Rollup merge of #136118 - oli-obk:push-qsslxsopnrmr, r=Zalathar

Change `collect_and_partition_mono_items` tuple return type to a struct

https://github.com/rust-lang/rust/pull/133429 will add a new field to this tuple, so it seems prudent to turn it into a struct first to avoid confusion about what the tuple elements mean.