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Type covariance absent when used in struct

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评估

难度
4/5
预计耗时
3-5 天
新手友好度
48/100
Issue 类型
缺陷
描述清晰度
基本清楚
活跃度
冷清
技术栈
rust
领域
data

调研方向

Start with the reported Test struct and the covariant and invariant function examples in this issue. Compare variance behavior for Array2 fields with Box, tuples, and Rust arrays; done means identifying and correcting the discrepancy and adding a regression test for the lifetime conversion.

由索引模型根据 Issue 内容生成。

描述

Arrays are covariant, but structs containing them don't seem to be. If I create a struct

struct Test<T> (ndarray::Array2<T>);

then, even though Array2 is covariant over T, my struct isn't:

// works
pub fn covariant<'long: 'short, 'short>(input: Box<ndarray::Array2<&'long u8>>) -> Box<ndarray::Array2<&'short u8>> {
    input
}

// compiler error
pub fn invariant<'long: 'short, 'short>(input: Box<Test<&'long u8>>) -> Box<Test<&'short u8>> {
    input
}

This doesn't make sense to me. From the rust reference, I would expect Test's variance to depend only on the types of its field, but it doesn't. Moreover, only structs that internally contain Array2 are affected: structs that take the array as a parameter (like the Box above) are fine, as are tuples and rust arrays. This is also unaffected by the complexity of the type inside the array, as long as it references a lifetime.

主要语言
Rust
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4.3k
派生
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30 天内没有已合并 PR

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  4. 提交 Pull Request，并在描述里引用这个 Issue 编号。

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