Auto merge of #39252 - alexcrichton:less-exports, r=nrc

Hide a few more standard library symbols

These commits touch up some of the symbol visibility rules for some crates related to the standard library, notably:

* Symbols that are `pub extern` and `#[no_mangle]` which are internal-to-rust ABI things are no longer at the `C` export level, but the `Rust` export level. This includes allocators, panic runtimes, and compiler builtins.
* The libbacktrace library is now compiled with `-fvisibility=hidden` to ensure that we don't export those symbols.

3 files changed, 28 insertions, 1 deletions

diff --git a/src/liballoc_jemalloc/build.rs b/src/liballoc_jemalloc/build.rs
index 1edcb0b1f24..1143df0c630 100644
--- a/src/liballoc_jemalloc/build.rs
+++ b/src/liballoc_jemalloc/build.rs
@@ -181,4 +181,15 @@ fn main() {
     } else if !target.contains("windows") && !target.contains("musl") {
         println!("cargo:rustc-link-lib=pthread");
     }
+
+    // The pthread_atfork symbols is used by jemalloc on android but the really
+    // old android we're building on doesn't have them defined, so just make
+    // sure the symbols are available.
+    if target.contains("androideabi") {
+        println!("cargo:rerun-if-changed=pthread_atfork_dummy.c");
+        gcc::Config::new()
+            .flag("-fvisibility=hidden")
+            .file("pthread_atfork_dummy.c")
+            .compile("libpthread_atfork_dummy.a");
+    }
 }
diff --git a/src/liballoc_jemalloc/lib.rs b/src/liballoc_jemalloc/lib.rs
index 241f8149d24..fc8a5455d1d 100644
--- a/src/liballoc_jemalloc/lib.rs
+++ b/src/liballoc_jemalloc/lib.rs
@@ -143,7 +143,7 @@ mod imp {
     // we're building on doesn't have them defined, so just make sure the symbols
     // are available.
     #[no_mangle]
-    #[cfg(target_os = "android")]
+    #[cfg(all(target_os = "android", not(cargobuild)))]
     pub extern "C" fn pthread_atfork(_prefork: *mut u8,
                                      _postfork_parent: *mut u8,
                                      _postfork_child: *mut u8)
diff --git a/src/liballoc_jemalloc/pthread_atfork_dummy.c b/src/liballoc_jemalloc/pthread_atfork_dummy.c
new file mode 100644
index 00000000000..4e3df0ab26c
--- /dev/null
+++ b/src/liballoc_jemalloc/pthread_atfork_dummy.c
@@ -0,0 +1,16 @@
+// 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.
+
+// See comments in build.rs for why this exists
+int pthread_atfork(void* prefork,
+                   void* postfork_parent,
+                   void* postfork_child) {
+  return 0;
+}