Add an option to opt-out of #[used] statics (#1021)

This is not needed for the wasi_snapshot_preview1 adapter in Wasmtime
and is causing issues with LLVM 19, so add an option to turn it off
which will be enabled in Wasmtime.

Author: alexcrichton

crates/guest-rust/macro/src/lib.rs

@@ -137,6 +137,9 @@ impl Parse for Config {
                     Opt::Features(f) => {
                         features.extend(f.into_iter().map(|f| f.value()));
                     }
+                    Opt::DisableCustomSectionLinkHelpers(disable) => {
+                        opts.disable_custom_section_link_helpers = disable.value();
+                    }
                 }
             }
         } else {
@@ -309,6 +312,7 @@ mod kw {
     syn::custom_keyword!(pub_export_macro);
     syn::custom_keyword!(generate_unused_types);
     syn::custom_keyword!(features);
+    syn::custom_keyword!(disable_custom_section_link_helpers);
 }
 
 #[derive(Clone)]
@@ -361,6 +365,7 @@ enum Opt {
     PubExportMacro(syn::LitBool),
     GenerateUnusedTypes(syn::LitBool),
     Features(Vec<syn::LitStr>),
+    DisableCustomSectionLinkHelpers(syn::LitBool),
 }
 
 impl Parse for Opt {
@@ -504,6 +509,10 @@ impl Parse for Opt {
             syn::bracketed!(contents in input);
             let list = Punctuated::<_, Token![,]>::parse_terminated(&contents)?;
             Ok(Opt::Features(list.into_iter().collect()))
+        } else if l.peek(kw::disable_custom_section_link_helpers) {
+            input.parse::<kw::disable_custom_section_link_helpers>()?;
+            input.parse::<Token![:]>()?;
+            Ok(Opt::DisableCustomSectionLinkHelpers(input.parse()?))
         } else {
             Err(l.error())
         }

crates/guest-rust/src/lib.rs

@@ -812,6 +812,12 @@
 ///     //
 ///     // By default this is an empty list.
 ///     features: ["foo", "bar", "baz"],
+///
+///     // Disables generation of a `#[used]` static to try harder to get the
+///     // custom section describing WIT types linked into the binary when
+///     // used in library-like situations. This is `false` by default with
+///     // `#[used]` statics being emitted.
+///     disable_custom_section_link_helpers: false,
 /// });
 /// ```
 ///

crates/rust/src/interface.rs

@@ -425,14 +425,23 @@ macro_rules! {macro_name} {{
     pub fn finish_append_submodule(mut self, snake: &str, module_path: Vec<String>) {
         let module = self.finish();
         let path_to_root = self.path_to_root();
+        let used_static = if self.gen.opts.disable_custom_section_link_helpers {
+            String::new()
+        } else {
+            format!(
+                "\
+                    #[used]
+                    #[doc(hidden)]
+                    static __FORCE_SECTION_REF: fn() =
+                        {path_to_root}__link_custom_section_describing_imports;
+                "
+            )
+        };
         let module = format!(
             "\
                 #[allow(dead_code, clippy::all)]
                 pub mod {snake} {{
-                    #[used]
-                    #[doc(hidden)]
-                    #[cfg(target_arch = \"wasm32\")]
-                    static __FORCE_SECTION_REF: fn() = {path_to_root}__link_custom_section_describing_imports;
+                    {used_static}
                     {module}
                 }}
 ",

crates/rust/src/lib.rs

@@ -226,6 +226,14 @@ pub struct Opts {
     /// Whether to generate unused structures, not generated by default (false)
     #[cfg_attr(feature = "clap", arg(long))]
     pub generate_unused_types: bool,
+
+    /// Whether or not to generate helper function/constants to help link custom
+    /// sections into the final output.
+    ///
+    /// Disabling this can shave a few bytes off a binary but makes
+    /// library-based usage of `generate!` prone to breakage.
+    #[cfg_attr(feature = "clap", arg(long))]
+    pub disable_custom_section_link_helpers: bool,
 }
 
 impl Opts {
@@ -816,7 +824,6 @@ macro_rules! __export_{world_name}_impl {{
                 "
                 #[inline(never)]
                 #[doc(hidden)]
-                #[cfg(target_arch = \"wasm32\")]
                 pub fn {func_name}() {{
                     {rt}::maybe_link_cabi_realloc();
                 }}
@@ -1075,7 +1082,11 @@ impl WorldGenerator for RustWasm {
             resolve_to_encode,
             world_to_encode,
             "encoded world",
-            Some("__link_custom_section_describing_imports"),
+            if self.opts.disable_custom_section_link_helpers {
+                None
+            } else {
+                Some("__link_custom_section_describing_imports")
+            },
         );
 
         if self.opts.stubs {

crates/rust/tests/codegen.rs

@@ -572,3 +572,19 @@ mod multiple_paths {
         generate_all,
     });
 }
+
+#[allow(unused)]
+mod generate_custom_section_link_helpers {
+    wit_bindgen::generate!({
+        inline: r#"
+            package a:b;
+
+            world test {
+                import a: interface {
+                    x: func();
+                }
+            }
+        "#,
+        disable_custom_section_link_helpers: true,
+    });
+}