Auto merge of #689 - fitzgen:Kowasaki-master, r=fitzgen

Emit `PhantomData<UnsafeCell<T>>` members for all generic parameters

This makes generated generic structs lifetime invariant, since we cannot know
the C++ type's true variance.

Fixes #506

Rebased + squashed version of #635; cc @Kowasaki

To resolve the conflicts and rebase and squash, I did essentially these commands:

```
$ git fetch servo
$ git checkout -b Kowasaki-master servo/master
$ git merge --squash --edit Kowasaki/master
$ cargo test
$ cd tests/expectations
$ cargo test
$ cd -
$ git commit --author="Kowasaki"
$ git push fitzgen Kowasaki-master
```

Hopefully that is useful for you in the future :) Thanks for fixing this issue!

1 files changed, 10 insertions, 1 deletions

diff --git a/src/codegen/mod.rs b/src/codegen/mod.rs
index 697c140f..7f0a822a 100644
--- a/src/codegen/mod.rs
+++ b/src/codegen/mod.rs
@@ -1465,11 +1465,20 @@ impl CodeGenerator for CompInfo {
         let mut generics = aster::AstBuilder::new().generics();
 
         if let Some(ref params) = used_template_params {
-            for ty in params.iter() {
+            for (idx, ty) in params.iter().enumerate() {
                 let param = ctx.resolve_type(*ty);
                 let name = param.name().unwrap();
                 let ident = ctx.rust_ident(name);
+
                 generics = generics.ty_param_id(ident);
+
+                let prefix = ctx.trait_prefix();
+                let phantom_ty = quote_ty!(
+                    ctx.ext_cx(),
+                    ::$prefix::marker::PhantomData<::$prefix::cell::UnsafeCell<$ident>>);
+                let phantom_field = StructFieldBuilder::named(format!("_phantom_{}", idx))
+                    .build_ty(phantom_ty);
+                fields.push(phantom_field);
             }
         }