More details in this zulip thread detailing the requirements to get ToOwned into core: #t-libs > core::borrow::Cow (cowcore)
Essentially, the impl in question is impl From<Cow<'_, str>> for Box<dyn Error>, where Cow and Error are both defined in core, but alloc defines Box, which is marked fundamental.
It allows this for From<&str> interestingly enough, but not Cow, presumably because it's not a primitive type.
This definitely is totally fine under the orphan rule because core is a dependency of alloc, and thus there would never be any scenario where a impl could be defined in core, even though Box is fundamental.
My guess is that this is a very low-priority bug to fix since this just blocks a change that needs larger stuff to be done first, but this feels like it could potentially show up in other ways if we're not careful, and is worth documenting.
More details in this zulip thread detailing the requirements to get
ToOwnedintocore: #t-libs > core::borrow::Cow (cowcore)Essentially, the impl in question is
impl From<Cow<'_, str>> for Box<dyn Error>, whereCowandErrorare both defined incore, butallocdefinesBox, which is marked fundamental.It allows this for
From<&str>interestingly enough, but notCow, presumably because it's not a primitive type.This definitely is totally fine under the orphan rule because
coreis a dependency ofalloc, and thus there would never be any scenario where a impl could be defined incore, even thoughBoxis fundamental.My guess is that this is a very low-priority bug to fix since this just blocks a change that needs larger stuff to be done first, but this feels like it could potentially show up in other ways if we're not careful, and is worth documenting.