Fix conditional #[wasm_bindgen] in impls

Reported in #1191 the fix requires us to get a bit creative I think. The
general gist is that a block like this:

    #[wasm_bindgen]
    impl Foo {
        pub fn foo() {}
    }

was previously expanded all in one go. Now, however, it's expanded into:

    impl Foo {
        #[__wasm_bindgen_class_marker(Foo = "Foo")]
        pub fn foo() {}
    }

    // goop generated by orginal #[wasm_bindgen]

This method of expansion takes advantage of rustc's recursive expansion
feature. It also allows us to expand `impl` blocks and allow inner items
to not be fully expanded yet, such as still having `#[cfg]` attributes
(like in the original bug report).

We use theinternal `__wasm_bindgen_class_marker` to indicate that we're
parsing an `ImplItemMethod` unconditionally, and then generation
proceeds as usual. The only final catch is that when we're expanding in
an `impl` block we have to generate tokens for the `Program`
(wasm-bindgen injected goop like the custom section) inside the body
of the function itself instead of next to it. Otherwise we'd get syntax
errors inside of impl blocks!

Closes #1191

crates/macro-support/src/lib.rs

@@ -13,8 +13,11 @@ extern crate wasm_bindgen_shared as shared;
 
 use backend::{Diagnostic, TryToTokens};
 pub use parser::BindgenAttrs;
+use quote::ToTokens;
 use parser::MacroParse;
 use proc_macro2::TokenStream;
+use syn::parse::{Parse, ParseStream, Result as SynResult};
+use quote::TokenStreamExt;
 
 mod parser;
 
@@ -36,3 +39,68 @@ pub fn expand(attr: TokenStream, input: TokenStream) -> Result<TokenStream, Diag
 
     Ok(tokens)
 }
+
+/// Takes the parsed input from a `#[wasm_bindgen]` macro and returns the generated bindings
+pub fn expand_class_marker(attr: TokenStream, input: TokenStream) -> Result<TokenStream, Diagnostic> {
+    parser::reset_attrs_used();
+    let mut item = syn::parse2::<syn::ImplItemMethod>(input)?;
+    let opts: ClassMarker = syn::parse2(attr)?;
+
+    let mut program = backend::ast::Program::default();
+    item.macro_parse(&mut program, (&opts.class, &opts.js_class))?;
+    parser::assert_all_attrs_checked(); // same as above
+
+    // This is where things are slightly different, we are being expanded in the
+    // context of an impl so we can't inject arbitrary item-like tokens into the
+    // output stream. If we were to do that then it wouldn't parse!
+    //
+    // Instead what we want to do is to generate the tokens for `program` into
+    // the header of the function. This'll inject some no_mangle functions and
+    // statics and such, and they should all be valid in the context of the
+    // start of a function.
+    //
+    // We manually implement `ToTokens for ImplItemMethod` here, injecting our
+    // program's tokens before the actual method's inner body tokens.
+    let mut tokens = proc_macro2::TokenStream::new();
+    tokens.append_all(item.attrs.iter().filter(|attr| {
+        match attr.style {
+            syn::AttrStyle::Outer => true,
+            _ => false,
+        }
+    }));
+    item.vis.to_tokens(&mut tokens);
+    item.sig.to_tokens(&mut tokens);
+    let mut err = None;
+    item.block.brace_token.surround(&mut tokens, |tokens| {
+        if let Err(e) = program.try_to_tokens(tokens) {
+            err = Some(e);
+        }
+        tokens.append_all(item.attrs.iter().filter(|attr| {
+            match attr.style {
+                syn::AttrStyle::Inner(_) => true,
+                _ => false,
+            }
+        }));
+        tokens.append_all(&item.block.stmts);
+    });
+
+    if let Some(err) = err {
+        return Err(err)
+    }
+
+    Ok(tokens)
+}
+
+struct ClassMarker {
+    class: syn::Ident,
+    js_class: String,
+}
+
+impl Parse for ClassMarker {
+    fn parse(input: ParseStream) -> SynResult<Self> {
+        let class = input.parse::<syn::Ident>()?;
+        input.parse::<Token![=]>()?;
+        let js_class = input.parse::<syn::LitStr>()?.value();
+        Ok(ClassMarker { class, js_class })
+    }
+}