Fix printing of the preamble in generated Rust code. (bytecodealliance#1087)

In the generated Rust bindings, Generate the preamble comments in a
separate string, because `prettyplease` does not preserve comments.

Also, add several more options to the preamble printing code.

Fixes bytecodealliance#1086.

Author: sunfishcode

crates/rust/src/lib.rs

@@ -27,6 +27,7 @@ struct InterfaceName {
 #[derive(Default)]
 struct RustWasm {
     types: Types,
+    src_preamble: Source,
     src: Source,
     opts: Opts,
     import_modules: Vec<(String, Vec<String>)>,
@@ -837,44 +838,90 @@ macro_rules! __export_{world_name}_impl {{
 
 impl WorldGenerator for RustWasm {
     fn preprocess(&mut self, resolve: &Resolve, world: WorldId) {
-        wit_bindgen_core::generated_preamble(&mut self.src, env!("CARGO_PKG_VERSION"));
+        wit_bindgen_core::generated_preamble(&mut self.src_preamble, env!("CARGO_PKG_VERSION"));
 
         // Render some generator options to assist with debugging and/or to help
         // recreate it if the original generation command is lost.
-        uwriteln!(self.src, "// Options used:");
+        uwriteln!(self.src_preamble, "// Options used:");
         if self.opts.std_feature {
-            uwriteln!(self.src, "//   * std_feature");
+            uwriteln!(self.src_preamble, "//   * std_feature");
         }
         if self.opts.raw_strings {
-            uwriteln!(self.src, "//   * raw_strings");
+            uwriteln!(self.src_preamble, "//   * raw_strings");
         }
         if !self.opts.skip.is_empty() {
-            uwriteln!(self.src, "//   * skip: {:?}", self.opts.skip);
+            uwriteln!(self.src_preamble, "//   * skip: {:?}", self.opts.skip);
+        }
+        if self.opts.stubs {
+            uwriteln!(self.src_preamble, "//   * stubs");
+        }
+        if let Some(export_prefix) = &self.opts.export_prefix {
+            uwriteln!(
+                self.src_preamble,
+                "//   * export_prefix: {:?}",
+                export_prefix
+            );
+        }
+        if let Some(runtime_path) = &self.opts.runtime_path {
+            uwriteln!(self.src_preamble, "//   * runtime_path: {:?}", runtime_path);
+        }
+        if let Some(bitflags_path) = &self.opts.bitflags_path {
+            uwriteln!(
+                self.src_preamble,
+                "//   * bitflags_path: {:?}",
+                bitflags_path
+            );
         }
         if !matches!(self.opts.ownership, Ownership::Owning) {
-            uwriteln!(self.src, "//   * ownership: {:?}", self.opts.ownership);
+            uwriteln!(
+                self.src_preamble,
+                "//   * ownership: {:?}",
+                self.opts.ownership
+            );
         }
         if !self.opts.additional_derive_attributes.is_empty() {
             uwriteln!(
-                self.src,
+                self.src_preamble,
                 "//   * additional derives {:?}",
                 self.opts.additional_derive_attributes
             );
         }
         for (k, v) in self.opts.with.iter() {
-            uwriteln!(self.src, "//   * with {k:?} = {v}");
+            uwriteln!(self.src_preamble, "//   * with {k:?} = {v}");
+        }
+        if let Some(type_section_suffix) = &self.opts.type_section_suffix {
+            uwriteln!(
+                self.src_preamble,
+                "//   * type_section_suffix: {:?}",
+                type_section_suffix
+            );
         }
         if let Some(default) = &self.opts.default_bindings_module {
-            uwriteln!(self.src, "//   * default-bindings-module: {default:?}");
+            uwriteln!(
+                self.src_preamble,
+                "//   * default-bindings-module: {default:?}"
+            );
         }
         if self.opts.disable_run_ctors_once_workaround {
-            uwriteln!(self.src, "//   * disable-run-ctors-once-workaround");
+            uwriteln!(
+                self.src_preamble,
+                "//   * disable-run-ctors-once-workaround"
+            );
         }
         if let Some(s) = &self.opts.export_macro_name {
-            uwriteln!(self.src, "//   * export-macro-name: {s}");
+            uwriteln!(self.src_preamble, "//   * export-macro-name: {s}");
         }
         if self.opts.pub_export_macro {
-            uwriteln!(self.src, "//   * pub-export-macro");
+            uwriteln!(self.src_preamble, "//   * pub-export-macro");
+        }
+        if self.opts.generate_unused_types {
+            uwriteln!(self.src_preamble, "//   * generate_unused_types");
+        }
+        if self.opts.disable_custom_section_link_helpers {
+            uwriteln!(
+                self.src_preamble,
+                "//   * disable_custom_section_link_helpers"
+            );
         }
         self.types.analyze(resolve);
         self.world = Some(world);
@@ -1101,6 +1148,11 @@ impl WorldGenerator for RustWasm {
             *src.as_mut_string() = prettyplease::unparse(&syntax_tree);
         }
 
+        // Prepend the preamble. We do this after formatting because
+        // `syn::parse_file` + `prettyplease::unparse` does not preserve comments.
+        let src_preamble = mem::take(&mut self.src_preamble);
+        *src.as_mut_string() = format!("{}{}", src_preamble.as_str(), src.as_str());
+
         let module_name = name.to_snake_case();
         files.push(&format!("{module_name}.rs"), src.as_bytes());