diff --git a/src/Cargo.lock b/src/Cargo.lock
index ad96ff40cd687..123c884585c19 100644
--- a/src/Cargo.lock
+++ b/src/Cargo.lock
@@ -1327,7 +1327,6 @@ dependencies = [
  "graphviz 0.0.0",
  "log 0.3.8 (registry+https://github.com/rust-lang/crates.io-index)",
  "rustc 0.0.0",
- "rustc_data_structures 0.0.0",
  "rustc_errors 0.0.0",
  "rustc_mir 0.0.0",
  "syntax 0.0.0",
@@ -1464,7 +1463,6 @@ dependencies = [
  "proc_macro 0.0.0",
  "rustc 0.0.0",
  "rustc_back 0.0.0",
- "rustc_const_math 0.0.0",
  "rustc_data_structures 0.0.0",
  "rustc_errors 0.0.0",
  "serialize 0.0.0",
@@ -1605,6 +1603,7 @@ dependencies = [
 name = "rustc_tsan"
 version = "0.0.0"
 dependencies = [
+ "alloc 0.0.0",
  "alloc_system 0.0.0",
  "build_helper 0.1.0",
  "cmake 0.1.24 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -1859,7 +1858,6 @@ name = "syntax_ext"
 version = "0.0.0"
 dependencies = [
  "fmt_macros 0.0.0",
- "log 0.3.8 (registry+https://github.com/rust-lang/crates.io-index)",
  "proc_macro 0.0.0",
  "rustc_errors 0.0.0",
  "syntax 0.0.0",
diff --git a/src/liballoc/tests/lib.rs b/src/liballoc/tests/lib.rs
index 86309dd87de9b..8f3e71ef79446 100644
--- a/src/liballoc/tests/lib.rs
+++ b/src/liballoc/tests/lib.rs
@@ -10,7 +10,6 @@
 
 #![deny(warnings)]
 
-#![feature(alloc)]
 #![feature(attr_literals)]
 #![feature(box_syntax)]
 #![feature(inclusive_range_syntax)]
@@ -27,14 +26,10 @@
 #![feature(splice)]
 #![feature(str_escape)]
 #![feature(string_retain)]
-#![feature(test)]
 #![feature(unboxed_closures)]
 #![feature(unicode)]
 
-extern crate alloc;
-extern crate test;
 extern crate std_unicode;
-extern crate core;
 
 use std::hash::{Hash, Hasher};
 use std::collections::hash_map::DefaultHasher;
diff --git a/src/liballoc_jemalloc/lib.rs b/src/liballoc_jemalloc/lib.rs
index 513b299fcf37a..efefabc974c7f 100644
--- a/src/liballoc_jemalloc/lib.rs
+++ b/src/liballoc_jemalloc/lib.rs
@@ -19,7 +19,7 @@
 #![feature(libc)]
 #![feature(linkage)]
 #![feature(staged_api)]
-#![cfg_attr(dummy_jemalloc, allow(dead_code))]
+#![cfg_attr(dummy_jemalloc, allow(dead_code, unused_extern_crates))]
 #![cfg_attr(not(dummy_jemalloc), feature(allocator_api))]
 
 extern crate alloc;
diff --git a/src/libcore/tests/lib.rs b/src/libcore/tests/lib.rs
index 84a3be99c275f..ab2022b1824ca 100644
--- a/src/libcore/tests/lib.rs
+++ b/src/libcore/tests/lib.rs
@@ -26,7 +26,6 @@
 #![feature(inclusive_range)]
 #![feature(inclusive_range_syntax)]
 #![feature(iter_rfind)]
-#![feature(libc)]
 #![feature(nonzero)]
 #![feature(ord_max_min)]
 #![feature(rand)]
@@ -41,13 +40,10 @@
 #![feature(test)]
 #![feature(trusted_len)]
 #![feature(try_from)]
-#![feature(unicode)]
 #![feature(unique)]
 
 extern crate core;
 extern crate test;
-extern crate libc;
-extern crate std_unicode;
 extern crate rand;
 
 mod any;
diff --git a/src/libpanic_unwind/lib.rs b/src/libpanic_unwind/lib.rs
index 5a340f1323df5..558286f4ec070 100644
--- a/src/libpanic_unwind/lib.rs
+++ b/src/libpanic_unwind/lib.rs
@@ -34,7 +34,9 @@
 #![feature(core_intrinsics)]
 #![feature(lang_items)]
 #![feature(libc)]
-#![feature(panic_unwind)]
+#![cfg_attr(not(any(target_env = "msvc",
+                    all(windows, target_arch = "x86_64", target_env = "gnu"))),
+            feature(panic_unwind))]
 #![feature(raw)]
 #![feature(staged_api)]
 #![feature(unwind_attributes)]
@@ -45,6 +47,7 @@
 
