allow v2 to always compile in MSRV with no features enabled

[alejandro-vaz]

yeah as of now, no default features = nightly only. once the allocator API stabilizes then it will use the proper API when the feature is disabled

I'm not sure but it seems wrong to me, I know no other crate that fails to compile with no default feature which usually means less strict, You might want to get some other feedback about it from the community.

Originally posted by @fereidani in #630 (comment)

instead, what we would do is have a allocator-api feature that does not then require the allocator-api2 dependency and use alloc, whilst when it is disabled it defaults to allocator-api2

it's just the inverse of what we have

and update the ./README.md feature description

[alejandro-vaz]

me and @fereidani agree

@bolshoytoster let me know your stance

[bolshoytoster]

instead, what we would do is have a allocator-api feature that does not then require the allocator-api2 dependency and use alloc, whilst when it is disabled it defaults to allocator-api2

I don't think we can do 'negative dependencies'.

We could do it the hashbrown way, where they have 2 features, nightly and allocator-api2, and when neither is on, it has its own internal Allocator trait.

[alejandro-vaz]

hmmm but we could do that it always has allocator-api2 in the dependency graph and when allocator-api is enabled then the dependency is simply not used

[alejandro-vaz]

the hashbrown way looks way too complicated for me, implementing an Allocator trait just because the libraries dissatisfies us falls out of the scope of this crate and is fixing someone else's stuff