Is Cargo able to infer multiple versions of the same library?

Question

Let's say there is a library fancy_math.

fancy_calculus 1.0 depends on fancy_math 1.8.

However, due to some breaking changs introduced in fancy_math 2.0, fancy_calculus 1.0 can only depend on fancy_math 1.8.

There's also a library fancy_physics 1.0 that is built upon fancy_math 2.0, and will not work with earlier versions.

With Cargo, if I wanted to use both fancy_calculus 1.0 and fancy_physics 1.0, will Cargo be able to infer that I need both fancy_math versions 1.8 and 2.0 and build them separately? Is there some namespace-mangling done?

Answer 1

From the Cargo reference:

If multiple packages have a common dependency with semver-incompatible versions, then Cargo will allow this, but will build two separate copies of the dependency. For example:

# Package A
[dependencies]
rand = "0.7"

# Package B
[dependencies]
rand = "0.6"

The above will result in Package A using the greatest 0.7 release (0.7.3 at the time of this writing) and Package B will use the greatest 0.6 release (0.6.5 for example). This can lead to potential problems, see the Version-incompatibility hazards section for more details.

However, as specified in the "Version-incompatibility hazards" section, you will not be able to use types of fancy_math that fancy_calculus exports where a type of fancy_math that fancy_physics exports is expected, or the opposite.