Traits for transforming bits of IR.
| Fold | Applies the given Folder to a value, producing a folded result
of type Self::Result. The result is in the interner
TI. The result type is typically the same as the source type
(modulo interner), but in some cases we convert from borrowed
to owned as well (e.g., the folder for &T will fold to a fresh
T; well, actually T::Result).
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| Folder | A "folder" is a transformer that can be used to make a copy of
some term -- that is, some bit of IR, such as a Goal -- with
certain changes applied. The idea is that it contains methods that
let you swap types/lifetimes for new types/lifetimes; meanwhile,
each bit of IR implements the Fold trait which, given a
Folder, will reconstruct itself, invoking the folder's methods
to transform each of the types/lifetimes embedded within.
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| SuperFold | For types where "fold" invokes a callback on the Folder, the
SuperFold trait captures the recursive behavior that folds all
the contents of the type.
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