 extern crate alloc;
 extern crate libc;
+#[cfg(not(any(target_env = "msvc", all(windows, target_arch = "x86_64", target_env = "gnu"))))]
 extern crate unwind;
 
 use core::intrinsics;
diff --git a/src/librustc/dep_graph/dep_node.rs b/src/librustc/dep_graph/dep_node.rs
index 9e5d40812311d..01fff60528394 100644
--- a/src/librustc/dep_graph/dep_node.rs
+++ b/src/librustc/dep_graph/dep_node.rs
@@ -522,6 +522,8 @@ define_dep_nodes!( <'tcx>
     [] DylibDepFormats(DefId),
     [] IsAllocator(DefId),
     [] IsPanicRuntime(DefId),
+    [] IsCompilerBuiltins(DefId),
+    [] HasGlobalAllocator(DefId),
     [] ExternCrate(DefId),
     [] LintLevels,
 );
diff --git a/src/librustc/lib.rs b/src/librustc/lib.rs
index 03dc2f3c1ac76..152b2e2aa5ebc 100644
--- a/src/librustc/lib.rs
+++ b/src/librustc/lib.rs
@@ -26,7 +26,7 @@
 #![feature(core_intrinsics)]
 #![feature(discriminant_value)]
 #![feature(i128_type)]
-#![feature(libc)]
+#![cfg_attr(windows, feature(libc))]
 #![feature(never_type)]
 #![feature(nonzero)]
 #![feature(quote)]
@@ -45,6 +45,7 @@ extern crate core;
 extern crate fmt_macros;
 extern crate getopts;
 extern crate graphviz;
+#[cfg(windows)]
 extern crate libc;
 extern crate owning_ref;
 extern crate rustc_back;
@@ -62,7 +63,9 @@ extern crate serialize as rustc_serialize; // used by deriving
 
 // Note that librustc doesn't actually depend on these crates, see the note in
 // `Cargo.toml` for this crate about why these are here.
+#[allow(unused_extern_crates)]
 extern crate flate2;
+#[allow(unused_extern_crates)]
 extern crate test;
 
 #[macro_use]
diff --git a/src/librustc/lint/builtin.rs b/src/librustc/lint/builtin.rs
index cbe642a9a76a6..811bf9776101d 100644
--- a/src/librustc/lint/builtin.rs
+++ b/src/librustc/lint/builtin.rs
@@ -30,7 +30,7 @@ declare_lint! {
 
 declare_lint! {
     pub UNUSED_EXTERN_CRATES,
-    Allow,
+    Warn,
     "extern crates that are never used"
 }
 
diff --git a/src/librustc/ty/context.rs b/src/librustc/ty/context.rs
index da1039c5de176..6ce2232eb3e5d 100644
--- a/src/librustc/ty/context.rs
+++ b/src/librustc/ty/context.rs
@@ -797,6 +797,8 @@ pub struct GlobalCtxt<'tcx> {
 
     pub maybe_unused_trait_imports: NodeSet,
 
+    pub maybe_unused_extern_crates: Vec<(NodeId, Span)>,
+
     // Internal cache for metadata decoding. No need to track deps on this.
     pub rcache: RefCell<FxHashMap<ty::CReaderCacheKey, Ty<'tcx>>>,
 
@@ -1038,6 +1040,7 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
             mir_passes,
             freevars: RefCell::new(resolutions.freevars),
             maybe_unused_trait_imports: resolutions.maybe_unused_trait_imports,
+            maybe_unused_extern_crates: resolutions.maybe_unused_extern_crates,
             rcache: RefCell::new(FxHashMap()),
             normalized_cache: RefCell::new(FxHashMap()),
             inhabitedness_cache: RefCell::new(FxHashMap()),
diff --git a/src/librustc/ty/maps.rs b/src/librustc/ty/maps.rs
index 042ec49b0bda1..a73202ced61e6 100644
--- a/src/librustc/ty/maps.rs
+++ b/src/librustc/ty/maps.rs
@@ -516,6 +516,18 @@ impl<'tcx> QueryDescription for queries::is_panic_runtime<'tcx> {
     }
 }
 
