Module rustc_trait_selection::traits::wf

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  • Elaborate 🔒
    Controls whether we “elaborate” supertraits and so forth on the WF predicates. This is a kind of hack to address #43784. The underlying problem in that issue was a trait structure like:


  • Returns the requirements for clause to be well-formed.
  • Points the cause span of a super predicate at the relevant associated type.
  • Given an object type like SomeTrait + Send, computes the lifetime bounds that must hold on the elided self type. These are derived from the declarations of SomeTrait, Send, and friends – if they declare trait SomeTrait : 'static, for example, then 'static would appear in the list. The hard work is done by infer::required_region_bounds, see that for more information.
  • Returns the set of obligations needed to make arg well-formed. If arg contains unresolved inference variables, this may include further WF obligations. However, if arg IS an unresolved inference variable, returns None, because we are not able to make any progress at all. This is to prevent “livelock” where we say “$0 is WF if $0 is WF”.
  • Given a set of predicates that apply to an object type, returns the region bounds that the (erased) Self type must outlive. Precisely because the Self type is erased, the parameter erased_self_ty must be supplied to indicate what type has been used to represent Self in the predicates themselves. This should really be a unique type; FreshTy(0) is a popular choice.
  • Returns the obligations that make this trait reference well-formed. For example, if there is a trait Set defined like trait Set<K: Eq>, then the trait bound Foo: Set<Bar> is WF if Bar: Eq.
  • Compute the predicates that are required for a type to be well-formed.