Moving repository to a virtual manifest can cause surprising dependency resolution errors due to resolver fallback #11811
Description
I am not entirely sure this is a bug report or a feature request but I'll describe anyway what happened.
We have a fairly large workspace over at https://github.com/libp2p/rust-libp2p. Previously, the root Cargo.toml was both a crate and a workspace manifest. For various reasons, I wanted to move away from that.
That root crate is edition = 2021 which implies the new dependency resolver (version 2). Thus, we did not specify resolver = 2 explicitly as part of the [workspace] table. We need the new feature resolver because we have some target-specific features to activiate:
What happened now is that by splitting the root manifest into a separate crate, the edition = 2021 disappeared and cargo was now falling back using v1 of the resolver, resulting in a failing build because the features of the snow dependency within the Cargo.toml of libp2p-noise were incorrectly merged.
This was especially surprising because I was running cargo build --package libp2p which does have edition = 2021. I guess this could be considered a bug? I am not sure.
The problem was fixed by explicitly specifying resolver = 2 in the [workspace] table.
Here is the PR: libp2p/rust-libp2p#3536