+impl<'tcx> QueryDescription for queries::is_compiler_builtins<'tcx> {
+    fn describe(_: TyCtxt, _: DefId) -> String {
+        "checking if the crate is_compiler_builtins".to_string()
+    }
+}
+
+impl<'tcx> QueryDescription for queries::has_global_allocator<'tcx> {
+    fn describe(_: TyCtxt, _: DefId) -> String {
+        "checking if the crate has_global_allocator".to_string()
+    }
+}
+
 impl<'tcx> QueryDescription for queries::extern_crate<'tcx> {
     fn describe(_: TyCtxt, _: DefId) -> String {
         "getting crate's ExternCrateData".to_string()
@@ -1079,6 +1091,8 @@ define_maps! { <'tcx>
 
     [] is_allocator: IsAllocator(DefId) -> bool,
     [] is_panic_runtime: IsPanicRuntime(DefId) -> bool,
+    [] is_compiler_builtins: IsCompilerBuiltins(DefId) -> bool,
+    [] has_global_allocator: HasGlobalAllocator(DefId) -> bool,
 
     [] extern_crate: ExternCrate(DefId) -> Rc<Option<ExternCrate>>,
 
diff --git a/src/librustc/ty/mod.rs b/src/librustc/ty/mod.rs
index 852bd48a5eeed..6597dccf25816 100644
--- a/src/librustc/ty/mod.rs
+++ b/src/librustc/ty/mod.rs
@@ -131,6 +131,7 @@ pub struct Resolutions {
     pub freevars: FreevarMap,
     pub trait_map: TraitMap,
     pub maybe_unused_trait_imports: NodeSet,
+    pub maybe_unused_extern_crates: Vec<(NodeId, Span)>,
     pub export_map: ExportMap,
 }
 
diff --git a/src/librustc_borrowck/Cargo.toml b/src/librustc_borrowck/Cargo.toml
index af99c0e938724..25f02537490fa 100644
--- a/src/librustc_borrowck/Cargo.toml
+++ b/src/librustc_borrowck/Cargo.toml
@@ -15,6 +15,5 @@ syntax = { path = "../libsyntax" }
 syntax_pos = { path = "../libsyntax_pos" }
 graphviz = { path = "../libgraphviz" }
 rustc = { path = "../librustc" }
-rustc_data_structures = { path = "../librustc_data_structures" }
 rustc_mir = { path = "../librustc_mir" }
 rustc_errors = { path = "../librustc_errors" }
diff --git a/src/librustc_borrowck/lib.rs b/src/librustc_borrowck/lib.rs
index e7f4f9caee335..9bedbfed5db6d 100644
--- a/src/librustc_borrowck/lib.rs
+++ b/src/librustc_borrowck/lib.rs
@@ -28,9 +28,7 @@ extern crate rustc_errors as errors;
 extern crate graphviz as dot;
 #[macro_use]
 extern crate rustc;
-extern crate rustc_data_structures;
 extern crate rustc_mir;
-extern crate core; // for NonZero
 
 pub use borrowck::check_crate;
 pub use borrowck::build_borrowck_dataflow_data_for_fn;
diff --git a/src/librustc_driver/driver.rs b/src/librustc_driver/driver.rs
index 163c698e3ff31..96688c6ac9cdb 100644
--- a/src/librustc_driver/driver.rs
+++ b/src/librustc_driver/driver.rs
@@ -877,6 +877,7 @@ pub fn phase_2_configure_and_expand<F>(sess: &Session,
             export_map: resolver.export_map,
             trait_map: resolver.trait_map,
             maybe_unused_trait_imports: resolver.maybe_unused_trait_imports,
+            maybe_unused_extern_crates: resolver.maybe_unused_extern_crates,
         },
         hir_forest,
     })
diff --git a/src/librustc_driver/lib.rs b/src/librustc_driver/lib.rs
index e56c989b843d1..1915a1c86484a 100644
--- a/src/librustc_driver/lib.rs
+++ b/src/librustc_driver/lib.rs
@@ -20,7 +20,7 @@
 #![deny(warnings)]
 
