@@ -82,8 +82,33 @@ type fn_ctxt =
8282 locals : hashmap < ast:: node_id , int > ,
8383 next_var_id : @mut int ,
8484 next_region_var_id : @mut int ,
85+
86+ // While type checking a function, the intermediate types for the
87+ // expressions, blocks, and so forth contained within the function are
88+ // stored in these tables. These types may contain unresolved type
89+ // variables. After type checking is complete, the functions in the
90+ // writeback module are used to take the types from this table, resolve
91+ // them, and then write them into their permanent home in the type
92+ // context `ccx.tcx`.
93+ //
94+ // This means that during inferencing you should use `fcx.write_ty()`
95+ // and `fcx.expr_ty()` / `fcx.node_ty()` to write/obtain the types of
96+ // nodes within the function.
97+ //
98+ // The types of top-level items, which never contain unbound type
99+ // variables, are stored directly into the `tcx` tables.
100+ //
101+ // n.b.: A type variable is not the same thing as a type parameter. A
102+ // type variable is rather an "instance" of a type parameter: that is,
103+ // given a generic function `fn foo<T>(t: T)`: while checking the
104+ // function `foo`, the type `ty_param(0)` refers to the type `T`, which
105+ // is treated in abstract. When `foo()` is called, however, `T` will be
106+ // substituted for a fresh type variable `ty_var(N)`. This variable will
107+ // eventually be resolved to some concrete type (which might itself be
108+ // type parameter).
85109 node_types : smallintmap:: smallintmap < ty:: t > ,
86110 node_type_substs : hashmap < ast:: node_id , [ ty:: t ] > ,
111+
87112 ccx : @crate_ctxt } ;
88113
89114// Determines whether the given node ID is a use of the def of
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