Query cycle before ast lowered turns into dumpster fire

[camsteffen]

This is mostly just a rustc developer experience issue that crops up if you accidentally introduce a query cycle before ast lowering completes. After this happened to me a few times, I finally understand what is happening so I figured I should document it.

If a query cycle occurs before or during ast lowering, then rustc winds up querying for the HIR while trying to collect info about the cycle to report it, and this leads to another cycle and more panics, which is rather confusing to behold. The root issue is that we try to call the def_span and def_kind queries in the following places:

rust/compiler/rustc_query_impl/src/plumbing.rs

Line 319 in e70cbef

Some(key.default_span(*tcx))

(default_span is implemented using def_span if the key is DefId or LocalDefId)

rust/compiler/rustc_query_impl/src/plumbing.rs

Line 326 in e70cbef

def_id.and_then(|def_id| def_id.as_local()).and_then(|def_id| tcx.opt_def_kind(def_id))

Some ideas:

1. Use source_span instead of def_span
2. Add a way to get def_span and/or def_kind from AST (probably only for diagnostics cases like this)
3. Add a way to detect if AST has been lowered, and skip those queries if not