Limit obligation processing depth#153338
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Related to rust-lang#152857 It's possible for `ObligationForest::process_cycles` to miss more complex cycles and this can lead to an endless fixpoint loop since it never reaches a settled state. It's desirable to adjust `find_cycles_from_node` to catch the cycle in rust-lang#152857. At the same time the nature of the code risks an endless loop by some other unexpected or unhandled cycle, so this PR aims to provide a basic limit to the depth of the loop. The value of the constant was derived to be ~20x the highest value seen when compiling rustc itself. The top values are: ``` ┌───────┐ │ 10852 │ │ 10852 │ │ 10433 │ │ 10433 │ │ 10433 │ │ 10433 │ │ 8498 │ │ 8498 │ │ 6361 │ │ 6361 │ └───────┘ ```
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r? @fee1-dead rustbot has assigned @fee1-dead. Use Why was this reviewer chosen?The reviewer was selected based on:
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Voultapher
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Mar 3, 2026
| // It's possible for nodes to recursively add obligations in an endless cycle, | ||
| // for example https://github.com/rust-lang/rust/issues/152857. | ||
| self.error_at(index); | ||
| return outcome; |
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Currently this creates an ICE, which strictly speaking is still preferable to an endless loop but I'd prefer to issue a better error something similar to the other overflow errors, but not sure what's the best way to do that here.
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@rustbot reroll |
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Does it add new obligations at each step in the cycle? If so, we should make sure that we increase the |
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r? lcnr |
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Related to #152857
It's possible for
ObligationForest::process_cyclesto miss more complex cycles and this can lead to an endless fixpoint loop since it never reaches a settled state. It's desirable to adjustfind_cycles_from_nodeto catch the cycle in#152857. At the same time the nature of the code risks an endless loop by some other unexpected or unhandled cycle, so this PR aims to provide a basic limit to the depth of the loop.
The value of the constant was derived to be ~20x the highest value seen when compiling rustc itself. The top 10 values out of ~70 million invocations are: