Robust error explanation index and error code uniqueness checking

[pnkfelix]

The PR's #25380 #25592 #25603 were motivated by brittleness such as #25364.

But removing the diagnostics metadata output and checking represents a step backward, in that we no longer have an auto-generated error index (nor uniqueness checking).

This issue is a work-item to put those two features back.

[mcast]

[...] we no longer have an auto-generated error index (nor uniqueness checking).
This issue is a work-item to put those two features back.

Are these two both covered by #26009 ?

It'd be nice to hang on to the error index in the Rust 1.2 release... currently it's disabled due to
[...]

• In the error-index-generator - which runs once all crates have been compiled - we can check uniqueness.

(Via #26240)

[michaelsproul]

Are these two both covered by #26009 ?

Yeah, this a "duplicate", but it's my fault, I created #26009 because I hadn't seen this.