Auto merge of #42588 - ishitatsuyuki:patch-1, r=petrochenkov

Make unused-extern-crate warn-by-default

Apart from enabling the lint, this pull request also removes existing unused crates in the codebase, and fix some amount of false positives on crates with special purposes.

Now that all false positive issues are closed, it should be possible to make it available to wider users.

Quote:
> Now that macro modularization is implemented, this is true today! *https://github.com/rust-lang/rust/issues/30849#issuecomment-286573218*

Concerns: can break some `#[deny(warnings)]`.

Close #42591

51 files changed, 95 insertions, 37 deletions

diff --git a/src/Cargo.lock b/src/Cargo.lock
index ad96ff40cd6..123c884585c 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 86309dd87de..8f3e71ef794 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 513b299fcf3..efefabc974c 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 84a3be99c27..ab2022b1824 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 5a340f1323d..558286f4ec0 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 9e5d4081231..01fff605283 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 03dc2f3c1ac..152b2e2aa5e 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 cbe642a9a76..811bf977610 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 da1039c5de1..6ce2232eb3e 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 042ec49b0bd..a73202ced61 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 852bd48a5ee..6597dccf258 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 af99c0e9387..25f02537490 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 e7f4f9caee3..9bedbfed5db 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 163c698e3ff..96688c6ac9c 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 e56c989b843..1915a1c8648 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 2f5aac65b92..870bb01bb9f 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 67b459ea18f..40b75be36fe 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 cfba11b5151..ad320a7ff3d 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 7d796dbd00f..f79abecf9da 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 05c227340d8..a66d1ce0859 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 ee349e31128..a83ac9bb633 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 97c1181e3a2..7b83985ba67 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 54941362e84..3429e3bda0f 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 e95d49f00bf..3da154e0689 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 cf1eb5cd52e..b57067e35e9 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 49cdba21a1d..538753d8692 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 a94585723a1..f44b9aa9615 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 bdcec26cb83..1c470240288 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 fa39095d329..42bbb4ae0cb 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 652bfa82b5c..642eb285564 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 edbb77fe390..bb292e2e52a 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 89227f6b010..996a6b97cd9 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 60bcd2ae076..c285c4d9e00 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 5e802bbbe3d..ad8888e58f7 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 00000000000..fc4bca865c9
--- /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 c65b2366bf1..bc9e2345f0e 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 e05dc066199..0f5aa24926b 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 8443518b3f5..9bc2c021904 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 b12ef6277bb..a3cfa134983 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 ea2dfca0714..a2778a83130 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 844955fb7e6..c5af9e3799d 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 381c22854e6..d72d1ee004e 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 39c09c69779..a07cc834411 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 11a0108b99b..89004f16897 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 f384eafa34b..49064853d21 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 cc73cbe15fe..35695efe1a9 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 87a17c0f19a..6a04a2c3704 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 09951348448..a89fd802baf 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 6416e1cacdc..e20d03478ec 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
-Subproject 7847f75bb6fc4fee6f511ca740bbe6872c08502
+Subproject 3d3f2c05d742e5f907e951aa8849b03f0bc1a89
diff --git a/src/tools/compiletest/src/main.rs b/src/tools/compiletest/src/main.rs
index 20239e97478..15216f52d91 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;