Module rustc_type_ir::visit

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A visiting traversal mechanism for complex data structures that contain type information.

This is a read-only traversal of the data structure.

This traversal has limited flexibility. Only a small number of “types of interest” within the complex data structures can receive custom visitation. These are the ones containing the most important type-related information, such as Ty, Predicate, Region, and Const.

There are three traits involved in each traversal.

  • TypeVisitable. This is implemented once for many types, including:
    • Types of interest, for which the methods delegate to the visitor.
    • All other types, including generic containers like Vec and Option. It defines a “skeleton” of how they should be visited.
  • TypeSuperVisitable. This is implemented only for recursive types of interest, and defines the visiting “skeleton” for these types. (This excludes Region because it is non-recursive, i.e. it never contains other types of interest.)
  • TypeVisitor. This is implemented for each visitor. This defines how types of interest are visited.

This means each visit is a mixture of (a) generic visiting operations, and (b) custom visit operations that are specific to the visitor.

  • The TypeVisitable impls handle most of the traversal, and call into TypeVisitor when they encounter a type of interest.
  • A TypeVisitor may call into another TypeVisitable impl, because some of the types of interest are recursive and can contain other types of interest.
  • A TypeVisitor may also call into a TypeSuperVisitable impl, because each visitor might provide custom handling only for some types of interest, or only for some variants of each type of interest, and then use default traversal for the remaining cases.

For example, if you have struct S(Ty, U) where S: TypeVisitable and U: TypeVisitable, and an instance s = S(ty, u), it would be visited like so:

s.visit_with(visitor) calls
- ty.visit_with(visitor) calls
  - visitor.visit_ty(ty) may call
    - ty.super_visit_with(visitor)
- u.visit_with(visitor)


  • This trait is implemented for every type that can be visited, providing the skeleton of the traversal.
  • This trait is implemented for every visiting traversal. There is a visit method defined for every type of interest. Each such method has a default that recurses into the type’s fields in a non-custom fashion.