Auto merge of #38165 - Yamakaky:better-backtrace, r=petrochenkov

Improve backtrace formating while panicking.

Fixes #37783.

Done:

- Fix alignment of file paths for better readability
- `RUST_BACKTRACE=full` prints all the informations (current behaviour)
- `RUST_BACKTRACE=(short|yes)` is the default and does:
  - Skip irrelevant frames at the beginning and the end
  - Remove function address
  - Remove the current directory from the absolute paths
  - Remove `::hfabe6541873` at the end of the symbols
- `RUST_BACKTRACE=(0|no)` disables the backtrace.
- `RUST_BACKTRACE=<everything else>` is equivalent to `short` for
  backward compatibility.
- doc
- More uniform printing across platforms.

Removed, TODO in a new PR:

- Remove path prefix for libraries and libstd

Example of short backtrace:
```rust
fn fail() {
    panic!();
}

fn main() {
    let closure = || fail();
    closure();
}
```
Short:
```
thread 'main' panicked at 'explicit panic', t.rs:2
Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
stack backtrace:
   0: t::fail
            at ./t.rs:2
   1: t::main::{{closure}}
            at ./t.rs:6
   2: t::main
            at ./t.rs:7
```
Full:
```
thread 'main' panicked at 'This function never returns!', t.rs:2
stack backtrace:
   0:     0x558ddf666478 - std::sys::imp::backtrace::tracing::imp::unwind_backtrace::hec84c9dd8389cc5d
                               at /home/yamakaky/dev/rust/rust/src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
   1:     0x558ddf65d90e - std::sys_common::backtrace::_print::hfa25f8b31f4b4353
                               at /home/yamakaky/dev/rust/rust/src/libstd/sys_common/backtrace.rs:71
   2:     0x558ddf65cb5e - std::sys_common::backtrace::print::h9b711e11ac3ba805
                               at /home/yamakaky/dev/rust/rust/src/libstd/sys_common/backtrace.rs:60
   3:     0x558ddf66796e - std::panicking::default_hook::{{closure}}::h736d216e74748044
                               at /home/yamakaky/dev/rust/rust/src/libstd/panicking.rs:355
   4:     0x558ddf66743c - std::panicking::default_hook::h16baff397e46ea10
                               at /home/yamakaky/dev/rust/rust/src/libstd/panicking.rs:371
   5:     0x558ddf6682bc - std::panicking::rust_panic_with_hook::h6d5a9bb4eca42c80
                               at /home/yamakaky/dev/rust/rust/src/libstd/panicking.rs:559
   6:     0x558ddf64ea93 - std::panicking::begin_panic::h17dc549df2f10b99
                               at /home/yamakaky/dev/rust/rust/src/libstd/panicking.rs:521
   7:     0x558ddf64ec42 - t::diverges::he6bc43fc925905f5
                               at /tmp/p/t.rs:2
   8:     0x558ddf64ec5a - t::main::h0ffc20356b8a69c0
                               at /tmp/p/t.rs:6
   9:     0x558ddf6687f5 - core::ops::FnOnce::call_once::hce41f19c0db56f93
  10:     0x558ddf667cde - std::panicking::try::do_call::hd4c8c97efb4291df
                               at /home/yamakaky/dev/rust/rust/src/libstd/panicking.rs:464
  11:     0x558ddf698d77 - __rust_try
  12:     0x558ddf698c57 - __rust_maybe_catch_panic
                               at /home/yamakaky/dev/rust/rust/src/libpanic_unwind/lib.rs:98
  13:     0x558ddf667adb - std::panicking::try::h2c56ed2a59ec1d12
                               at /home/yamakaky/dev/rust/rust/src/libstd/panicking.rs:440
  14:     0x558ddf66cc9a - std::panic::catch_unwind::h390834e0251cc9af
                               at /home/yamakaky/dev/rust/rust/src/libstd/panic.rs:361
  15:     0x558ddf6809ee - std::rt::lang_start::hb73087428e233982
                               at /home/yamakaky/dev/rust/rust/src/libstd/rt.rs:57
  16:     0x558ddf64ec92 - main
  17:     0x7fecb869e290 - __libc_start_main
  18:     0x558ddf64e8b9 - _start
  19:                0x0 - <unknown>
```

12 files changed, 319 insertions, 323 deletions

diff --git a/src/libstd/sys/redox/backtrace.rs b/src/libstd/sys/redox/backtrace.rs
index 6f53841502a..961148fb6b4 100644
--- a/src/libstd/sys/redox/backtrace.rs
+++ b/src/libstd/sys/redox/backtrace.rs
@@ -10,9 +10,14 @@
 
 use libc;
 use io;
-use sys_common::backtrace::output;
+use sys_common::backtrace::Frame;
+
+pub use sys_common::gnu::libbacktrace::*;
+pub struct BacktraceContext;
 
