rust-lang/rust-clippy

New lint: Use `Into`/`TryInto` in bounds as opposed to `From`/`TryFrom`

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#4894 opened on Dec 10, 2019

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 (29 comments) (1 reaction) (0 assignees)Rust (10,406 stars) (1,391 forks)batch import
A-lintL-stylegood first issue

Description

When writing generic bounds such as:

fn foo<T>(a: T) where u32: From<T>;

Into should be preferred, like this:

fn foo<T>(a: T) where T: Into<u32>;

Why the former is bad: Into is a superset of From. In some cases coherence rules prevent implementing From but do allow implementing Into. As a result Into is more generic bound than From.

Category: Style, I guess?

Contributor guide

Tech stack
rust
Domain
toolingdeveloper experience
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.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Basic RustUnderstanding of trait boundsClippy development setup
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40
Research direction
Study the Clippy developer guide for adding a new lint. Look at existing lints that suggest trait usage, such as the 'trait usage' lints in clippy lints/src/. The issue comments may contain further rationale; review them for consensus on exact implementation. The lint should detect generic bounds like `u32: From<T>` and suggest replacing with `T: Into<u32>`. Explore the clippy lints crate to understand naming conventions and testing patterns.