Linking operates only on Public functions; so we have to make functions Public to use linking, and then we are gonna have those functions lying around still public when we have no more wish to link them. A particular case is following ReplaceTypes that do linearization (arrays, bools). There are a number of possibilities:

• After we've finished (e.g. linearization), go and hide any functions that were inserted. This requires finding them via...
  • name being the mangled form of __discard, so beginning $__discard or something like that, which is a bit, ugh.
  • Add some kind of state mechanism for the ReplaceTypes callback to record the functions as it generates them.
    • Inside the callback - I guess via a RefCell since the callback is Fn not FnMut (and should be, for exactly the same reason as why RefCell is a bad idea: a single ReplaceTypes can operate on multiple Hugrs, etc. etc.)
    • Into ReplaceTypes... feat!: Allow making (and sharing) new FuncDecls in linearization #2108 started on this but it's pretty hard/nasty so I'm not clear I want to go that way
• Record the set of functions in the Hugr before we start; any not in that set, must have been new....
  • We could add this as a general facility to ReplaceTypes
    • Perhaps an fn ReplaceTypes::set_hide_new_funcs(&mut self, FuncsToHide) (an enum with variants None, maybe-default OnlyDefns, All) - so ReplaceTypes snapshots the module children before it starts, then after it's done goes through the module-children and hides appropriate ones not in the snapshot. One nice feature is that (with default None) this is a non-breaking change.
    • Or, we make ReplaceTypes return the new functions/module-children added. (Just does a snapshot before and then filters out.) Breaking as ReplaceTypes returns a bool atm (could become an Option<Vec<Node>>.)
    • The latter would be easy enough for any caller to ReplaceTypes to do the snapshotting itself! I think the only caller is ReplaceBoolPass (tket2) so we could just do it there.
• Or, tag new functions created during ReplaceTypes via metadata, and then look for that at the end (same variations: ReplaceTypes hides them; ReplaceTypes return them; just do it all in the caller). One question here is lining up the metadata set by the hugr-core linearization handlers with the later hiding (is there one metadata that says this or is it configurable).
• A more general facility (not specific to ReplaceTypes) is to add a new variants of enum Visibility. (This is a bunch of code: we have distinct enum Visibilitys in hugr-core, hugr-model and capnp.) E.g. Visibility::Tag(String), and then NameLinkingPolicy gains consider_as_public(&must self, String).