LocalDefId <-> HirId conversion functions may not be incremental safe

[cjgillot]

The conversion between LocalDefId and HirId relies on mappings that are built during AST->HIR lowering. Those mappings are not tracked anywhere. As a consequence, a node could see its HirId change without notice: for instance a closure above which an extra statement is inserted.

The consequences of this untracked data access are not clear at the moment:

• are there queries whose result store an HirId?
• do HirId really change without notice?

[cjgillot]

Those maps have been removed and integrated into the HIR queries.