Module rustc_trait_selection::traits

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Trait Resolution. See the rustc dev guide for more information on how this works.



  • Support code for rustdoc and external tools. You really don’t want to be using this unless you need to.
  • coherence 🔒
    See Rustc Dev Guide chapters on trait-resolution and trait-specialization for more info on how this works.
  • Checking that constant values used in types can be successfully evaluated.
  • engine 🔒
  • fulfill 🔒
  • Miscellaneous type-system utilities that are too small to deserve their own modules.
  • Deeply normalize types using the old trait solver.
  • “Object safety” refers to the ability for a trait to be converted to an object. In general, traits may only be converted to an object if all of their methods meet certain criteria. In particular, they must:
  • Code for projecting associated types out of trait references.
  • Experimental types for the trait query interface. The methods defined in this module are all based on canonicalization, which makes a canonical query by replacing unbound inference variables and regions, so that results can be reused more broadly. The providers for the queries defined here can be found in rustc_traits.
  • select 🔒
    Candidate selection. See the rustc dev guide for more information on how this works.
  • specialize 🔒
    Logic and data structures related to impl specialization, explained in greater detail below.
  • util 🔒


  • The fulfillment context is used to drive trait resolution. It consists of a list of obligations that must be (eventually) satisfied. The job is to track which are satisfied, which yielded errors, and which are still pending. At any point, users can call select_where_possible, and the fulfillment context will try to do selection, retaining only those obligations that remain ambiguous. This may be helpful in pushing type inference along. Once all type inference constraints have been generated, the method select_all_or_error can be used to report any remaining ambiguous cases as errors.
  • Identifies a particular impl in the source, along with a set of generic parameters from the impl’s type/lifetime parameters. The nested vector corresponds to the nested obligations attached to the impl’s type parameters.
  • An Obligation represents some trait reference (e.g., i32: Eq) for which the “impl_source” must be found. The process of finding an “impl_source” is called “resolving” the Obligation. This process consists of either identifying an impl (e.g., impl Eq for i32) that satisfies the obligation, or else finding a bound that is in scope. The eventual result is usually a Selection (defined below).
  • The reason why we incurred this obligation; used for error reporting.
  • Used if you want to have pleasant experience when dealing with obligations outside of hir or mir typeck.
  • Information pertinent to an overlapping impl error.
  • The inverse of BoundVarReplacer: replaces placeholders with the bound vars from which they came.
  • The projection cache. Unlike the standard caches, this can include infcx-dependent type variables, therefore we have to roll the cache back each time we roll a snapshot back, to avoid assumptions on yet-unresolved inference variables. Types with placeholder regions also have to be removed when the respective snapshot ends.
  • “Trait alias expansion” is the process of expanding a sequence of trait references into another sequence by transitively following all trait aliases. e.g. If you have bounds like Foo + Send, a trait alias trait Foo = Bar + Sync;, and another trait alias trait Bar = Read + Write, then the bounds would expand to Read + Write + Sync + Send. Expansion is done via a DFS (depth-first search), and the visited field is used to avoid cycles.
  • Stores information about the expansion of a trait via a path of zero or more trait aliases.




  • Normalize a type and process all resulting obligations, returning any errors.
  • Given an upcast trait object described by object, returns the index of the method method_def_id (which should be part of object.upcast_trait_ref) within the vtable for object.
  • Returns the object safety violations that affect HIR ty lowering.
  • Normalizes the predicates and checks whether they hold in an empty environment. If this returns true, then either normalize encountered an error or one of the predicates did not hold. Used when creating vtables to check for unsatisfiable methods.
  • Checks whether a trait’s associated item is impossible to reference on a given impl.
  • We say a method is vtable safe if it can be invoked on a trait object. Note that object-safe traits can have some non-vtable-safe methods, so long as they require Self: Sized or otherwise ensure that they cannot be used when Self = Trait.
  • Normalizes the parameter environment, reporting errors if they occur.
  • Returns Some(_) if this item makes the containing trait not object safe.
  • Checks the coherence orphan rules. impl_def_id should be the DefId of a trait impl. To pass, either the trait must be local, or else two conditions must be satisfied:
  • If there are types that satisfy both impls, returns Some with a suitably-freshened ImplHeader with those types instantiated. Otherwise, returns None.
  • FIXME(@lcnr): this function doesn’t seem right and shouldn’t exist?
  • Creates predicate obligations from the generic bounds.
  • This method traverses the structure of ty, trying to find an instance of an ADT (i.e. struct or enum) that doesn’t implement the structural-match traits, or a generic type parameter (which cannot be determined to be structural-match).
  • A specialized variant of elaborate that only elaborates trait references that may define the given associated item with the name assoc_name. It uses the super_predicates_that_define_assoc_item query to avoid enumerating super-predicates that aren’t related to assoc_item. This is used when resolving types like Self::Item or T::Item and helps to avoid cycle errors (see e.g. #35237).
  • Given the generic parameters for the requested impl, translate it to the generic parameters appropriate for the actual item definition (whether it be in that impl, a parent impl, or the trait).
  • Like translate_args, but obligations from the parent implementation are registered with the provided ObligationCause.
  • Determines whether the type ty is known to meet bound and returns true if so. Returns false if ty either does not meet bound or is not known to meet bound (note that this is conservative towards no impl, which is the opposite of the evaluate methods).
  • Casts a trait reference into a reference to one of its super traits; returns None if target_trait_def_id is not a supertrait.
  • Executes f on value after replacing all escaping bound variables with placeholders and then replaces these placeholders with the original bound variables in the result.

Type Aliases§