(For those of you who don't pay super close attention to Rust development, that means libraries like Serde and Diesel will be as convenient to use on stable as they are on nightly, today.)
Let's take a step back. Macros 1.1 is RFC 1681: https://github.com/rust-lang/rfcs/blob/master/text/1681-macr...
See the section saying "The initial implementation of librustc_macro is proposed to be incredibly bare bones:" it really is very, very small. So it landed, and while there was some questions that came up, people migrated to it, and it works well.
22 days ago, it was to be reviewed: https://github.com/rust-lang/rust/issues/35900#issuecomment-...
However, two things happened: one, the Mozilla all-hands in Hawaii, which shook things up, but secondly, that meant a lot of people, including one of the reviewers, took PTO around that time, to actually see some of the islands. That includes someone who was on the reviewers list. So in general, "oh hey this is waiting on one person who forgot to check a box but isn't reachable now" is awkward.
Luckily, in this case, we know that it was just forgetting to check the box; Felix did not have any objections that we were aware of. So we decided to sign off for him, and put it into FCP. This happened so that FCP technically passed the deadline for 1.15, but given how important it is to ship, we're planning on nominating it for a backport, and don't anticipate complaints.
So: tiny feature, code wise. Big win for being stable. Procedural mishap involving awkward scheduling. All these things played into it. Make sense?