This is an alternative to `-Zminimal-versions` as discussed in #5657.
The problem with `-Zminimal-versions` is it requires the root most
dependencies to verify it and we then percolate that up the stack. This
requires a massive level of cooperation to accomplish and so far there
have been mixed results with it to the point that cargo's unstable
documentation discourages its use.
`-Zdirect-minimal-versions` instead only applies this rule to your
direct dependencies, allowing anyone in the stack to immediately adopt
it, independent of everyone else.
Special notes
- Living up to the name and the existing design, this ignores yanked
crates. This makes sense for `^1.1` version requirements but might
look weird for `^1.1.1` version requirements as it could select
`1.1.2`.
- This will error if an indirect dependency requires a newer version.
Your version requirement will need to capture what you use **and** all
of you dependencies. An alternative design would have tried to merge
the result of minimum versions for direct dependencies and maximum
versions for indirect dependencies. This would have been complex and
led to weird corner cases, making it harder to predict. I also suspect
the value gained would be relatively low as you can't verify that
version requirement in any other way.
- The error could be improved to call out that this was from minimal
versions but I felt getting this out now and starting to collect
feedback was more important.
