rust-lang/rust-clippy

False positive for inherent_to_string_shadow_display?

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#4396 opened on Aug 16, 2019

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C-bugC-enhancementI-false-positiveL-suggestiongood first issue

Description

I think I found a false positive or at least questionable case of inherent_to_string_shadow_display rule triggering.

The place this rule is (or rather "was", since I disabled it) triggered: https://github.com/huxi/rusty_ulid/blob/72ac82736311981d12c339b78756049f48406179/src/lib.rs#L557

The place where the method in question is used to actually implement Display: https://github.com/huxi/rusty_ulid/blob/72ac82736311981d12c339b78756049f48406179/src/lib.rs#L571

This boils down to a situation where a type has an efficient implementation of fn to_string(&self) -> String that is used to implement fn fmt(&self, f: &mut fmt::Formatter<'_>) -> Result<(), fmt::Error> of fmt::Display.

I think this case (method is used to implement trait) shouldn't trigger the rule.

Contributor guide

Tech stack
rust
Domain
toolingdeveloper experience
Issue type
bug
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.
mostly clear
Prerequisites
Rust programmingclippy internals
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40
Research direction
Examine the clippy lint implementation for `inherent to string shadow display` in the clippy source. The issue points to a case where an inherent `to string` is used to implement `Display`, so the lint should not trigger. Investigate the lint's logic to see if there is an existing exception for this pattern. The fix would likely involve modifying the lint to check whether the inherent method is used to satisfy the `Display` trait. Consult relevant comments and the Rust documentation on trait implementation.