Mention the rust version the spec targets

[Veykril]

Will require changes in ferrocene when this gets mirrored I think?

[pietroalbini]

In ferrocene/ferrocene the extension part is already fully implemented and hooked up. Can we just add the substitution in the monorepo?

[Veykril]

The build here will fail though if we don't set the substitution locally (and we can't hook up with substitution tooling from ferrocene_qualification given it defines a bunch of extra required args)

[pietroalbini]

bors merge

[bors-ferrocene]

Build succeeded:

• CI