 #![feature(box_syntax)]
-#![feature(libc)]
+#![cfg_attr(unix, feature(libc))]
 #![feature(quote)]
 #![feature(rustc_diagnostic_macros)]
 #![feature(set_stdio)]
@@ -29,6 +29,7 @@ extern crate arena;
 extern crate getopts;
 extern crate graphviz;
 extern crate env_logger;
+#[cfg(unix)]
 extern crate libc;
 extern crate rustc;
 extern crate rustc_allocator;
diff --git a/src/librustc_errors/lib.rs b/src/librustc_errors/lib.rs
index 2f5aac65b923d..870bb01bb9ffb 100644
--- a/src/librustc_errors/lib.rs
+++ b/src/librustc_errors/lib.rs
@@ -16,10 +16,11 @@
 #![feature(custom_attribute)]
 #![allow(unused_attributes)]
 #![feature(range_contains)]
-#![feature(libc)]
+#![cfg_attr(unix, feature(libc))]
 #![feature(conservative_impl_trait)]
 
 extern crate term;
+#[cfg(unix)]
 extern crate libc;
 extern crate serialize as rustc_serialize;
 extern crate syntax_pos;
diff --git a/src/librustc_metadata/Cargo.toml b/src/librustc_metadata/Cargo.toml
index 67b459ea18f4a..40b75be36fefb 100644
--- a/src/librustc_metadata/Cargo.toml
+++ b/src/librustc_metadata/Cargo.toml
@@ -15,7 +15,6 @@ owning_ref = "0.3.3"
 proc_macro = { path = "../libproc_macro" }
 rustc = { path = "../librustc" }
 rustc_back = { path = "../librustc_back" }
-rustc_const_math = { path = "../librustc_const_math" }
 rustc_data_structures = { path = "../librustc_data_structures" }
 rustc_errors = { path = "../librustc_errors" }
 serialize = { path = "../libserialize" }
diff --git a/src/librustc_metadata/cstore_impl.rs b/src/librustc_metadata/cstore_impl.rs
index cfba11b5151a2..ad320a7ff3d1e 100644
--- a/src/librustc_metadata/cstore_impl.rs
+++ b/src/librustc_metadata/cstore_impl.rs
@@ -137,6 +137,8 @@ provide! { <'tcx> tcx, def_id, cdata,
 
     dylib_dependency_formats => { Rc::new(cdata.get_dylib_dependency_formats(&tcx.dep_graph)) }
     is_panic_runtime => { cdata.is_panic_runtime(&tcx.dep_graph) }
+    is_compiler_builtins => { cdata.is_compiler_builtins(&tcx.dep_graph) }
+    has_global_allocator => { cdata.has_global_allocator(&tcx.dep_graph) }
     extern_crate => { Rc::new(cdata.extern_crate.get()) }
 }
 
diff --git a/src/librustc_metadata/lib.rs b/src/librustc_metadata/lib.rs
index 7d796dbd00f8f..f79abecf9da4b 100644
--- a/src/librustc_metadata/lib.rs
+++ b/src/librustc_metadata/lib.rs
@@ -39,7 +39,6 @@ extern crate proc_macro;
 #[macro_use]
 extern crate rustc;
 extern crate rustc_back;
-extern crate rustc_const_math;
 extern crate rustc_data_structures;
 
 mod diagnostics;
diff --git a/src/librustc_resolve/check_unused.rs b/src/librustc_resolve/check_unused.rs
index 05c227340d868..a66d1ce0859b7 100644
--- a/src/librustc_resolve/check_unused.rs
+++ b/src/librustc_resolve/check_unused.rs
@@ -121,9 +121,7 @@ pub fn check_crate(resolver: &mut Resolver, krate: &ast::Crate) {
                  directive.vis.get() == ty::Visibility::Public ||
                  directive.span.source_equal(&DUMMY_SP) => {}
             ImportDirectiveSubclass::ExternCrate => {
-                let lint = lint::builtin::UNUSED_EXTERN_CRATES;
-                let msg = "unused extern crate";
-             ;   resolver.session.buffer_lint(lint, directive.id, directive.span, msg)
+                resolver.maybe_unused_extern_crates.push((directive.id, directive.span));
             }
             ImportDirectiveSubclass::MacroUse => {
                 let lint = lint::builtin::UNUSED_IMPORTS;
diff --git a/src/librustc_resolve/lib.rs b/src/librustc_resolve/lib.rs
index ee349e3112889..a83ac9bb63369 100644
--- a/src/librustc_resolve/lib.rs
+++ b/src/librustc_resolve/lib.rs
@@ -1250,6 +1250,7 @@ pub struct Resolver<'a> {
 
     used_imports: FxHashSet<(NodeId, Namespace)>,
     pub maybe_unused_trait_imports: NodeSet,
+    pub maybe_unused_extern_crates: Vec<(NodeId, Span)>,
 
