Split 'use' inside worlds into 'use import' and 'use export'

Author: lukewagner

design/mvp/WIT.md

@@ -897,13 +897,16 @@ Concretely, the structure of a world is:
 ```ebnf
 world-item ::= 'world' id '{' world-items* '}'
 
-world-items ::= export-item | import-item | use-item | typedef-item | include-item
+world-items ::= export-item | import-item | use-export-item | use-import-item | typedef-item | include-item
 
 export-item ::= 'export' id ':' extern-type
               | 'export' use-path ';'
 import-item ::= 'import' id ':' extern-type
               | 'import' use-path ';'
 
+use-import-item ::= 'use' 'import' use-item-body
+use-export-item ::= 'use' 'export' use-item-body
+
 extern-type ::= func-type ';' | 'interface' '{' interface-items* '}'
 ```
 
@@ -912,6 +915,27 @@ from the root of a component and used within functions imported and exported.
 The `interface` item here additionally defines the grammar for IDs used to refer
 to `interface` items.
 
+The `use export`/`use import` items work just like `use` inside an `interface`
+except that they exclusively refer to exports/imports, respectively. This
+allows a world to import and export the same interface and be able to
+independently refer to same type in both. For example, the following world
+defines a single function using both an imported and exported version of the
+same interface's resource type:
+
+```wit
+interface i {
+  resource r;
+}
+
+world w {
+  import i;
+  export i;
+  use import i.{r as r1};
+  use export i.{r as r2};
+  export transform: func(in: r1) -> r2;
+}
+```
+
 ## Item: `include`
 
 A `include` statement enables the union of the current world with another world. The structure of an `include` statement is:
@@ -983,7 +1007,9 @@ use my:dependency/the-interface.{more, names as foo}
 Specifically the structure of this is:
 
 ```ebnf
-use-item ::= 'use' use-path '.' '{' use-names-list '}' ';'
+use-item ::= 'use' use-item-body
+
+use-item-body ::= use-path '.' '{' use-names-list '}' ';'
 
 use-names-list ::= use-names-item
                  | use-names-item ',' use-names-list?