E0271 rust type_error ai_generated true

error[E0271]: type mismatch resolving `<X as Y>::Z` -- expected bound lifetime parameter `'a`, found concrete lifetime

ID: rust/e0271-expected-bound-lifetime-parameter

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78%Fix Rate
85%Confidence
1Evidence
2023-08-15First Seen

Version Compatibility

VersionStatusIntroducedDeprecatedNotes
rustc 1.72.0 active — — —
rustc 1.75.0 active — — —
rustc 1.80.0 active — — —

Root Cause

A trait bound or associated type expects a generic lifetime parameter, but the implementation provides a concrete lifetime, causing a mismatch in higher-ranked trait bounds.

generic

中文

trait 约束或关联类型期望一个泛型生命周期参数，但实现提供了具体生命周期，导致高阶 trait 约束不匹配。

Official Documentation

https://doc.rust-lang.org/error_codes/E0271.html

Workarounds

  1. 85% success Explicitly annotate the higher-ranked lifetime using `for<'a>` syntax in the trait bound, e.g., `T: for<'a> MyTrait<'a>`. 
    Explicitly annotate the higher-ranked lifetime using `for<'a>` syntax in the trait bound, e.g., `T: for<'a> MyTrait<'a>`.
  2. 80% success Refactor the associated type to use a generic lifetime parameter instead of a concrete one, e.g., change `type Output = &i32` to `type Output<'a> = &'a i32`. 
    Refactor the associated type to use a generic lifetime parameter instead of a concrete one, e.g., change `type Output = &i32` to `type Output<'a> = &'a i32`.
  3. 75% success Use a boxed trait object with dynamic dispatch to avoid lifetime parameter mismatches. 
    Use a boxed trait object with dynamic dispatch to avoid lifetime parameter mismatches.

中文步骤

  1. Explicitly annotate the higher-ranked lifetime using `for<'a>` syntax in the trait bound, e.g., `T: for<'a> MyTrait<'a>`.
  2. Refactor the associated type to use a generic lifetime parameter instead of a concrete one, e.g., change `type Output = &i32` to `type Output<'a> = &'a i32`.
  3. Use a boxed trait object with dynamic dispatch to avoid lifetime parameter mismatches.

Dead Ends

Common approaches that don't work:

  1. 65% fail

    Forces all references to be 'static, which is often too strict and may cause new borrow-checker errors.

  2. 55% fail

    Lifetime elision often defaults to '_, which may still not match the expected bound, and the error may shift to a different location.