     /// privacy errors are delayed until the end in order to deduplicate them
     privacy_errors: Vec<PrivacyError<'a>>,
@@ -1457,6 +1458,7 @@ impl<'a> Resolver<'a> {
 
             used_imports: FxHashSet(),
             maybe_unused_trait_imports: NodeSet(),
+            maybe_unused_extern_crates: Vec::new(),
 
             privacy_errors: Vec::new(),
             ambiguity_errors: Vec::new(),
diff --git a/src/librustc_tsan/Cargo.toml b/src/librustc_tsan/Cargo.toml
index 97c1181e3a220..7b83985ba6731 100644
--- a/src/librustc_tsan/Cargo.toml
+++ b/src/librustc_tsan/Cargo.toml
@@ -14,5 +14,6 @@ build_helper = { path = "../build_helper" }
 cmake = "0.1.18"
 
 [dependencies]
+alloc = { path = "../liballoc" }
 alloc_system = { path = "../liballoc_system" }
 core = { path = "../libcore" }
diff --git a/src/librustc_tsan/lib.rs b/src/librustc_tsan/lib.rs
index 54941362e8450..3429e3bda0f67 100644
--- a/src/librustc_tsan/lib.rs
+++ b/src/librustc_tsan/lib.rs
@@ -9,8 +9,10 @@
 // except according to those terms.
 
 #![sanitizer_runtime]
-#![feature(sanitizer_runtime)]
 #![feature(alloc_system)]
+#![feature(allocator_api)]
+#![feature(global_allocator)]
+#![feature(sanitizer_runtime)]
 #![feature(staged_api)]
 #![no_std]
 #![unstable(feature = "sanitizer_runtime_lib",
@@ -18,3 +20,8 @@
             issue = "0")]
 
 extern crate alloc_system;
+
+use alloc_system::System;
+
+#[global_allocator]
+static ALLOC: System = System;
diff --git a/src/librustc_typeck/check_unused.rs b/src/librustc_typeck/check_unused.rs
index e95d49f00bf76..3da154e0689ad 100644
--- a/src/librustc_typeck/check_unused.rs
+++ b/src/librustc_typeck/check_unused.rs
@@ -72,4 +72,15 @@ pub fn check_crate<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>) {
 
     let mut visitor = CheckVisitor { tcx, used_trait_imports };
     tcx.hir.krate().visit_all_item_likes(&mut visitor);
+
+    for &(id, span) in &tcx.maybe_unused_extern_crates {
+        let cnum = tcx.sess.cstore.extern_mod_stmt_cnum(id).unwrap().as_def_id();
+        if !tcx.is_compiler_builtins(cnum)
+            && !tcx.is_panic_runtime(cnum)
+            && !tcx.has_global_allocator(cnum) {
+                let lint = lint::builtin::UNUSED_EXTERN_CRATES;
+                let msg = "unused extern crate";
+                tcx.lint_node(lint, id, span, msg);
+            }
+    }
 }
diff --git a/src/libstd/lib.rs b/src/libstd/lib.rs
index cf1eb5cd52ea4..b57067e35e9d9 100644
--- a/src/libstd/lib.rs
+++ b/src/libstd/lib.rs
@@ -358,6 +358,7 @@ extern crate std_unicode;
 extern crate libc;
 
 // We always need an unwinder currently for backtraces
+#[allow(unused_extern_crates)]
 extern crate unwind;
 
 // compiler-rt intrinsics
diff --git a/src/libstd/prelude/mod.rs b/src/libstd/prelude/mod.rs
index 49cdba21a1d37..538753d86923e 100644
--- a/src/libstd/prelude/mod.rs
+++ b/src/libstd/prelude/mod.rs
@@ -23,6 +23,7 @@
 //! On a technical level, Rust inserts
 //!
 //! ```
+//! # #[allow(unused_extern_crates)]
 //! extern crate std;
 //! ```
 //!
diff --git a/src/libstd/sys/unix/ext/fs.rs b/src/libstd/sys/unix/ext/fs.rs
index a94585723a1ee..f44b9aa961568 100644
--- a/src/libstd/sys/unix/ext/fs.rs
+++ b/src/libstd/sys/unix/ext/fs.rs
@@ -154,8 +154,6 @@ pub trait OpenOptionsExt {
     /// # Examples
     ///
     /// ```no_run
-    /// # #![feature(libc)]
-    /// extern crate libc;
     /// use std::fs::OpenOptions;
     /// use std::os::unix::fs::OpenOptionsExt;
     ///
diff --git a/src/libsyntax_ext/Cargo.toml b/src/libsyntax_ext/Cargo.toml
index bdcec26cb838b..1c4702402886d 100644
--- a/src/libsyntax_ext/Cargo.toml
+++ b/src/libsyntax_ext/Cargo.toml
@@ -10,7 +10,6 @@ crate-type = ["dylib"]
 
