feat!: add support for conditional dependencies

[baszalmstra]

This PR adds support for conditional dependencies. A conditional dependency is a dependency on a package that is only active if another requirement is satisfied. E.g. A requires B if C. Conditions can be combined with and and or. This allows depending on packages that are only active in certain scenarios, e.g.

• Depending on the platform (using conda virtual packages). E.g. linux-specific; if __linux
• Depending on the presence of other packages (e.g. numpy <=1.22; if python<3.12, numpy >1.22; if python >=3.13).

Care is taken that conditional dependencies only incur an overhead when used. So this PR should not affect the performance of problems without conditional dependencies. Even when they are used the overhead should be minimal.

I've added a lot of documentation that describes how conditional dependencies are implemented.

Todo

• Benchmark
• Cpp bindings

[baszalmstra]

@prsabahrami @jjerphan If you have time I would appreciate a few more eyes on this PR! 👍

[Copilot]

Pull Request Overview

This PR implements support for conditional dependencies by introducing new types and methods to capture conditions and their conversion into SAT clauses. Key changes include:

• Adding new data structures (e.g. ConditionalSpec, ConditionalRequirement, Condition, LogicalOperator) to encode conditions.
• Extending the solver internals to handle conditional requirements via disjunctions and auxiliary variables.
• Updating C++ bindings and tests to account for the new conditional dependency support.

Reviewed Changes

Copilot reviewed 59 out of 59 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/solver/bundle_box/conditional_spec.rs	New test support for conditional dependencies.
tests/snapshots/solver__root_constraints.snap	Snapshot changes reflecting updated solver behavior.
src/solver/variable_map.rs	Added variable variant and allocation function for at-least-one semantics.
src/solver/mod.rs	Updated API and internal state to carry ConditionalRequirement and disjunctions.
src/solver/encoding.rs	Major changes to encode conditional requirements including disjunction conversion and clause registration.
src/solver/conditions.rs	New module implementing condition conversion to DNF and related types.
src/solver/clause.rs	Modified clause representation to support optional condition disjunctions.
Other files (binary_encoding.rs, conditional_requirement.rs, C++ bindings, Cargo.toml, etc.)	Adapted to support conditional dependencies and updated language features.

Comments suppressed due to low confidence (2)

src/solver/encoding.rs:292

• [nitpick] The variable name 'conditions' is used to store disjunction IDs produced from the DNF conversion of a requirement condition. To improve clarity, consider renaming it to 'disjunction_ids' or similar.

let mut conditions = Vec::with_capacity(condition.as_ref().map_or(0, |(_, dnf)| dnf.len()));

cpp/src/lib.rs:646

• [nitpick] Using .cloned() instead of .copied() is appropriate here because ConditionalRequirement may not implement Copy; verify that this change aligns with the intended type traits for ConditionalRequirement.

.cloned()

[tdejager]

Phew! That was a read, but what excellent excellent work! Can't wait for this to land :) Just a few nits.

[wolfv]

I added some benchmarking here: conda/rattler#1541

[baszalmstra]

can you also post the benchmark results here?

[baszalmstra]

is this pub crate needed?

[wolfv]

I just kinda copied it from other fields, but probabbly it's not needed. Let me remove it for now.

[wolfv]

Made it non-pub.

[wolfv]

Benchmark results: