feat: update message in stubbed go exports

Signed-off-by: Andrew Steurer <94206073+asteurer@users.noreply.github.com>

Author: asteurer

crates/go/src/lib.rs

@@ -157,6 +157,7 @@ struct InterfaceData {
     need_unsafe: bool,
     need_runtime: bool,
     need_math: bool,
+    has_stubs: bool,
 }
 
 impl InterfaceData {
@@ -166,6 +167,7 @@ impl InterfaceData {
         self.need_unsafe |= data.need_unsafe;
         self.need_runtime |= data.need_runtime;
         self.need_math |= data.need_math;
+        self.has_stubs |= data.has_stubs;
     }
 
     fn imports(&self) -> String {
@@ -186,6 +188,7 @@ impl InterfaceData {
             need_unsafe: generator.need_unsafe,
             need_runtime: generator.need_pinner,
             need_math: generator.need_math,
+            has_stubs: false,
         }
     }
 }
@@ -198,6 +201,7 @@ impl From<InterfaceGenerator<'_>> for InterfaceData {
             need_unsafe: generator.need_unsafe,
             need_runtime: generator.need_runtime,
             need_math: false,
+            has_stubs: generator.has_stubs,
         }
     }
 }
@@ -899,6 +903,18 @@ impl WorldGenerator for Go {
 "
         );
 
+        let stub_header = &format!(
+            "// The exported functions below are stubs which should be 
+// replaced with your own implementation.
+//
+// WARNING: Regenerating the bindings with `--generate-stubs` will 
+// regenerate this file and discard any edits.
+//
+// Generated by `wit-bindgen` {version} from the following packages:
+{packages}
+"
+        );
+
         let src = mem::take(&mut self.src);
         let align = self.return_area_align.format(POINTER_SIZE_EXPRESSION);
         let size = self.return_area_size.format(POINTER_SIZE_EXPRESSION);
@@ -974,6 +990,7 @@ var {SYNC_EXPORT_PINNER} = runtime.Pinner{{}}
             for (name, data) in interfaces {
                 let imports = data.imports();
                 let code = &data.code;
+                let header = if data.has_stubs { stub_header } else { header };
 
                 files.push(
                     &format!("{prefix}{name}/wit_bindings.go"),
@@ -1384,12 +1401,14 @@ func wasm_export_post_return_{name}(result {results}) {{
                 .extend(InterfaceData {
                     code: format!(
                         r#"
+// TODO: Implement
 func {camel}({params}) {results} {{
         panic("not implemented")
 }}
 "#
                     ),
                     imports,
+                    has_stubs: true,
                     ..InterfaceData::default()
                 });
         }
@@ -2636,6 +2655,7 @@ struct InterfaceGenerator<'a> {
     imports: BTreeSet<String>,
     need_unsafe: bool,
     need_runtime: bool,
+    has_stubs: bool,
 }
 
 impl<'a> InterfaceGenerator<'a> {
@@ -2654,6 +2674,7 @@ impl<'a> InterfaceGenerator<'a> {
             imports: BTreeSet::new(),
             need_unsafe: false,
             need_runtime: false,
+            has_stubs: false,
         }
     }