Allow dependency resolution with multiple incompatible major versions

[tachyonics]

Motivation

The immediate motivation for this change is to enable greater flexibility for Swift Macros. Currently a library that uses Swift Macros depends on a Swift-version-specific major version of swift-syntax. This results in library combinations that are impossible to use together even though they would be compatible with each other at a public API level (that is they are not exposing types from any conflicting major versions, particularly in this case as swift-syntax is effectively a build dependency).

Although Swift Macros are the immediate motivation for such a feature, as the Swift ecosystem evolves and the number of major versions for libraries increases, the likelihood of library and application developers running into artificial limitations caused by the single-major-version-per-dependency-graph rule also increases.

There is prior-art in this space. The Rust language cargo’s resolver will attempt to unify the version of dependencies where possible but allow for conflicting private dependencies.

https://doc.rust-lang.org/cargo/reference/resolver.html

Introducing the concept of public and private dependencies is also a mechanism that could be used to introduce this feature is a understandable way

https://rust-lang.github.io/rfcs/3516-public-private-dependencies.html#dependency-visibility-and-the-resolver-1

"Cargo will specifically reject graphs that contain two different versions of the same crate being publicly depended upon and reachable from each other."

Proposed solution

No firm proposed solution but I (as part of the Swift Server Work Group) will raise it in the forum to get feedback on possible approaches.

Alternatives considered

No response

Additional information

No response