Eliminate duplicate types in the type section

This commit updates the `wasm-gc` pass of wasm-bindgen to eliminate
duplicate types in the type section, effectively enabling a gc of the
type section itself. The main purpose here is ensure that code generated
by `wasm-bindgen` itself doesn't have to go too far out of its way to
deduplicate at generation time, but rather it can rely on the gc pass to
clean up.

Note that this currently depends on paritytech/parity-wasm#231, but this
can be updated if that ends up not landing.

Cargo.toml

@@ -82,3 +82,4 @@ members = [
 
 [patch.crates-io]
 wasm-bindgen = { path = '.' }
+parity-wasm = { git = 'https://github.com/alexcrichton/parity-wasm', branch = 'hash' }

crates/gc/src/lib.rs

@@ -9,7 +9,7 @@ extern crate log;
 extern crate rustc_demangle;
 
 use std::mem;
-use std::collections::HashSet;
+use std::collections::{HashSet, HashMap};
 
 use parity_wasm::elements::*;
 
@@ -457,14 +457,20 @@ impl<'a> RemapContext<'a> {
         let mut memories = Vec::new();
 
         if let Some(s) = m.type_section() {
-            let mut removed = 0;
-            for i in 0..(s.types().len() as u32) {
-                if analysis.types.contains(&i) {
-                    types.push(i - removed);
+            let mut ntypes = 0;
+            let mut map = HashMap::new();
+            for (i, ty) in s.types().iter().enumerate() {
+                if analysis.types.contains(&(i as u32)) {
+                    if let Some(prev) = map.get(&ty) {
+                        types.push(*prev);
+                        continue
+                    }
+                    map.insert(ty, ntypes);
+                    types.push(ntypes);
+                    ntypes += 1;
                 } else {
                     debug!("gc type {}", i);
                     types.push(u32::max_value());
-                    removed += 1;
                 }
             }
         }