crdb conditional upsert (`insert_into().on_conflict().filter_target()`) doesn't apply the `filter_target` predicate

tl;dr: let's say you want to:

• upsert a row into a database
• but only update it if a particular condition matches

The most obvious way to do this is with Diesel's DecoratableTarget::filter_target, but if you try and use that, CockroachDB silently drops the condition associated with filter_target if this is a full index. This is pretty bad.

For a full explanation, see this comment.

More details

I discovered this while working on #5032.

The Postgres documentation on INSERT documents two possible formulations for a conditional upsert:

1. ON CONFLICT ... WHERE ... DO UPDATE
2. ON CONFLICT ... DO UPDATE ... WHERE

filter_target becomes 1, not 2. It is possible to write 2 in Diesel via manually importing the FilterDsl trait, though it's not part of rustdoc for some reason.

Pattern 1 is only present to support partial indexes. If the index is a full index, CockroachDB silently ignores the WHERE clause in queries like 1. (This behavior makes sense -- see the linked full explanation for why.)

See this gist for a repro.

As the comment there indicates, you can even pass in a completely non-existent column -- and Cockroach completely ignores that.

To do:

• Figure out whether the bug still exists in the latest version of Cockroach -- yes, it does with v23.2.0
• If so, file a bug upstream: https://github.com/cockroachdb/cockroach/issues/119117
• Write a Diesel extension that implements the ON CONFLICT ... DO UPDATE ... WHERE pattern -- not necessary because .filter() is supported, though it doesn't show up in the documentation
• Define a replacement for DecoratableTarget::filter_target, and add a clippy lint: https://github.com/oxidecomputer/omicron/pull/5052
• File an issue with Diesel (or rustdoc?) talking about the implementations not being visible in FilterDsl.
• File an issue with Cockroach about crashes if you pass in an invalid expression like ON CONFLICT ... WHERE 'x' = 1