Rollup merge of rust-lang#32469 - nikomatsakis:shared-cgu, r=eddyb

make available monomorphizations shared by CGU

The current setup means that all generics are local to a codegen-unit,
which means massive duplication.

src/librustc_trans/trans/context.rs

@@ -80,6 +80,7 @@ pub struct SharedCrateContext<'a, 'tcx: 'a> {
     mir_map: &'a MirMap<'tcx>,
     mir_cache: RefCell<DefIdMap<Rc<mir::Mir<'tcx>>>>,
 
+    available_monomorphizations: RefCell<FnvHashSet<String>>,
     available_drop_glues: RefCell<FnvHashMap<DropGlueKind<'tcx>, String>>,
     use_dll_storage_attrs: bool,
 
@@ -105,7 +106,6 @@ pub struct LocalCrateContext<'tcx> {
     /// Cache instances of monomorphic and polymorphic items
     instances: RefCell<FnvHashMap<Instance<'tcx>, ValueRef>>,
     monomorphizing: RefCell<DefIdMap<usize>>,
-    available_monomorphizations: RefCell<FnvHashSet<String>>,
     /// Cache generated vtables
     vtables: RefCell<FnvHashMap<ty::PolyTraitRef<'tcx>, ValueRef>>,
     /// Cache of constant strings,
@@ -357,6 +357,7 @@ impl<'b, 'tcx> SharedCrateContext<'b, 'tcx> {
             },
             check_overflow: check_overflow,
             check_drop_flag_for_sanity: check_drop_flag_for_sanity,
+            available_monomorphizations: RefCell::new(FnvHashSet()),
             available_drop_glues: RefCell::new(FnvHashMap()),
             use_dll_storage_attrs: use_dll_storage_attrs,
             translation_items: RefCell::new(FnvHashMap()),
@@ -474,7 +475,6 @@ impl<'tcx> LocalCrateContext<'tcx> {
                 external_srcs: RefCell::new(NodeMap()),
                 instances: RefCell::new(FnvHashMap()),
                 monomorphizing: RefCell::new(DefIdMap()),
-                available_monomorphizations: RefCell::new(FnvHashSet()),
                 vtables: RefCell::new(FnvHashMap()),
                 const_cstr_cache: RefCell::new(FnvHashMap()),
                 const_unsized: RefCell::new(FnvHashMap()),
@@ -723,7 +723,7 @@ impl<'b, 'tcx> CrateContext<'b, 'tcx> {
     }
 
     pub fn available_monomorphizations<'a>(&'a self) -> &'a RefCell<FnvHashSet<String>> {
-        &self.local.available_monomorphizations
+        &self.shared.available_monomorphizations
     }
 
     pub fn available_drop_glues(&self) -> &RefCell<FnvHashMap<DropGlueKind<'tcx>, String>> {