Rust variance

The question: does a value that holds 'b fit where the compiler asks for 'a, given 'b: 'a? The type constructor around the lifetime decides, not the lifetime. Variance belongs to type constructors only: a trait bound matches by equality, so nothing coerces through it.

Every quoted diagnostic comes from rustc 1.98.1, edition 2024, aarch64-apple-darwin.

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