cli-support: Ignore missing descriptor functions

This can happen when a nested dependency crate exports things but the root crate
doesn't use them. In these cases, it is fine to ignore the missing descriptor,
because the thing it describes was removed as dead code.

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

@@ -29,7 +29,7 @@ pub struct Context<'a> {
     pub imported_names: HashSet<String>,
     pub exported_classes: HashMap<String, ExportedClass>,
     pub function_table_needed: bool,
-    pub run_descriptor: &'a Fn(&str) -> Vec<u32>,
+    pub run_descriptor: &'a Fn(&str) -> Option<Vec<u32>>,
     pub module_versions: Vec<(String, String)>,
 }
 
@@ -474,7 +474,10 @@ impl<'a> Context<'a> {
         for field in class.fields.iter() {
             let wasm_getter = shared::struct_field_get(name, &field.name);
             let wasm_setter = shared::struct_field_set(name, &field.name);
-            let descriptor = self.describe(&wasm_getter);
+            let descriptor = match self.describe(&wasm_getter) {
+                None => continue,
+                Some(d) => d,
+            };
 
             let set = {
                 let mut cx = Js2Rust::new(&field.name, self);
@@ -1393,10 +1396,9 @@ impl<'a> Context<'a> {
         Ok(())
     }
 
-    fn describe(&self, name: &str) -> Descriptor {
+    fn describe(&self, name: &str) -> Option<Descriptor> {
         let name = format!("__wbindgen_describe_{}", name);
-        let ret = (self.run_descriptor)(&name);
-        Descriptor::decode(&ret)
+        (self.run_descriptor)(&name).map(|d| Descriptor::decode(&d))
     }
 
     fn global(&mut self, s: &str) {
@@ -1474,7 +1476,12 @@ impl<'a, 'b> SubContext<'a, 'b> {
         if let Some(ref class) = export.class {
             return self.generate_export_for_class(class, export);
         }
-        let descriptor = self.cx.describe(&export.function.name);
+
+        let descriptor = match self.cx.describe(&export.function.name) {
+            None => return Ok(()),
+            Some(d) => d,
+        };
+
         let (js, ts) = Js2Rust::new(&export.function.name, self.cx)
             .process(descriptor.unwrap_function())?
             .finish("function", &format!("wasm.{}", export.function.name));
@@ -1492,7 +1499,12 @@ impl<'a, 'b> SubContext<'a, 'b> {
         export: &shared::Export,
     ) -> Result<(), Error> {
         let wasm_name = shared::struct_function_export_name(class_name, &export.function.name);
-        let descriptor = self.cx.describe(&wasm_name);
+
+        let descriptor = match self.cx.describe(&wasm_name) {
+            None => return Ok(()),
+            Some(d) => d,
+        };
+
         let (js, ts) = Js2Rust::new(&export.function.name, self.cx)
             .method(export.method)
             .process(descriptor.unwrap_function())?
@@ -1603,7 +1615,10 @@ impl<'a, 'b> SubContext<'a, 'b> {
                                 import: &shared::ImportFunction)
         -> Result<(), Error>
     {
-        let descriptor = self.cx.describe(&import.shim);
+        let descriptor = match self.cx.describe(&import.shim) {
+            None => return Ok(()),
+            Some(d) => d,
+        };
 
         let target = match import.class {
             Some(ref class) if import.js_new => {

crates/cli-support/src/lib.rs

@@ -140,11 +140,19 @@ impl Bindgen {
                 module_versions: Default::default(),
                 run_descriptor: &|name| {
                     let mut v = MyExternals(Vec::new());
-                    let ret = instance
-                        .invoke_export(name, &[], &mut v)
-                        .expect("failed to run export");
-                    assert!(ret.is_none());
-                    v.0
+                    match instance.invoke_export(name, &[], &mut v) {
+                        Ok(None) => Some(v.0),
+                        Ok(Some(_)) => {
+                            unreachable!(
+                                "there is only one export, and we only return None from it"
+                            )
+                        },
+                        // Allow missing exported describe functions. This can
+                        // happen when a nested dependency crate exports things
+                        // but the root crate doesn't use them.
+                        Err(wasmi::Error::Function(_)) => None,
+                        Err(e) => panic!("unexpected error running descriptor: {}", e),
+                    }
                 },
             };
             for program in programs.iter() {
@@ -342,6 +350,7 @@ impl wasmi::ImportResolver for MyResolver {
 }
 
 struct MyExternals(Vec<u32>);
+
 #[derive(Debug)]
 struct MyError(String);