resolver in Cargo.toml changed MSRV

[TheBlueMatt]

More an FYI than a bug, I think, but #2602 said the following, which doesn't appear true for us:

But that's fine and the published serde crate won't be affected.

For our CI, building serde on rustc 1.48 and 1.49 now fails with:

   Downloaded serde v1.0.187
error: failed to parse manifest at `/home/runner/.cargo/registry/src/github.com-1ecc6299db9ec823/serde-1.0.187/Cargo.toml`

Caused by:
  feature `resolver` is required

  this Cargo does not support nightly features, but if you
  switch to nightly channel you can add
  `cargo-features = ["resolver"]` to enable this feature

[dtolnay]

Thanks, I didn't realize Cargo would propagate that field from the workspace manifest into serde's manifest during publish. I don't understand why it does that — the choice of resolver is controlled by the top-level crate/workspace being built; serde as a dependency has no say in this.

I'll revert that change.

[epage]

We propagate the resolver version so that cargo install of a bin carries forward the expected behavior from the developer. See rust-lang/cargo#11047

[dtolnay]

I'd prefer if it did not do that for packages not containing any bin crates.

[epage]

At this time, the code responsible does not have that knowledge. However, I've been wanting to change our publish code to to explicitly list all auto targets at which point it would be possible to determine that. There still might be enough nuance to be annoying because examples are considered installable.

[dtolnay]

As an easier fix that still eliminates resolver-related breakage for libraries, could we make sure Cargo disregards any value of resolver in the manifest of library dependencies?

I tried editing $CARGO_HOME/registry/src/index.crates.io-6f17d22bba15001f/serde-1.0.188/Cargo.toml to add resolver = "100", and that currently makes crates depending on serde as a library fail with this error, on current nightly:

error: failed to download `serde v1.0.188`

Caused by:
  unable to get packages from source

Caused by:
  failed to download replaced source registry `crates-io`

Caused by:
  failed to parse manifest at `$CARGO_HOME/registry/src/index.crates.io-6f17d22bba15001f/serde-1.0.188/Cargo.toml`

Caused by:
  `resolver` setting `100` is not valid, valid options are "1" or "2"