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typeck: check phase

Within the check phase of type check, we check each item one at a time (bodies of function expressions are checked as part of the containing function). Inference is used to supply types wherever they are unknown.

By far the most complex case is checking the body of a function. This can be broken down into several distinct phases:

  • gather: creates type variables to represent the type of each local variable and pattern binding.

  • main: the main pass does the lion’s share of the work: it determines the types of all expressions, resolves methods, checks for most invalid conditions, and so forth. In some cases, where a type is unknown, it may create a type or region variable and use that as the type of an expression.

    In the process of checking, various constraints will be placed on these type variables through the subtyping relationships requested through the demand module. The infer module is in charge of resolving those constraints.

  • regionck: after main is complete, the regionck pass goes over all types looking for regions and making sure that they did not escape into places where they are not in scope. This may also influence the final assignments of the various region variables if there is some flexibility.

  • writeback: writes the final types within a function body, replacing type variables with their final inferred types. These final types are written into the tcx.node_types table, which should never contain any reference to a type variable.

Intermediate types

While type checking a function, the intermediate types for the expressions, blocks, and so forth contained within the function are stored in fcx.node_types and fcx.node_substs. These types may contain unresolved type variables. After type checking is complete, the functions in the writeback module are used to take the types from this table, resolve them, and then write them into their permanent home in the type context tcx.

This means that during inferencing you should use fcx.write_ty() and fcx.expr_ty() / fcx.node_ty() to write/obtain the types of nodes within the function.

The types of top-level items, which never contain unbound type variables, are stored directly into the tcx typeck_results.

N.B., a type variable is not the same thing as a type parameter. A type variable is an instance of a type parameter. That is, given a generic function fn foo<T>(t: T), while checking the function foo, the type ty_param(0) refers to the type T, which is treated in abstract. However, when foo() is called, T will be substituted for a fresh type variable N. This variable will eventually be resolved to some concrete type (which might itself be a type parameter).