Q: Trunk-based development with feature flags and breaking changes in library

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Trunk-based development pushes all commits to a single branch (e.g. master) and uses feature flags to disable features that are not yet ready for release.

Let's say my current release uses a library (nuget package) 1.1 and my new feature requires the very same library in version 2.0, but this version of the library has breaking changes.

How would I handle that?
I can't use the same library twice within my project.
I can't really change all code to use library version 2.0 because those changes can't be flagged and therefore not tested later in test-environments and therefore not commited on the normal daily basis.