Function sandbox_posture 

pub fn sandbox_posture(
    sandbox: &SandboxConfig,
    runtime_kind: RuntimeKind,
    workspace_dir: Option<&Path>,
    extra_roots: &SandboxExtraRoots,
) -> SandboxPosture

Inspect sandbox backend selection without returning a usable sandbox.

This does not enforce anything, but on Linux it is not a minimal or side-effect-free probe: landlock_available reaches LandlockSandbox::with_roots, which builds the full ruleset execution will use — opening every configured path and emitting the same DEBUG/WARN diagnostics for absent, unopenable, or unenforceable roots. That is deliberate. Deciding availability from a cheaper probe is what let posture report a backend as active while every subsequent spawn failed; validating through the real construction keeps the two answers in step.

The ruleset is dropped without restrict_self, so the calling process is never confined.