 [dependencies]
 fmt_macros = { path = "../libfmt_macros" }
-log = "0.3"
 proc_macro = { path = "../libproc_macro" }
 rustc_errors = { path = "../librustc_errors" }
 syntax = { path = "../libsyntax" }
diff --git a/src/libsyntax_ext/lib.rs b/src/libsyntax_ext/lib.rs
index fa39095d3298b..42bbb4ae0cbee 100644
--- a/src/libsyntax_ext/lib.rs
+++ b/src/libsyntax_ext/lib.rs
@@ -18,7 +18,6 @@
 #![feature(proc_macro_internals)]
 
 extern crate fmt_macros;
-extern crate log;
 #[macro_use]
 extern crate syntax;
 extern crate syntax_pos;
diff --git a/src/libtest/lib.rs b/src/libtest/lib.rs
index 652bfa82b5c71..642eb28556408 100644
--- a/src/libtest/lib.rs
+++ b/src/libtest/lib.rs
@@ -35,13 +35,14 @@
 #![deny(warnings)]
 
 #![feature(asm)]
-#![feature(libc)]
+#![cfg_attr(unix, feature(libc))]
 #![feature(set_stdio)]
 #![feature(panic_unwind)]
 #![feature(staged_api)]
 
 extern crate getopts;
 extern crate term;
+#[cfg(unix)]
 extern crate libc;
 extern crate panic_unwind;
 
diff --git a/src/test/compile-fail-fulldeps/plugin-as-extern-crate.rs b/src/test/compile-fail-fulldeps/plugin-as-extern-crate.rs
index edbb77fe39063..bb292e2e52a95 100644
--- a/src/test/compile-fail-fulldeps/plugin-as-extern-crate.rs
+++ b/src/test/compile-fail-fulldeps/plugin-as-extern-crate.rs
@@ -15,6 +15,7 @@
 // libsyntax is not compiled for it.
 
 #![deny(plugin_as_library)]
+#![allow(unused_extern_crates)]
 
 extern crate macro_crate_test; //~ ERROR compiler plugin used as an ordinary library
 
diff --git a/src/test/compile-fail/E0254.rs b/src/test/compile-fail/E0254.rs
index 89227f6b0100c..996a6b97cd998 100644
--- a/src/test/compile-fail/E0254.rs
+++ b/src/test/compile-fail/E0254.rs
@@ -9,6 +9,7 @@
 // except according to those terms.
 
 #![feature(alloc)]
+#![allow(unused_extern_crates)]
 
 extern crate alloc;
 //~^ NOTE previous import of the extern crate `alloc` here
diff --git a/src/test/compile-fail/E0259.rs b/src/test/compile-fail/E0259.rs
index 60bcd2ae07649..c285c4d9e00c1 100644
--- a/src/test/compile-fail/E0259.rs
+++ b/src/test/compile-fail/E0259.rs
@@ -9,6 +9,7 @@
 // except according to those terms.
 
 #![feature(alloc, libc)]
+#![allow(unused_extern_crates)]
 
 extern crate alloc;
 //~^ NOTE previous import of the extern crate `alloc` here
diff --git a/src/test/compile-fail/E0260.rs b/src/test/compile-fail/E0260.rs
index 5e802bbbe3dda..ad8888e58f79a 100644
--- a/src/test/compile-fail/E0260.rs
+++ b/src/test/compile-fail/E0260.rs
@@ -9,6 +9,7 @@
 // except according to those terms.
 
 #![feature(alloc)]
+#![allow(unused_extern_crates)]
 
 extern crate alloc;
 //~^ NOTE previous import of the extern crate `alloc` here
diff --git a/src/test/compile-fail/auxiliary/lint_unused_extern_crate5.rs b/src/test/compile-fail/auxiliary/lint_unused_extern_crate5.rs
new file mode 100644
index 0000000000000..fc4bca865c932
--- /dev/null
+++ b/src/test/compile-fail/auxiliary/lint_unused_extern_crate5.rs
@@ -0,0 +1,9 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
diff --git a/src/test/compile-fail/enable-unstable-lib-feature.rs b/src/test/compile-fail/enable-unstable-lib-feature.rs
index c65b2366bf10f..bc9e2345f0ebb 100644
--- a/src/test/compile-fail/enable-unstable-lib-feature.rs
+++ b/src/test/compile-fail/enable-unstable-lib-feature.rs
@@ -16,6 +16,7 @@
 #![deny(non_snake_case)] // To trigger a hard error
 
