Solver is overzealous with fundamental impls in the standard library

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.

Start with the linked Zulip thread and investigate the compiler's orphan-rule handling for impl From<Cow<'_, str>> for Box<dyn Error>. Compare it with the accepted From<&str> case and determine the intended behavior for fundamental types; completion requires a confirmed fix or documented limitation.