Forbid lowering the same `NodeId` multiple times

[cjgillot]

During AST->HIR lowering, the method lower_node_id is tasked to transform NodeIds that identify AST nodes into HirIds that identify HIR nodes. This method maintains a node_id_to_local_id: FxHashMap<NodeId, hir::ItemLocalId> to remember this mapping.

However, this mapping is not entirely useful, and mostly exists (1) to make the developer's life easier, (2) to lower resolutions to local bindings Res::Local in lower_res. The mapping node_id_to_local_id should be removed, and multiple calls to local_node_id with the same NodeId should be forbidden. For usage (2), Res::Local only appears for ident patterns (PatKind::Ident) which are lowered by lower_pat_ident. Hence, lower_res should use a dedicated hash-map filled by lower_pat_ident.

Furthermore, next_id calls lower_node_id with a node_id: NodeId which is entirely local to that function, and will never be known to any other code. Manipulations of node_id_to_local_id there are entirely useless.

Instructions:

• inline lower_node_id into next_id and skip manipulations of node_id_to_local_id;
• replace the Entry::Occupied branch in lower_node_id by a panic!, and fix all the ICEs;
• create another mapping node_id_to_local_id_for_res (name to bikeshed) which is only filled by lower_pat_ident and read by lower_res; this mapping should be saved by with_hir_id_owner like node_id_to_local_id is;
• remove node_id_to_local_id, or eventually keep it only to debug-assert that we don't call lower_node_id twice on the same NodeId.

[kckeiks]

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[raiyansayeed]

Hey @kckeiks , are you still working on this task by chance?

[kckeiks]

@raiyansayeed I was but I have a couple of other issues that I'm working on so you can take this one if you want.

[spastorino]

@raiyansayeed feel free to ping me too on Zulip if you need help or something.

[raiyansayeed]

@rustbot claim

[trevyn]

@raiyansayeed are you still working on this?

[Noratrieb]

No activity, feel free to claim it again. @rustbot release-assignment

[azadnn]

@rustbot claim