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`u` (and `findAndModify`'s `update`) may be an array of aggregation stages
instead of a document of operators. The stages themselves already existed --
M2 built `compile_rewrite` and `apply_rewrite` for `aggregate` -- so this is
mostly about which of them are allowed here and what happens around them.
Six are: `$addFields`/`$set`, `$project`, `$unset`, `$replaceRoot`/
`$replaceWith`. Each rewrites one document into one document, which is the
property that makes them usable: `$match` could drop it, `$group` and
`$unwind` could change how many there are, and `$sort` means nothing to one.
mongod refuses those four by name with InvalidOptions (72) and a name that is
no stage at all with 40324, and the difference is worth keeping -- "not here"
and "not at all" are different things to be told.
`$replaceWith` did not exist here at all and now does, in `compile_rewrite`,
so `aggregate` gets it too: it is `$replaceRoot` with the expression in place
of the `{newRoot: ...}` wrapper, one stage under two spellings.
**The `_id` survives every stage.** `$replaceRoot: {newRoot: "$t"}` drops it
and `$project: {_id: 0}` asks to, and it comes back either way, because a
pipeline update rewrites a document rather than replacing one document with
another. A stage that sets it to a *different* value is ImmutableField (66);
restating the same one is fine. Measured on all six stages, and the mutation
that reddens the test is deleting the restore -- the document becomes
unfindable by the id it is stored under.
`EvalCtx.coll` becomes optional, which is what lets the upsert path run a
pipeline over a document held in memory before the collection it will be
inserted into exists. Only an `.offsets` stream reads the slab; a `.docs` one
never touched the field.
`arrayFilters` beside a pipeline is FailedToParse (9), not ignored: an
identifier a pipeline cannot spell would make the update silently different
from the one the client wrote.
pipeline.json 0/23 -> 23/23, operator corpus 125/125.
Pinned crud scorecard **218 -> 228 pass, 73 -> 63 fail**: the five pipeline
files, the four `-rawdata` files that use pipelines, and
`findOneAndUpdate-comment`, which is two more pipeline cases.
249/249 unit tests in ReleaseFast and ReleaseSafe, 83/83 fuzz.
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