std: Internalize almost all of std::rt

This commit does some refactoring to make almost all of the `std::rt` private.
Specifically, the following items are no longer part of its API:

* DEFAULT_ERROR_CODE
* backtrace
* unwind
* args
* at_exit
* cleanup
* heap (this is just alloc::heap)
* min_stack
* util

The module is now tagged as `#[doc(hidden)]` as the only purpose it's serve is
an entry point for the `panic!` macro via the `begin_unwind` and
`begin_unwind_fmt` reexports.

src/libstd/sys/common/backtrace.rs

@@ -8,10 +8,14 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-use io;
+use env;
 use io::prelude::*;
-use str;
+use io;
 use libc;
+use str;
+use sync::atomic::{self, Ordering};
+
+pub use sys::backtrace::write;
 
 #[cfg(target_pointer_width = "64")]
 pub const HEX_WIDTH: usize = 18;
@@ -19,6 +23,23 @@ pub const HEX_WIDTH: usize = 18;
 #[cfg(target_pointer_width = "32")]
 pub const HEX_WIDTH: usize = 10;
 
+// For now logging is turned off by default, and this function checks to see
+// whether the magical environment variable is present to see if it's turned on.
+pub fn log_enabled() -> bool {
+    static ENABLED: atomic::AtomicIsize = atomic::AtomicIsize::new(0);
+    match ENABLED.load(Ordering::SeqCst) {
+        1 => return false,
+        2 => return true,
+        _ => {}
+    }
+
+    let val = match env::var_os("RUST_BACKTRACE") {
+        Some(..) => 2,
+        None => 1,
+    };
+    ENABLED.store(val, Ordering::SeqCst);
+    val == 2
+}
 
 // These output functions should now be used everywhere to ensure consistency.
 pub fn output(w: &mut Write, idx: isize, addr: *mut libc::c_void,
@@ -163,3 +184,43 @@ pub fn demangle(writer: &mut Write, s: &str) -> io::Result<()> {
 
     Ok(())
 }
+
+#[cfg(test)]
+mod tests {
+    use prelude::v1::*;
+    use sys_common;
+    macro_rules! t { ($a:expr, $b:expr) => ({
+        let mut m = Vec::new();
+        sys_common::backtrace::demangle(&mut m, $a).unwrap();
+        assert_eq!(String::from_utf8(m).unwrap(), $b);
+    }) }
+
+    #[test]
+    fn demangle() {
+        t!("test", "test");
+        t!("_ZN4testE", "test");
+        t!("_ZN4test", "_ZN4test");
+        t!("_ZN4test1a2bcE", "test::a::bc");
+    }
+
+    #[test]
+    fn demangle_dollars() {
+        t!("_ZN4$RP$E", ")");
+        t!("_ZN8$RF$testE", "&test");
+        t!("_ZN8$BP$test4foobE", "*test::foob");
+        t!("_ZN9$u20$test4foobE", " test::foob");
+    }
+
+    #[test]
+    fn demangle_many_dollars() {
+        t!("_ZN13test$u20$test4foobE", "test test::foob");
+        t!("_ZN12test$BP$test4foobE", "test*test::foob");
+    }
+
+    #[test]
+    fn demangle_windows() {
+        t!("ZN4testE", "test");
+        t!("ZN13test$u20$test4foobE", "test test::foob");
+        t!("ZN12test$RF$test4foobE", "test&test::foob");
+    }
+}