Separate definitions and HIR owners in the type system

[cjgillot]

Both definitions and HIR owners are represented by a LocalDefId,
but they don't have the same semantics.

HIR owners are the owner field of a HirId,
and the argument passed to the hir_owner* queries.
HIR owners contain the following HIR nodes: Item, TraitItem, ImplItem, ForeignItem, MacroDef,
and GenericParams which come from impl Trait desugaring.

LocalDefId enumerate definitions in the local crate.
Definitions are a broader concept than HIR owners.
For instance, closures are definitions but not HIR owners.

On the other hand, all HIR owners are definitions.

We need to introduce a new type HirOwner type (name to bikeshed),
whose usage will be the following:

• HirOwner is defined as struct HirOwner { def_id: LocalDefId };
• HirId becomes { owner: HirOwner, local_id: ItemLocalId };
• Item, TraitItem, ImplItem, ForeignItem, MacroDef have a HirOwner as their def_id field;
• hir_owner* queries take a HirOwner as argument;
• the passage between HirOwner and LocalDefId happens through
  the local_def_id_to_hir_id: LocalDefId -> HirId
  and local_def_id: HirId -> LocalDefId methods.

I am available on Zulip for further information.

[osa1]

@rustbot claim

I'd like to do this. @cjgillot I'll ping you on Zulip with questions.

[osa1]

I don't understand HIR ownership and I'm working on other issues currently. I'm hoping to come back to this but if anyone wants to do it in the meantime please feel free to pick this up.

[0xPoe]

@rustbot claim

[ABouttefeux]

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