Use feature conditions

Conditional dependencies, sources, and directories in besa.toml use structural feature conditions. The three operators may be nested.

all

when = { all = ["toolchain-cuda", "mpi"] }

Every operand must match.

any

when = { any = ["toolchain-cuda", "toolchain-hip"] }

At least one operand must match.

not

when = { all = ["toolchain-cpp", { not = "toolchain-cuda" }] }

not may contain one feature name or another nested condition.

For feature logic that is not naturally expressible with these operators, register a portable Python feature constraint in besa.toml; see Add configuration constraints.

The CMake module still exposes its lower-level selector API for backend-specific CMake code, but normal project topology should be expressed in besa.toml so another backend can consume the same project declaration.