 // Shouldn't generate a warning about unstable features
+#[allow(unused_extern_crates)]
 extern crate stability_cfg2;
 
 pub fn BOGUS() { } //~ ERROR
diff --git a/src/test/compile-fail/issue-36881.rs b/src/test/compile-fail/issue-36881.rs
index e05dc06619969..0f5aa24926beb 100644
--- a/src/test/compile-fail/issue-36881.rs
+++ b/src/test/compile-fail/issue-36881.rs
@@ -11,6 +11,7 @@
 // aux-build:issue-36881-aux.rs
 
 fn main() {
+    #[allow(unused_extern_crates)]
     extern crate issue_36881_aux;
     use issue_36881_aux::Foo; //~ ERROR unresolved import
 }
diff --git a/src/test/compile-fail/lint-stability-deprecated.rs b/src/test/compile-fail/lint-stability-deprecated.rs
index 8443518b3f568..9bc2c021904aa 100644
--- a/src/test/compile-fail/lint-stability-deprecated.rs
+++ b/src/test/compile-fail/lint-stability-deprecated.rs
@@ -14,7 +14,7 @@
 // aux-build:stability_cfg2.rs
 
 #![warn(deprecated)]
-#![allow(dead_code)]
+#![allow(dead_code, unused_extern_crates)]
 #![feature(staged_api, test_feature, rustc_attrs)]
 
 #![stable(feature = "rust1", since = "1.0.0")]
diff --git a/src/test/compile-fail/lint-unused-extern-crate.rs b/src/test/compile-fail/lint-unused-extern-crate.rs
index b12ef6277bb4b..a3cfa1349831d 100644
--- a/src/test/compile-fail/lint-unused-extern-crate.rs
+++ b/src/test/compile-fail/lint-unused-extern-crate.rs
@@ -12,12 +12,15 @@
 // aux-build:lint_unused_extern_crate2.rs
 // aux-build:lint_unused_extern_crate3.rs
 // aux-build:lint_unused_extern_crate4.rs
+// aux-build:lint_unused_extern_crate5.rs
 
 #![deny(unused_extern_crates)]
 #![allow(unused_variables)]
 #![allow(deprecated)]
 
-extern crate lint_unused_extern_crate4; //~ ERROR: unused extern crate
+extern crate lint_unused_extern_crate5; //~ ERROR: unused extern crate
+
+pub extern crate lint_unused_extern_crate4; // no error, it is reexported
 
 extern crate lint_unused_extern_crate3; // no error, it is used
 
diff --git a/src/test/compile-fail/macro-reexport-malformed-1.rs b/src/test/compile-fail/macro-reexport-malformed-1.rs
index ea2dfca0714fc..a2778a831306b 100644
--- a/src/test/compile-fail/macro-reexport-malformed-1.rs
+++ b/src/test/compile-fail/macro-reexport-malformed-1.rs
@@ -11,5 +11,6 @@
 #![no_std]
 #![feature(macro_reexport)]
 
+#[allow(unused_extern_crates)]
 #[macro_reexport]  //~ ERROR bad macro reexport
 extern crate std;
diff --git a/src/test/compile-fail/macro-reexport-malformed-2.rs b/src/test/compile-fail/macro-reexport-malformed-2.rs
index 844955fb7e664..c5af9e3799de7 100644
--- a/src/test/compile-fail/macro-reexport-malformed-2.rs
+++ b/src/test/compile-fail/macro-reexport-malformed-2.rs
@@ -11,5 +11,6 @@
 #![no_std]
 #![feature(macro_reexport)]
 
+#[allow(unused_extern_crates)]
 #[macro_reexport="foo"]  //~ ERROR bad macro reexport
 extern crate std;
diff --git a/src/test/compile-fail/macro-reexport-malformed-3.rs b/src/test/compile-fail/macro-reexport-malformed-3.rs
index 381c22854e654..d72d1ee004ef7 100644
--- a/src/test/compile-fail/macro-reexport-malformed-3.rs
+++ b/src/test/compile-fail/macro-reexport-malformed-3.rs
@@ -11,5 +11,6 @@
 #![no_std]
 #![feature(macro_reexport)]
 
+#[allow(unused_extern_crates)]
 #[macro_reexport(foo="bar")]  //~ ERROR bad macro reexport
 extern crate std;
diff --git a/src/test/compile-fail/macro-use-bad-args-1.rs b/src/test/compile-fail/macro-use-bad-args-1.rs
index 39c09c6977963..a07cc83441173 100644
--- a/src/test/compile-fail/macro-use-bad-args-1.rs
+++ b/src/test/compile-fail/macro-use-bad-args-1.rs
@@ -10,5 +10,6 @@
 
