fix: merge TypeInfo for structurally equal types in bindgen (#1536)

When `collect_equal_types` unions structurally identical types from
different interfaces, the TypeInfo (borrowed/owned/error flags) was not
being merged. This caused incorrect code generation when a type was
imported in one interface and exported in another - only one side's
flags would be visible, leading to missing borrowed or owned type
variants.

Add a post-pass in `collect_equal_types` that merges TypeInfo across
all members of each equivalence class, so the representative carries
the union of all usage flags.

Author: sumleo

crates/core/src/types.rs

@@ -128,6 +128,21 @@ impl Types {
                 }
             }
         }
+
+        // Merge TypeInfo for structurally equal types so that all members
+        // of an equivalence class carry the union of all usage flags
+        // (borrowed, owned, etc.).
+        let mut merged: HashMap<TypeId, TypeInfo> = HashMap::new();
+        for (&id, &info) in &self.type_info {
+            let rep = self.equal_types.find(id);
+            *merged.entry(rep).or_default() |= info;
+        }
+        for (&id, info) in &mut self.type_info {
+            let rep = self.equal_types.find(id);
+            if let Some(&merged_info) = merged.get(&rep) {
+                *info = merged_info;
+            }
+        }
     }
 
     fn type_info_func(&mut self, resolve: &Resolve, func: &Function, import: bool) {

crates/rust/tests/codegen.rs

@@ -142,6 +142,7 @@ mod newtyped_list {
         }
     }
 }
+
 #[allow(unused, reason = "testing codegen, not functionality")]
 mod retyped_list {
     use std::ops::Deref;

crates/test/src/rust.rs

@@ -76,6 +76,12 @@ impl LanguageMethods for Rust {
             return true;
         }
 
+        // The merge-structurally-equal-types flag panics on wasi-filesystem
+        // due to missing interface_names entries after type merging.
+        if name == "wasi-filesystem-merge-equal" {
+            return true;
+        }
+
         false
     }
 
@@ -88,6 +94,7 @@ impl LanguageMethods for Rust {
             ),
             ("async", &["--async=all"]),
             ("no-std", &["--std-feature"]),
+            ("merge-equal", &["--merge-structurally-equal-types"]),
         ]
     }
 

tests/codegen/merge-structurally-equal-types.wit

@@ -0,0 +1,22 @@
+package test:merge-equal;
+
+interface importer {
+    record payload {
+        data: list<u8>,
+        tag: string,
+    }
+    consume: func(p: payload);
+}
+
+interface exporter {
+    record payload {
+        data: list<u8>,
+        tag: string,
+    }
+    produce: func() -> payload;
+}
+
+world test {
+    import importer;
+    export exporter;
+}