Function lower_body_to_mir 

pub fn lower_body_to_mir<'db>(
    db: &'db (dyn HirDatabase + 'static),
    owner: DefWithBodyId,
    body: &Body,
    infer: &InferenceResult,
    root_expr: Idx<Expr>,
) -> Result<MirBody, MirLowerError>

Extracts params from body.params/body.self_param and the callable signature, then delegates to lower_to_mir_with_store.