than 64bit", and then neither of them does anything.

I would be happy to either have one of these lints also trigger on usize-to-u64 casts, or to have a new lint against all integer casts.

Lint Name

cast_integer

Category

pedantic

Advantage

Integer casts are subtle and should be done via From/TryFrom, never via as, so I want to rule out all of them in my codebase.

Drawbacks

No response

Example

pub fn foo(x: usize) -> u64 { x as u64 }

Could be written as:

pub fn foo(x: usize) -> u64 { u64::try_from(x).unwrap() }

Contributor guide

Tech stack
rust
Domain
tooling
Issue type
feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-2 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
needs maintainer response
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
knowledge of Rustunderstanding of Clippy internals
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
50
Research direction
To implement this feature, start by examining existing casting lints in clippy lints/src/casts/. The new lint `cast integer` would need to detect all integer to integer casts using `as`. Look at how `cast possible truncation` and `cast lossless` are implemented to understand the pattern. The issue requests a pedantic lint that catches all integer casts, overriding the existing per case approach. Check comments for any design decisions. The lint should be registered in clippy lints/src/lib.rs and added to the pedantic group. Implementation involves walking the AST for ExprKind::Cast with integer types and emitting a warning. Ensure to handle platform dependent pointer sizes correctly.