Consider only libs that aren't excluded by #[link(cfg=...)]

src/librustc_metadata/creader.rs

@@ -113,6 +113,13 @@ fn register_native_lib(sess: &Session,
     cstore.add_used_library(lib);
 }
 
+fn relevant_lib(sess: &Session, lib: &NativeLibrary) -> bool {
+    match lib.cfg {
+        Some(ref cfg) => attr::cfg_matches(cfg, &sess.parse_sess, None),
+        None => true,
+    }
+}
+
 // Extra info about a crate loaded for plugins or exported macros.
 struct ExtensionCrate {
     metadata: PMDSource,
@@ -290,7 +297,7 @@ impl<'a> CrateLoader<'a> {
 
         let cnum_map = self.resolve_crate_deps(root, &crate_root, &metadata, cnum, span, dep_kind);
 
-        let cmeta = Rc::new(cstore::CrateMetadata {
+        let mut cmeta = cstore::CrateMetadata {
             name: name,
             extern_crate: Cell::new(None),
             key_map: metadata.load_key_map(crate_root.index),
@@ -308,9 +315,18 @@ impl<'a> CrateLoader<'a> {
                 rlib: rlib,
                 rmeta: rmeta,
             },
-            dllimport_foreign_items: RefCell::new(None),
-        });
+            dllimport_foreign_items: FxHashSet(),
+        };
 
+        let dllimports: Vec<_> = cmeta.get_native_libraries().iter()
+                            .filter(|lib| relevant_lib(self.sess, lib) &&
+                                          lib.kind == cstore::NativeLibraryKind::NativeUnknown)
+                            .flat_map(|lib| &lib.foreign_items)
+                            .map(|id| *id)
+                            .collect();
+        cmeta.dllimport_foreign_items.extend(dllimports);
+
+        let cmeta = Rc::new(cmeta);
         self.cstore.set_crate_data(cnum, cmeta.clone());
         (cnum, cmeta)
     }
@@ -643,7 +659,7 @@ impl<'a> CrateLoader<'a> {
         let mut items = vec![];
         let libs = self.cstore.get_used_libraries();
         for lib in libs.borrow().iter() {
-            if lib.kind == kind {
+            if relevant_lib(self.sess, lib) && lib.kind == kind {
                 items.extend(&lib.foreign_items);
             }
         }

src/librustc_metadata/cstore.rs

@@ -85,7 +85,7 @@ pub struct CrateMetadata {
 
     pub proc_macros: Option<Vec<(ast::Name, Rc<SyntaxExtension>)>>,
     // Foreign items imported from a dylib (Windows only)
-    pub dllimport_foreign_items: RefCell<Option<FxHashSet<DefIndex>>>,
+    pub dllimport_foreign_items: FxHashSet<DefIndex>,
 }
 
 pub struct CachedInlinedItem {

src/librustc_metadata/decoder.rs

@@ -11,13 +11,13 @@
 // Decoding metadata from a single crate's metadata
 
 use astencode::decode_inlined_item;
-use cstore::{self, CrateMetadata, MetadataBlob, NativeLibrary, NativeLibraryKind};
+use cstore::{self, CrateMetadata, MetadataBlob, NativeLibrary};
 use index::Index;
 use schema::*;
 
 use rustc::hir::map as hir_map;
 use rustc::hir::map::{DefKey, DefPathData};
-use rustc::util::nodemap::{FxHashMap, FxHashSet};
+use rustc::util::nodemap::FxHashMap;
 use rustc::hir;
 use rustc::hir::intravisit::IdRange;
 
@@ -36,7 +36,6 @@ use rustc::mir::Mir;
 use std::borrow::Cow;
 use std::cell::Ref;
 use std::io;
-use std::iter::FromIterator;
 use std::mem;
 use std::str;
 use std::u32;
@@ -1089,15 +1088,7 @@ impl<'a, 'tcx> CrateMetadata {
     }
 
     pub fn is_dllimport_foreign_item(&self, id: DefIndex) -> bool {
-        if self.dllimport_foreign_items.borrow().is_none() {
-            *self.dllimport_foreign_items.borrow_mut() = Some(FxHashSet::from_iter(
-                self.get_native_libraries().iter()
-                    .filter(|lib| lib.kind == NativeLibraryKind::NativeUnknown)
-                    .flat_map(|lib| &lib.foreign_items)
-                    .map(|id| *id)
-            ));
-        }
-        self.dllimport_foreign_items.borrow().as_ref().unwrap().contains(&id)
+        self.dllimport_foreign_items.contains(&id)
     }
 
     pub fn is_defaulted_trait(&self, trait_id: DefIndex) -> bool {