src/doc/rustc-dev-guide/src/type-checking.md

   1 # Type checking
   2
   3 The [`rustc_typeck`][typeck] crate contains the source for "type collection"
   4 and "type checking", as well as a few other bits of related functionality. (It
   5 draws heavily on the [type inference] and [trait solving].)
   6
   7 [typeck]: https://github.com/rust-lang/rust/tree/master/src/librustc_typeck
   8 [type inference]: type-inference.html
   9 [trait solving]: traits/resolution.html
  10
  11 ## Type collection
  12
  13 Type "collection" is the process of converting the types found in the HIR
  14 (`hir::Ty`), which represent the syntactic things that the user wrote, into the
  15 **internal representation** used by the compiler (`Ty<'tcx>`) – we also do
  16 similar conversions for where-clauses and other bits of the function signature.
  17
  18 To try and get a sense for the difference, consider this function:
  19
  20 ```rust,ignore
  21 struct Foo { }
  22 fn foo(x: Foo, y: self::Foo) { ... }
  23 //        ^^^     ^^^^^^^^^
  24 ```
  25
  26 Those two parameters `x` and `y` each have the same type: but they will have
  27 distinct `hir::Ty` nodes. Those nodes will have different spans, and of course
  28 they encode the path somewhat differently. But once they are "collected" into
  29 `Ty<'tcx>` nodes, they will be represented by the exact same internal type.
  30
  31 Collection is defined as a bundle of [queries] for computing information about
  32 the various functions, traits, and other items in the crate being compiled.
  33 Note that each of these queries is concerned with *interprocedural* things –
  34 for example, for a function definition, collection will figure out the type and
  35 signature of the function, but it will not visit the *body* of the function in
  36 any way, nor examine type annotations on local variables (that's the job of
  37 type *checking*).
  38
  39 For more details, see the [`collect`][collect] module.
  40
  41 [queries]: query.html
  42 [collect]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_typeck/collect/
  43
  44 **TODO**: actually talk about type checking...