Module rustc_middle::infer::canonical

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Canonicalization is the key to constructing a query in the middle of type inference. Ordinarily, it is not possible to store types from type inference in query keys, because they contain references to inference variables whose lifetimes are too short and so forth. Canonicalizing a value T1 using canonicalize_query produces two things:

  • a value T2 where each unbound inference variable has been replaced with a canonical variable;
  • a map M (of type CanonicalVarValues) from those canonical variables back to the original.

We can then do queries using T2. These will give back constraints on the canonical variables which can be translated, using the map M, into constraints in our source context. This process of translating the results back is done by the instantiate_query_result method.

For a more detailed look at what is happening here, check out the chapter in the rustc dev guide.



  • When we canonicalize a value to form a query, we wind up replacing various parts of it with canonical variables. This struct stores those replaced bits to remember for when we process the query result.
  • After we execute a query with a canonicalized key, we get back a Canonical<QueryResponse<..>>. You can use instantiate_query_result to access the data in this result.


  • Rust actually has more than one category of type variables; notably, the type variables we create for literals (e.g., 22 or 22.) can only be instantiated with integral/float types (e.g., usize or f32). In order to faithfully reproduce a type, we need to know what set of types a given type variable can be unified with.
  • Describes the “kind” of the canonical variable. This is a “kind” in the type-theory sense of the term – i.e., a “meta” type system that analyzes type-like values.
  • Indicates whether or not we were able to prove the query to be true.

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