rust - 在Rust中，为什么 `std::any::TypeId::of`不是 `const fn`？

Question

一个明显的问题:
为什么std::any::TypeId::of::<T>不是const fn？
由于std::any::type_name::<T>已经是const fn，我找不到任何原因。

Answer 1

如果查看the code(单击its documentation右侧的[src]按钮)，您会看到of标记为const，但是上面有rustc_const_unstable属性:

#[rustc_const_unstable(feature = "const_type_id", issue = "77125")]

这告诉我们，至少稳定地使 TypeId::of成为 const的跟踪问题是 #77125。在该期中，有一个指向 PR that reverted its initial stabilization的链接，进一步链接到一个 comment by eddyb:

TypeId is one a few types (mem::Discriminant is the only other one I can think of) that's a thin wrapper for a private integer, at least right now. This is ripe for abuse no matter what we say/document it as.

I'm not saying we should cater to this, but rather avoid stabilizing even worse abuse-enabling tools like const fn TypeId::of.Like I said, I am already aware of usecases where compile-time transmute(TypeId::of::<T>()) is an uniquely useful tool and someone might not think twice before releasing a crate which uses it somewhere internally and becomes widely-used.

因此，这里的主要问题似乎是不道德的代码滥用 TypeId::of来在编译时执行操作的可能性，这种方式使得将来更改 TypeId的实现很难在没有重大破损的情况下进行。