{
  "$type": "site.standard.document",
  "bskyPostRef": {
    "cid": "bafyreif4pxrysjzcpf6o7xi2ncooxl6mf2uxjlpocxnux3zd777wooe5wm",
    "uri": "at://did:plc:ivbknywyskln22er3nkssdhl/app.bsky.feed.post/3mlg5ewbtz2v2"
  },
  "path": "/t/type-inference-breaks-when-a-loop-is-added-between-inference-points/24232#post_7",
  "publishedAt": "2026-05-09T10:09:47.000Z",
  "site": "https://internals.rust-lang.org",
  "textContent": "Here is another minimal example similar to the original:\n\n\n    struct S {\n        elem: i32\n    }\n\n    fn foo(a: Vec<S>) -> Vec<S> {\n        let result = a.into_iter().collect();\n        for s in &result {\n            println!(\"{}\", s.elem);\n        }\n        result\n    }\n\n\n\n    error[E0282]: type annotations needed\n     --> src/lib.rs:8:24\n      |\n    8 |         println!(\"{}\", s.elem);\n      |                        ^ cannot infer type\n\n\nThe problem is that field accesses and method calls have to be resolved based on information known _before_ the call. This is just a limitation of the current type checker.\n\nAt `println`, the compiler knows that `result` is `FromIterator<S>`, and that `&result` is `IntoIterator<Item=X>` for some unknown type `X`. It only knows that `s` is of type `X`, and can't figure out what `s.elem` refers to because `X` is unknown.\n\nOnly later, in the last line (where we return `result`) does the compiler realize that `result` must be `Vec<S>` and thus that `X = &S`.\n\nI think this _could_ be fixed by having all the field accesses and method calls resolved later, when the necessary information becomes available after type checking the rest of the function.",
  "title": "Type inference breaks when a loop is added between inference points"
}