{
  "$type": "site.standard.document",
  "bskyPostRef": {
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  "path": "/t/the-hashtable-problem-a-litmus-test-for-external-impl-proposals/24396#post_4",
  "publishedAt": "2026-06-13T09:00:58.000Z",
  "site": "https://internals.rust-lang.org",
  "tags": [
    "rust-rfcs/text/3634-scoped-impl-trait-for-type.md at 251e4c443757ebe7c079a8a3627e32f9b79d4d0e · Tamschi/rust-rfcs · GitHub",
    "version-agnostic link, in case I update it",
    "https://github.com/rust-lang/rfcs/pull/3634#issuecomment-2153325753",
    "Scoped `impl Trait for Type` by Tamschi · Pull Request #3634 · rust-lang/rfcs · GitHub"
  ],
  "textContent": "There's another wrinkle to this which is type-erasing/type-keyed hashtables, which a \"plain\" solution over static typing may not catch. You can best read my attempt (not independently reviewed) at solving both here: rust-rfcs/text/3634-scoped-impl-trait-for-type.md at 251e4c443757ebe7c079a8a3627e32f9b79d4d0e · Tamschi/rust-rfcs · GitHub (version-agnostic link, in case I update it, in case I update it).\n\nThere's _another_ common problem with orphan-rule relaxations, which is external-global implementation mixing in multi-trait bounds breaking cross-impl proofs valid for existing code: https://github.com/rust-lang/rfcs/pull/3634#issuecomment-2153325753\nThis can be worked around without changes, but in my opinion that would break ergonomics badly for what in practice is a very rare problem. I think it's feasible to change what `impl` blocks can assume across an edition boundary though, along with an attribute to apply the previous mixing constraint explicitly. I haven't (_yet_ , hopefully) found the time to revise for that, though, so **my RFC is unsound in that regard**. You can find a sketch of a possible solution in the GitHub comments, starting here: Scoped `impl Trait for Type` by Tamschi · Pull Request #3634 · rust-lang/rfcs · GitHub",
  "title": "The Hashtable Problem: A Litmus Test for External Impl Proposals"
}