Auto merge of #37401 - eddyb:lazy-2, r=nikomatsakis

[2/n] rustc_metadata: move is_extern_item to trans.

*This is part of a series ([prev](https://github.com/rust-lang/rust/pull/37400) | [next](https://github.com/rust-lang/rust/pull/37402)) of patches designed to rework rustc into an out-of-order on-demand pipeline model for both better feature support (e.g. [MIR-based](https://github.com/solson/miri) early constant evaluation) and incremental execution of compiler passes (e.g. type-checking), with beneficial consequences to IDE support as well.
If any motivation is unclear, please ask for additional PR description clarifications or code comments.*
<hr>

Minor cleanup missed by #36551: `is_extern_item` is one of, if not the only `CrateStore` method who takes a `TyCtxt` but doesn't produce something cached in it, and such methods are going away.

src/librustc/middle/cstore.rs

@@ -164,7 +164,6 @@ pub trait CrateStore<'tcx> {
     fn is_const_fn(&self, did: DefId) -> bool;
     fn is_defaulted_trait(&self, did: DefId) -> bool;
     fn is_default_impl(&self, impl_did: DefId) -> bool;
-    fn is_extern_item<'a>(&self, tcx: TyCtxt<'a, 'tcx, 'tcx>, did: DefId) -> bool;
     fn is_foreign_item(&self, did: DefId) -> bool;
     fn is_statically_included_foreign_item(&self, id: ast::NodeId) -> bool;
 
@@ -331,8 +330,6 @@ impl<'tcx> CrateStore<'tcx> for DummyCrateStore {
     fn is_const_fn(&self, did: DefId) -> bool { bug!("is_const_fn") }
     fn is_defaulted_trait(&self, did: DefId) -> bool { bug!("is_defaulted_trait") }
     fn is_default_impl(&self, impl_did: DefId) -> bool { bug!("is_default_impl") }
-    fn is_extern_item<'a>(&self, tcx: TyCtxt<'a, 'tcx, 'tcx>, did: DefId) -> bool
-        { bug!("is_extern_item") }
     fn is_foreign_item(&self, did: DefId) -> bool { bug!("is_foreign_item") }
     fn is_statically_included_foreign_item(&self, id: ast::NodeId) -> bool { false }
 

src/librustc_metadata/cstore_impl.rs

@@ -206,11 +206,6 @@ impl<'tcx> CrateStore<'tcx> for cstore::CStore {
         self.get_crate_data(impl_did.krate).is_default_impl(impl_did.index)
     }
 
-    fn is_extern_item<'a>(&self, tcx: TyCtxt<'a, 'tcx, 'tcx>, did: DefId) -> bool {
-        self.dep_graph.read(DepNode::MetaData(did));
-        self.get_crate_data(did.krate).is_extern_item(did.index, tcx)
-    }
-
     fn is_foreign_item(&self, did: DefId) -> bool {
         self.get_crate_data(did.krate).is_foreign_item(did.index)
     }

src/librustc_metadata/decoder.rs

@@ -1009,30 +1009,6 @@ impl<'a, 'tcx> CrateMetadata {
         constness == hir::Constness::Const
     }
 
-    pub fn is_extern_item(&self, id: DefIndex, tcx: TyCtxt<'a, 'tcx, 'tcx>) -> bool {
-        let item = match self.maybe_entry(id) {
-            Some(item) => item.decode(self),
-            None => return false,
-        };
-        let applicable = match item.kind {
-            EntryKind::ImmStatic |
-            EntryKind::MutStatic |
-            EntryKind::ForeignImmStatic |
-            EntryKind::ForeignMutStatic => true,
-
-            EntryKind::Fn(_) |
-            EntryKind::ForeignFn(_) => self.get_generics(id, tcx).types.is_empty(),
-
-            _ => false,
-        };
-
-        if applicable {
-            attr::contains_extern_indicator(tcx.sess.diagnostic(), &self.get_attributes(&item))
-        } else {
-            false
-        }
-    }
-
     pub fn is_foreign_item(&self, id: DefIndex) -> bool {
         match self.entry(id).kind {
             EntryKind::ForeignImmStatic |

src/librustc_trans/base.rs

@@ -35,6 +35,7 @@ use back::link;
 use back::linker::LinkerInfo;
 use llvm::{Linkage, ValueRef, Vector, get_param};
 use llvm;
+use rustc::hir::def::Def;
 use rustc::hir::def_id::DefId;
 use middle::lang_items::{LangItem, ExchangeMallocFnLangItem, StartFnLangItem};
 use rustc::ty::subst::Substs;
@@ -1712,8 +1713,21 @@ pub fn trans_crate<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
     // `reachable_symbols` list later on so it should be ok.
     for cnum in sess.cstore.crates() {
         let syms = sess.cstore.reachable_ids(cnum);
-        reachable_symbols.extend(syms.into_iter().filter(|did| {
-            sess.cstore.is_extern_item(shared_ccx.tcx(), *did)
+        reachable_symbols.extend(syms.into_iter().filter(|&def_id| {
+            let applicable = match sess.cstore.describe_def(def_id) {
+                Some(Def::Static(..)) => true,
+                Some(Def::Fn(_)) => {
+                    shared_ccx.tcx().lookup_generics(def_id).types.is_empty()
+                }
+                _ => false
+            };
+
+            if applicable {
+                let attrs = shared_ccx.tcx().get_attrs(def_id);
+                attr::contains_extern_indicator(sess.diagnostic(), &attrs)
+            } else {
+                false
+            }
         }).map(|did| {
             symbol_for_def_id(did, &shared_ccx, &symbol_map)
         }));