From 3abf747a648eb64747245573aede9a2e194e0af5 Mon Sep 17 00:00:00 2001
From: Scott Andrews <scott@andrews.me>
Date: Mon, 8 Jun 2026 21:53:04 -0400
Subject: [PATCH] feat(rust): Expose merge_structurally_equal_types

The generate! macro now exposes `merge_structurally_equal_types`. This
functionality was previously only available via the
`--merge-structurally-equal-types` CLI flag.

The underlying functionality is unchanged.

Signed-off-by: Scott Andrews <scott@andrews.me>
---
 crates/guest-rust/macro/src/lib.rs |  9 ++++
 crates/guest-rust/src/lib.rs       |  9 ++++
 crates/rust/tests/codegen.rs       | 66 ++++++++++++++++++++++++++++++
 3 files changed, 84 insertions(+)

diff --git a/crates/guest-rust/macro/src/lib.rs b/crates/guest-rust/macro/src/lib.rs
index e5ae0b0ef..acf97ad53 100644
--- a/crates/guest-rust/macro/src/lib.rs
+++ b/crates/guest-rust/macro/src/lib.rs
@@ -168,6 +168,9 @@ impl Parse for Config {
                     Opt::EnableMethodChaining(enable) => {
                         opts.enable_method_chaining = enable.value();
                     }
+                    Opt::MergeStructurallyEqualTypes(enable) => {
+                        opts.merge_structurally_equal_types = Some(Some(enable.value()))
+                    }
                 }
             }
         } else {
@@ -322,6 +325,7 @@ mod kw {
     syn::custom_keyword!(imports);
     syn::custom_keyword!(debug);
     syn::custom_keyword!(enable_method_chaining);
+    syn::custom_keyword!(merge_structurally_equal_types);
 }
 
 #[derive(Clone)]
@@ -403,6 +407,7 @@ enum Opt {
     Async(AsyncFilterSet, Span),
     Debug(syn::LitBool),
     EnableMethodChaining(syn::LitBool),
+    MergeStructurallyEqualTypes(syn::LitBool),
 }
 
 impl Parse for Opt {
@@ -586,6 +591,10 @@ impl Parse for Opt {
                 }
                 Ok(Opt::Async(set, span))
             }
+        } else if l.peek(kw::merge_structurally_equal_types) {
+            input.parse::<kw::merge_structurally_equal_types>()?;
+            input.parse::<Token![:]>()?;
+            Ok(Opt::MergeStructurallyEqualTypes(input.parse()?))
         } else {
             Err(l.error())
         }
diff --git a/crates/guest-rust/src/lib.rs b/crates/guest-rust/src/lib.rs
index a98cc7a0b..1be14bead 100644
--- a/crates/guest-rust/src/lib.rs
+++ b/crates/guest-rust/src/lib.rs
@@ -865,6 +865,15 @@ extern crate std;
 ///     // returning `-> &Self`, to permit method chaining (e.g. for builder).
 ///     // This expectation is also imposed on exports.
 ///     enable_method_chaining: true,
+///
+///     // Find all structurally equal types and only generate one type
+///     // definition for each equivalence class.
+///     //
+///     // Other types in the same class will be type aliases to the generated
+///     // type. This avoids clone when converting between types that are
+///     // structurally equal, which is useful when import and export the same
+///     // interface.
+///     merge_structurally_equal_types: true,
 /// });
 /// ```
 ///
diff --git a/crates/rust/tests/codegen.rs b/crates/rust/tests/codegen.rs
index a6df22cf9..acc068a96 100644
--- a/crates/rust/tests/codegen.rs
+++ b/crates/rust/tests/codegen.rs
@@ -235,3 +235,69 @@ mod method_chaining {
         enable_method_chaining: true
     });
 }
+
+#[allow(unused, reason = "testing codegen, not functionality")]
+mod merge_structurally_equal_types {
+    wit_bindgen::generate!({
+        inline: r#"
+        package test:merge-structurally-equal-types;
+
+        interface blag {
+            variant kind1 { a, b(u64), c }
+            variant kind2 { a, b(u64), c }
+            record kind3 { a: input-stream }
+            record kind4 { a: input-stream }
+            record tree { l: t1, r: t1  }
+            record t1 { l: t2, r: t2 }
+            record t2 { l: t3, r: t3 }
+            record t3 { l: kind1, r: kind2 }
+            record t-stream  { tree: tree, %stream: option<borrow<input-stream>> }
+            resource input-stream {
+                read: func(len: u64) -> list<u8>;
+            }
+            f: func(x: kind1) -> kind2;
+            g: func(x: kind3) -> kind4;
+            h: func(x: t-stream) -> tree;
+        }
+
+        interface blah {
+            use blag.{input-stream, kind4, t-stream};
+            variant kind5 { a, b(u64), c }
+            variant kind6 { a, c, b(u64) }
+            record kind7 { a: borrow<input-stream> }
+            record tt { l: t2, r: t2  }
+            record t1 { l: t3, r: t3 }
+            record t2 { l: t1, r: t1 }
+            record t3 { l: kind5, r: kind5 }
+            variant custom-result { ok(tt), err }
+            f: func(x: kind6) -> kind5;
+            g: func(x: kind7) -> kind4;
+            h: func(x: t-stream) -> custom-result;
+
+            record r1 { a: u8 }
+            type a1 = u8;
+            record r2 { a: a1 }
+            alias-type: func(x: r1) -> r2;
+        }
+
+        interface resources {
+            resource r1;
+            type r2 = r1;
+
+            record t1 { a: r1 }
+            record t2 { a: r2 }
+            alias-own: func(x: t1) -> t2;
+            alias-aggregate: func(x: option<t1>) -> option<t2>;
+        }
+
+        world proxy {
+            import blag;
+            export blag;
+            import blah;
+            export blah;
+        }
+        "#,
+        generate_all,
+        merge_structurally_equal_types: true
+    });
+}