Add support for version specifications

This commit adds a `#[wasm_bindgen(version = "...")]` attribute support. This
information is eventually written into a `__wasm_pack_unstable` section.
Currently this is a strawman for the proposal in ashleygwilliams/wasm-pack#101

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

@@ -5,6 +5,7 @@ use std::mem;
 use failure::{Error, ResultExt};
 use parity_wasm::elements::*;
 use parity_wasm;
+use serde_json;
 use shared;
 use wasm_gc;
 
@@ -29,6 +30,7 @@ pub struct Context<'a> {
     pub exported_classes: HashMap<String, ExportedClass>,
     pub function_table_needed: bool,
     pub run_descriptor: &'a Fn(&str) -> Vec<u32>,
+    pub module_versions: Vec<(String, String)>,
 }
 
 #[derive(Default)]
@@ -341,6 +343,7 @@ impl<'a> Context<'a> {
 
         self.export_table();
         self.gc()?;
+        self.add_wasm_pack_section();
 
         while js.contains("\n\n\n") {
             js = js.replace("\n\n\n", "\n\n");
@@ -1333,6 +1336,28 @@ impl<'a> Context<'a> {
         self.globals.push_str(s);
         self.globals.push_str("\n");
     }
+
+    fn add_wasm_pack_section(&mut self) {
+        if self.module_versions.len() == 0 {
+            return
+        }
+
+        #[derive(Serialize)]
+        struct WasmPackSchema<'a> {
+            version: &'a str,
+            modules: &'a [(String, String)],
+        }
+
+        let contents = serde_json::to_string(&WasmPackSchema {
+            version: "0.0.1",
+            modules: &self.module_versions,
+        }).unwrap();
+
+        let mut section = CustomSection::default();
+        *section.name_mut() = "__wasm_pack_unstable".to_string();
+        *section.payload_mut() = contents.into_bytes();
+        self.module.sections_mut().push(Section::Custom(section));
+    }
 }
 
 impl<'a, 'b> SubContext<'a, 'b> {
@@ -1423,6 +1448,7 @@ impl<'a, 'b> SubContext<'a, 'b> {
     }
 
     fn generate_import(&mut self, import: &shared::Import) -> Result<(), Error> {
+        self.validate_import_module(import)?;
         match import.kind {
             shared::ImportKind::Function(ref f) => {
                 self.generate_import_function(import, f)
@@ -1443,6 +1469,40 @@ impl<'a, 'b> SubContext<'a, 'b> {
         Ok(())
     }
 
+    fn validate_import_module(&mut self, import: &shared::Import)
+        -> Result<(), Error>
+    {
+        let version = match import.version {
+            Some(ref s) => s,
+            None => return Ok(()),
+        };
+        let module = match import.module {
+            Some(ref s) => s,
+            None => return Ok(()),
+        };
+        if module.starts_with("./") {
+            return Ok(())
+        }
+        let pkg = if module.starts_with("@") {
+            // Translate `@foo/bar/baz` to `@foo/bar` and `@foo/bar` to itself
+            let first_slash = match module.find('/') {
+                Some(i) => i,
+                None => {
+                    bail!("packages starting with `@` must be of the form \
+                           `@foo/bar`, but found: `{}`", module)
+                }
+            };
+            match module[first_slash + 1..].find('/') {
+                Some(i) => &module[..i],
+                None => module,
+            }
+        } else {
+            module.split('/').next().unwrap()
+        };
+        self.cx.module_versions.push((pkg.to_string(), version.clone()));
+        Ok(())
+    }
+
     fn generate_import_static(
         &mut self,
         info: &shared::Import,

crates/cli-support/src/lib.rs

@@ -1,5 +1,7 @@
 extern crate parity_wasm;
 extern crate wasm_bindgen_shared as shared;
+#[macro_use]
+extern crate serde_derive;
 extern crate serde_json;
 extern crate wasm_gc;
 extern crate wasmi;
@@ -131,6 +133,7 @@ impl Bindgen {
                 config: &self,
                 module: &mut module,
                 function_table_needed: false,
+                module_versions: Default::default(),
                 run_descriptor: &|name| {
                     let mut v = MyExternals(Vec::new());
                     let ret = instance