emit errors during AbstractConst building
There changes are currently still untested, so I don't expect this to pass CI 😆
It seems to me like this is the direction we want to go in, though we didn't have too much of a discussion about this.
r? @oli-obk
Don't query stability data when `staged_api` is off
This data only needs to be encoded when `#![feature(staged_api)]` or `-Zforce-unstable-if-unmarked` is on. Running these queries takes measurable time on large crates with many items, so skip it when the unstable flags have not been enabled.
Previously, we would throw away the `SyntaxContext` of any span with a
dummy location during metadata encoding. This commit makes metadata Span
encoding consistent with incr-cache Span encoding - an 'invalid span'
tag is only used when it doesn't lose any information.
