Lifetime may not live enough regression

[iamsauravsharma]

Code

I tried this code:

struct Test {
    field: Vec<&'static str>
}

impl Test {
    fn field<'field>(&self) -> impl Iterator<Item = &&'field str> {
        self.field.iter()
    }
}

I expected to see this happen: no compilation error
Instead, this happened:

error: lifetime may not live long enough
  --> src/lib.rs:21:9
   |
20 |     fn field<'field>(&self) -> impl Iterator<Item = &&'field str> {
   |              ------  - let's call the lifetime of this reference `'1`
   |              |
   |              lifetime `'field` defined here
21 |         self.field.iter()
   |         ^^^^^^^^^^^^^^^^^ method was supposed to return data with lifetime `'1` but it is returning data with lifetime `'field`

error: could not compile `playground` (lib) due to 1 previous error

It most recently works on nightly-2024-03-06 but returns error on nightly-2024-03-07

[lqd]

I believe this should be due to the soundness fix in #121679.

[oli-obk]

Yes, that PR was a follow up to #115008 (comment)

Your code example can be used unsoundly, even if a smarter analysis could infer the right lifetimes.

We should have cratered the change and opened a fix PR against your repo, apologies about the unannounced breakage. I'll start a crater run now

[oli-obk]

@craterbot run name=opaque-soundness-fix start=80399064afa4a2cd153f30d02c25f7ea0383ed65 end=09bc67b9158392361780e779d32997f14cc75c39 mode=check

[craterbot]

🚨 Error: failed to parse the command

🆘 If you have any trouble with Crater please ping @rust-lang/infra!
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[oli-obk]

@craterbot run name=opaque-soundness-fix start=80399064afa4a2cd153f30d02c25f7ea0383ed65 end=09bc67b9158392361780e779d32997f14cc75c39 mode=check-only

[craterbot]

👌 Experiment opaque-soundness-fix created and queued.
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more