Comment by Diggsey

Within a crate graph, for any given major version of a crate (eg. D v1) only a single minor version can exist. So if B depends on D v1.x, and C depends on D v2.x, then two versions of D will exist. If B depends on Dv1.2 and C depends on Dv1.3, then only Dv1.3 will exist.

I'm over-simplifying a few things here:

1. Semver has special treatment of 0.x versions. For these crates the minor version depends like the major version and the patch version behaves like the minor version. So technically you could have v0.1 and v0.2 of a crate in the same crate graph.

2. I'm assuming all dependencies are specified "the default way", ie. as just a number. When a dependency looks like "1.3", cargo actually treats this as "^1.3", ie. the version must be at least 1.3, but can be any semver compatible version (eg. 1.4). When you specify an exact dependency like "=1.3" instead, the rules above still apply (you still can't have 1.3 and 1.4 in the same crate graph) but cargo will error if no version can be found that satisfies all constraints, instead of just picking a version that's compatible with all dependents.