Preserve the function table explicitly (#1970)

The main gc pass of unused items in wasm-bindgen was accidentally
removing the function table because we weren't properly rooting it in
the auxiliary section which has a few ways that imports can reference
the function table via intrinsics and closures.

Closes #1967

Author: alexcrichton

crates/cli-support/src/js/mod.rs

@@ -2889,7 +2889,7 @@ impl<'a> Context<'a> {
     }
 
     fn export_function_table(&mut self) -> Result<String, Error> {
-        match self.module.tables.main_function_table()? {
+        match self.aux.function_table {
             Some(id) => Ok(self.export_name_of(id)),
             None => bail!("no function table found in module"),
         }

crates/cli-support/src/wit/mod.rs

@@ -164,6 +164,12 @@ impl<'a> Context<'a> {
             // access to them while processing programs.
             self.descriptors.extend(descriptors);
 
+            // If any closures exist we need to prevent the function table from
+            // getting gc'd
+            if closure_imports.len() > 0 {
+                self.aux.function_table = self.module.tables.main_function_table()?;
+            }
+
             // Register all the injected closure imports as that they're expected
             // to manufacture a particular type of closure.
             //
@@ -310,6 +316,9 @@ impl<'a> Context<'a> {
     }
 
     fn bind_intrinsic(&mut self, id: ImportId, intrinsic: Intrinsic) -> Result<(), Error> {
+        if let Intrinsic::FunctionTable = intrinsic {
+            self.aux.function_table = self.module.tables.main_function_table()?;
+        }
         let id = self.import_adapter(id, intrinsic.signature(), AdapterJsImportKind::Normal)?;
         self.aux
             .import_map

crates/cli-support/src/wit/nonstandard.rs

@@ -50,6 +50,7 @@ pub struct WasmBindgenAux {
     /// Information about various internal functions used to manage the `anyref`
     /// table, later used to process JS bindings.
     pub anyref_table: Option<walrus::TableId>,
+    pub function_table: Option<walrus::TableId>,
     pub anyref_alloc: Option<walrus::FunctionId>,
     pub anyref_drop_slice: Option<walrus::FunctionId>,
 
@@ -366,6 +367,9 @@ impl walrus::CustomSection for WasmBindgenAux {
         if let Some(id) = self.anyref_table {
             roots.push_table(id);
         }
+        if let Some(id) = self.function_table {
+            roots.push_table(id);
+        }
         if let Some(id) = self.anyref_alloc {
             roots.push_func(id);
         }

crates/cli-support/src/wit/section.rs

@@ -50,6 +50,7 @@ pub fn add(module: &mut Module) -> Result<(), Error> {
         anyref_drop_slice: _,
         exn_store: _,
         shadow_stack_pointer: _,
+        function_table: _,
     } = *aux;
 
     let adapter_context = |id: AdapterId| {

crates/cli/tests/wasm-bindgen/main.rs

@@ -334,3 +334,21 @@ $",
     )?);
     Ok(())
 }
+
+#[test]
+fn function_table_preserved() {
+    let (mut cmd, _out_dir) = Project::new("function_table_preserved")
+        .file(
+            "src/lib.rs",
+            r#"
+                use wasm_bindgen::prelude::*;
+
+                #[wasm_bindgen]
+                pub fn bar() {
+                    Closure::wrap(Box::new(|| {}) as Box<dyn Fn()>);
+                }
+            "#,
+        )
+        .wasm_bindgen("");
+    cmd.assert().success();
+}