allow arbitrary inherent impls in the standard library · Issue #487 · rust-lang/compiler-team (original) (raw)

Proposal

We currently use a lot of different lang items to support inherent impls in the standard library, e.g.
https://github.com/rust-lang/rust/blob/6e5a6ffb14fc47051b0a23410c681ad6e4af045f/library/alloc/src/slice.rs#L659-L661

I intend to change both probe and coherence to deal with arbitrary inherent impls from all crates in the dependency crate with some special crate attribute, e.g. #[rustc_arbitrary_inherent_impls]. This change removes a lot of lang items and greatly improves the experience of library developers looking to move types into core. Alternatively, add 3 lang items for the crates core, alloc and std to only search these.

Having inherent impls for foreign types is sound and to my knowledge only forbidden because:

foreign crates adding inherent items for your type causes the addition of something with the same name to be a breaking change
if ever crate could add inherent impls for foreign types, we would have to always check all dependencies when searching for them, probably impacting perf
probably also just easier to reason about for users, so general lang design reasons ✨

All of these things do not get worse if we stop requiring lang items in the standard library for this.

Mentors or Reviewers

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Process

The main points of the Major Change Process are as follows:

File an issue describing the proposal.
A compiler team member or contributor who is knowledgeable in the area can second by writing @rustbot second.
- Finding a "second" suffices for internal changes. If however, you are proposing a new public-facing feature, such as a -C flag, then full team check-off is required.
- Compiler team members can initiate a check-off via @rfcbot fcp merge on either the MCP or the PR.
Once an MCP is seconded, the Final Comment Period begins. If no objections are raised after 10 days, the MCP is considered approved.

Comments

This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.