rust-lang / rust-lang/rfcs

Relax Trait Coherence rules to allow the implementation of a trait on generic types where the type must impl another trait owned by the current crate

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Description

The title is lengthy, but it boils down to this:

If I have a trait MyRead and I want to implement Read for all T: MyRead, I should be able to do so because I own MyRead and the impl of Read would (should?) only come into scope for type T: MyRead if and only if MyRead is also in scope.

There must be a major hole I am missing, but this would seriously reduce code in a lot of libraries, i.e. implementing traits onto owned types individually.

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Research direction

No files, tests, or entry points are named. Start by clarifying the proposed trait-coherence rule and checking its interaction with generic implementations and trait scope; done would require a concrete, reviewed design that addresses the stated coherence concerns.

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Assessment

Tech stack
rust
Domain
compilers
Issue type
Feature
Difficulty
5/5
Estimated time
Over a week
Activity status
Stale
Clarity
Needs clarification
Newbie friendliness
25/100

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