Fix bindings generation with unused types in Rust (#530)

* Fix bindings generation with unused types in Rust

This commit fixes an issue where if a type was imported into an
interface but wasn't actually used anywhere then bindings weren't
correctly generated because inference about whether it was owned or
borrowed didn't run. The fix here is to skip generating bindings for
unused types since they're not actually needed anywhere anyway. If
necessary in the future bindings can be forcibly generated but for now
this is hopefully largely "just" fixing an edge case.

* Ignore the new tests for Go

Author: alexcrichton

crates/core/src/lib.rs

@@ -166,7 +166,8 @@ impl Types {
             }
             TypeDefKind::Unknown => unreachable!(),
         }
-        self.type_info.insert(ty, info);
+        let prev = self.type_info.insert(ty, info);
+        assert!(prev.is_none());
         info
     }
 

crates/go/tests/codegen.rs

@@ -7,6 +7,7 @@ use std::process::Command;
 macro_rules! codegen_test {
     // TODO: should fix this test
     (lift_lower_foreign $name:tt $test:tt) => {};
+    (unused_import $name:tt $test:tt) => {};
 
     ($id:ident $name:tt $test:tt) => {
         #[test]

crates/rust-lib/src/lib.rs

@@ -381,6 +381,9 @@ pub trait RustGenerator<'a> {
 
     fn modes_of(&self, ty: TypeId) -> Vec<(String, TypeMode)> {
         let info = self.info(ty);
+        if !info.owned && !info.borrowed {
+            return Vec::new();
+        }
         let mut result = Vec::new();
         let first_mode = if info.owned || !info.borrowed {
             TypeMode::Owned

tests/codegen/unused-import.wit

@@ -0,0 +1,17 @@
+interface types {
+    record r {
+        a: list<u8>,
+    }
+}
+
+default world the-world {
+    import foo: interface {
+      use self.types.{r}
+
+      foo: func(data: r)
+    }
+
+    export bar: interface {
+      use self.types.{r}
+    }
+}