Fix single-element tuple generation in WIT bindings (#402)

* Fix single-element tuple generation in WIT bindings

Single-element tuples in Rust require a trailing comma to distinguish
them from parentheses used for grouping. The generator was producing
(String) instead of (String,) for single-element tuples, causing
compilation errors.

This fix adds logic to append a comma when generating single-element
tuples in the TypeDefKind::Tuple case.

* Simplify tuple generation logic per reviewer feedback

Author: colinrozzi

src/generator.rs

@@ -376,11 +376,9 @@ impl<'a> UnimplementedFunction<'a> {
             }
             TypeDefKind::Tuple(t) => {
                 source.push('(');
-                for (i, ty) in t.types.iter().enumerate() {
-                    if i > 0 {
-                        source.push_str(", ");
-                    }
+                for ty in t.types.iter() {
                     self.print_type(ty, trie, source)?;
+                    source.push_str(", ");
                 }
                 source.push(')');
             }

tests/new.rs

@@ -305,3 +305,59 @@ fn it_supports_the_proxy_option() -> Result<()> {
 
     Ok(())
 }
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 1)]
+async fn test_single_element_tuple_generation() -> Result<()> {
+    let (server, config, _task) = spawn_server(["test"]).await?;
+
+    publish_wit(
+        config,
+        "test:tuples",
+        "1.0.0",
+        r#"package test:tuples@1.0.0;
+
+world test-world {
+    export test-interface: interface {
+        test-function: func(params: tuple<string>) -> result<tuple<u32>, string>;
+        another-function: func(single: tuple<bool>) -> tuple<list<string>>;
+    }
+}
+"#,
+    )
+    .await?;
+
+    let project = server.project("test-tuple", true, ["--target", "test:tuples@1.0.0"])?;
+
+    project
+        .cargo_component(["build"])
+        .assert()
+        .stderr(contains(
+            "Finished `dev` profile [unoptimized + debuginfo] target(s)",
+        ))
+        .success();
+
+    let lib_rs = fs::read_to_string(project.root().join("src/lib.rs"))?;
+
+    assert!(
+        lib_rs.contains("params: (String,)"),
+        "Single-element tuple parameter should have trailing comma. Generated code:\n{}",
+        lib_rs
+    );
+    assert!(
+        lib_rs.contains("Result<(u32,), String>") || lib_rs.contains("-> (u32,)"),
+        "Single-element tuple return should have trailing comma. Generated code:\n{}",
+        lib_rs
+    );
+    assert!(
+        lib_rs.contains("single: (bool,)"),
+        "Single-element tuple parameter should have trailing comma. Generated code:\n{}",
+        lib_rs
+    );
+    assert!(
+        lib_rs.contains("-> (Vec<String>,)"),
+        "Single-element tuple return should have trailing comma. Generated code:\n{}",
+        lib_rs
+    );
+
+    Ok(())
+}