darling: bump for minimal version compatibility

[mathstuf]

This allows -Z minimal-versions to be used in derive_builder-using
crates.

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Needs a release of darling with TedDriggs/darling#83 included. Just making a guess at the version number it would use.

[TedDriggs]

Can you provide a link to this minimal-versions flag? I'm a bit confused and I'm not finding anything authoritative on Google.

[mathstuf]

rust-lang/cargo#5657

[TedDriggs]

Ah, it took me a minute but I think I finally understand what's going on here.

1. darling_core was using a function from syn@1.0.1, but just declared 1 as its dependency. This happened to work because it was valid to resolve that as the latest syn, which is what normally happened.
2. This minimal-versions flag says to choose the oldest satisfactory version of the dependency, rather than the newest. This caused darling_core to break when the flag was set.
3. The change in syn: bump minimum to 1.0.1 TedDriggs/darling#83 fixed that crate's dependencies, but now we need to republish darling_core with the fixed Cargo.toml, and then we need to republish this crate pointing to that dependency.

Did I get all that right?

Edit: I'll publish the darling crates as soon as CI passes, then we can re-run the Travis job for this PR.

[mathstuf]

Yep, that sounds right.

[TedDriggs]

@mathstuf CI is now passing, but at the moment the PR is still marked "WIP". Is it ready to merge, or were there other changes you planned to make?

[mathstuf]

It was WIP because I was unsure of the version for the suitable darling release. Now unmarked. Thanks!