Function emit_join 

pub fn emit_join(
    ctx: &mut EmitContext,
    pipeline: &mut CodegenPipeline,
    builder: &mut FunctionBuilder<'_>,
    vmctx: Value,
    gc_sig: SigRef,
    tree: &CoreExpr,
    label: &JoinId,
    params: &[VarId],
    rhs_idx: usize,
    body_idx: usize,
) -> Result<SsaVal, EmitError>

Emits a Join expression. Join { label, params, rhs, body } creates a join point (a parameterized block) that can be jumped to from within the body.