Type checking

The rustc_typeck crate contains the source for "type collection" and "type checking", as well as a few other bits of related functionality. (It draws heavily on the type inference and trait solving.)

Type collection

Type "collection" is the process of converting the types found in the HIR (hir::Ty), which represent the syntactic things that the user wrote, into the internal representation used by the compiler (Ty<'tcx>) – we also do similar conversions for where-clauses and other bits of the function signature.

To try and get a sense for the difference, consider this function:

struct Foo { }
fn foo(x: Foo, y: self::Foo) { ... }
//        ^^^     ^^^^^^^^^

Those two parameters x and y each have the same type: but they will have distinct hir::Ty nodes. Those nodes will have different spans, and of course they encode the path somewhat differently. But once they are "collected" into Ty<'tcx> nodes, they will be represented by the exact same internal type.

Collection is defined as a bundle of queries for computing information about the various functions, traits, and other items in the crate being compiled. Note that each of these queries is concerned with interprocedural things – for example, for a function definition, collection will figure out the type and signature of the function, but it will not visit the body of the function in any way, nor examine type annotations on local variables (that's the job of type checking).

For more details, see the collect module.

TODO: actually talk about type checking...