Cargo dependency conflict with multiple git submodules - rust

Question

We have a Rust Cargo project under Git that we have divided into multiple submodules each under their own git repo to allow controlled access by different teams, some external. Some teams will just work on one sub-module. Here is a simplified structure:
Project
---Module 1
---Sub-Module 1
---Sub-Module 2
---Sub-Module 3
Module 1 has a dependency on all 3 sub-modules; and Sub-Module 1 and Sub-Module 2 also have dependencies on Sub-Module 3.
The advantage of using sub-modules is that changes can be made to Module 1 and the sub-modules together and compiled togeher, as opposed to just keeping the sub-modules as separate repos and developing them separately.
Since Sub-Module 1 and Sub-Module 2 are independent repos, they have no direct knowledge of Sub-Module 3 and therefore must include it via the git repo.
Module 1 is including Sub-Module 3 as a direct path. This causes the conflict in Cargo as Module 1 has two versions of Sub-Module 3 - one direct dependency and one through Sub-Module 1 / Sub-Module 2.
Solution A would be to include Sub-Module 3 into Module 1 via the git repo (instead of via a direct path), but this defeats the object of having it as a submodule as any coding changes to Sub-Module 3 would have to be coded, committed and pushed to the repo before Module 1 can see them.
Solution B would be to add Sub-Module 3 in itself as a sub-module to Sub-Module 1 and Sub-Module 2 and this would then negate the need to define the depenency via the git repo. But then Sub-Module 3 would appear twice in the Project and this might get confusing. Also, we have not tested this but suspect that Cargo would still have the same conflict as it will still have two versions of Sub-Module 3.
This is the type of error being produced by Cargo:
= note: expected struct sub_module_3::ExampleStruct
found struct ExampleStruct
= note: perhaps two different versions of crate `sub_module_3` are being used?
Any advice on how to solve this much appreciated.
Thanks

Answer 1

To normalize the dependency graph where some crates are accessing each other by version or repository and others are accessing the same crates by path, you need to override the nested dependencies with a [patch] section in your Cargo.toml:
[patch.'https://github.com/sub-module-3'] # sub-module-1 and sub-module-2 use this dependency
sub-module-3 = { path '../sub-module-3' } # they should instead use the local path
This can either be done just for the module-1 crate or at the workspace root if you want to have the patches applied for developing the sub-modules.