 #![no_std]
 
+#[allow(unused_extern_crates)]
 #[macro_use(foo(bar))]  //~ ERROR bad macro import
 extern crate std;
diff --git a/src/test/compile-fail/macro-use-bad-args-2.rs b/src/test/compile-fail/macro-use-bad-args-2.rs
index 11a0108b99b89..89004f1689774 100644
--- a/src/test/compile-fail/macro-use-bad-args-2.rs
+++ b/src/test/compile-fail/macro-use-bad-args-2.rs
@@ -10,5 +10,6 @@
 
 #![no_std]
 
+#[allow(unused_extern_crates)]
 #[macro_use(foo="bar")]  //~ ERROR bad macro import
 extern crate std;
diff --git a/src/test/compile-fail/no-std-inject.rs b/src/test/compile-fail/no-std-inject.rs
index f384eafa34bdf..49064853d21d6 100644
--- a/src/test/compile-fail/no-std-inject.rs
+++ b/src/test/compile-fail/no-std-inject.rs
@@ -9,6 +9,7 @@
 // except according to those terms.
 
 #![no_std]
+#![allow(unused_extern_crates)]
 
 extern crate core; //~ ERROR: the name `core` is defined multiple times
 extern crate std;
diff --git a/src/test/compile-fail/placement-expr-unstable.rs b/src/test/compile-fail/placement-expr-unstable.rs
index cc73cbe15fe5f..35695efe1a9b2 100644
--- a/src/test/compile-fail/placement-expr-unstable.rs
+++ b/src/test/compile-fail/placement-expr-unstable.rs
@@ -12,8 +12,6 @@
 
 #![feature(placement_in_syntax)]
 
-extern crate core;
-
 fn main() {
     use std::boxed::HEAP; //~ ERROR use of unstable library feature
 
diff --git a/src/test/compile-fail/resolve-conflict-extern-crate-vs-extern-crate.rs b/src/test/compile-fail/resolve-conflict-extern-crate-vs-extern-crate.rs
index 87a17c0f19a6d..6a04a2c370430 100644
--- a/src/test/compile-fail/resolve-conflict-extern-crate-vs-extern-crate.rs
+++ b/src/test/compile-fail/resolve-conflict-extern-crate-vs-extern-crate.rs
@@ -8,6 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#[allow(unused_extern_crates)]
 extern crate std;
 //~^ ERROR the name `std` is defined multiple times
 
diff --git a/src/test/compile-fail/resolve_self_super_hint.rs b/src/test/compile-fail/resolve_self_super_hint.rs
index 099513484480b..a89fd802baf0c 100644
--- a/src/test/compile-fail/resolve_self_super_hint.rs
+++ b/src/test/compile-fail/resolve_self_super_hint.rs
@@ -9,6 +9,7 @@
 // except according to those terms.
 
 #![feature(alloc)]
+#![allow(unused_extern_crates)]
 
 mod a {
     extern crate alloc;
diff --git a/src/test/compile-fail/unused-attr.rs b/src/test/compile-fail/unused-attr.rs
index 6416e1cacdc0c..e20d03478ec18 100644
--- a/src/test/compile-fail/unused-attr.rs
+++ b/src/test/compile-fail/unused-attr.rs
@@ -9,7 +9,7 @@
 // except according to those terms.
 
 #![deny(unused_attributes)]
-#![allow(dead_code, unused_imports)]
+#![allow(dead_code, unused_imports, unused_extern_crates)]
 #![feature(custom_attribute)]
 
 #![foo] //~ ERROR unused attribute
diff --git a/src/tools/cargo b/src/tools/cargo
index 7847f75bb6fc4..3d3f2c05d742e 160000
--- a/src/tools/cargo
+++ b/src/tools/cargo
@@ -1 +1 @@
-Subproject commit 7847f75bb6fc4fee6f511ca740bbe6872c08502b
+Subproject commit 3d3f2c05d742e5f907e951aa8849b03f0bc1a895
diff --git a/src/tools/compiletest/src/main.rs b/src/tools/compiletest/src/main.rs
index 20239e974788b..15216f52d91fd 100644
--- a/src/tools/compiletest/src/main.rs
+++ b/src/tools/compiletest/src/main.rs
@@ -14,6 +14,7 @@
 
 #![deny(warnings)]
 
+#[cfg(any(target_os = "macos", target_os = "ios"))]
 extern crate libc;
 extern crate test;
 extern crate getopts;