 #[inline(never)]
-pub fn write(w: &mut io::Write) -> io::Result<()> {
-    output(w, 0, 0 as *mut libc::c_void, None)
+pub fn unwind_backtrace(frames: &mut [Frame])
+    -> io::Result<(usize, BacktraceContext)>
+{
+    Ok((0, BacktraceContext))
 }
diff --git a/src/libstd/sys/unix/backtrace/mod.rs b/src/libstd/sys/unix/backtrace/mod.rs
index 1eef89bf66f..29d4012dcdf 100644
--- a/src/libstd/sys/unix/backtrace/mod.rs
+++ b/src/libstd/sys/unix/backtrace/mod.rs
@@ -83,7 +83,8 @@
 /// to symbols. This is a bit of a hokey implementation as-is, but it works for
 /// all unix platforms we support right now, so it at least gets the job done.
 
-pub use self::tracing::write;
+pub use self::tracing::unwind_backtrace;
+pub use self::printing::{foreach_symbol_fileline, resolve_symname};
 
 // tracing impls:
 mod tracing;
@@ -100,3 +101,5 @@ pub mod gnu {
         Err(io::Error::new(io::ErrorKind::Other, "Not implemented"))
     }
 }
+
+pub struct BacktraceContext;
diff --git a/src/libstd/sys/unix/backtrace/printing/dladdr.rs b/src/libstd/sys/unix/backtrace/printing/dladdr.rs
index d9b759dc673..05a071a7978 100644
--- a/src/libstd/sys/unix/backtrace/printing/dladdr.rs
+++ b/src/libstd/sys/unix/backtrace/printing/dladdr.rs
@@ -9,33 +9,45 @@
 // except according to those terms.
 
 use io;
-use io::prelude::*;
+use intrinsics;
+use ffi::CStr;
 use libc;
+use sys::backtrace::BacktraceContext;
+use sys_common::backtrace::Frame;
 
-pub fn print(w: &mut Write, idx: isize, addr: *mut libc::c_void,
-             _symaddr: *mut libc::c_void) -> io::Result<()> {
-    use sys_common::backtrace::{output};
-    use intrinsics;
-    use ffi::CStr;
-
-    #[repr(C)]
-    struct Dl_info {
-        dli_fname: *const libc::c_char,
-        dli_fbase: *mut libc::c_void,
-        dli_sname: *const libc::c_char,
-        dli_saddr: *mut libc::c_void,
-    }
-    extern {
-        fn dladdr(addr: *const libc::c_void,
-                  info: *mut Dl_info) -> libc::c_int;
+pub fn resolve_symname<F>(frame: Frame,
+                          callback: F,
+                          _: &BacktraceContext) -> io::Result<()>
+    where F: FnOnce(Option<&str>) -> io::Result<()>
+{
+    unsafe {
+        let mut info: Dl_info = intrinsics::init();
+        let symname = if dladdr(frame.exact_position, &mut info) == 0 {
+            None
+        } else {
+            CStr::from_ptr(info.dli_sname).to_str().ok()
+        };
+        callback(symname)
     }
+}
 
-    let mut info: Dl_info = unsafe { intrinsics::init() };
-    if unsafe { dladdr(addr, &mut info) == 0 } {
-        output(w, idx,addr, None)
-    } else {
-        output(w, idx, addr, Some(unsafe {
-            CStr::from_ptr(info.dli_sname).to_bytes()
-        }))
-    }
+pub fn foreach_symbol_fileline<F>(_symbol_addr: Frame,
+                                  _f: F,
+                                  _: &BacktraceContext) -> io::Result<bool>
+    where F: FnMut(&[u8], libc::c_int) -> io::Result<()>
+{
+    Ok(false)
+}
+
+#[repr(C)]
+struct Dl_info {
+    dli_fname: *const libc::c_char,
+    dli_fbase: *mut libc::c_void,
+    dli_sname: *const libc::c_char,
+    dli_saddr: *mut libc::c_void,
+}
+
+extern {
+    fn dladdr(addr: *const libc::c_void,
+              info: *mut Dl_info) -> libc::c_int;
 }
diff --git a/src/libstd/sys/unix/backtrace/printing/mod.rs b/src/libstd/sys/unix/backtrace/printing/mod.rs
index 02e53854727..1ae82e01100 100644
--- a/src/libstd/sys/unix/backtrace/printing/mod.rs
+++ b/src/libstd/sys/unix/backtrace/printing/mod.rs
@@ -8,7 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-pub use self::imp::print;
+pub use self::imp::{foreach_symbol_fileline, resolve_symname};
 
 #[cfg(any(target_os = "macos", target_os = "ios",
           target_os = "emscripten"))]
@@ -17,5 +17,6 @@ mod imp;
 
 #[cfg(not(any(target_os = "macos", target_os = "ios",
               target_os = "emscripten")))]
-#[path = "gnu.rs"]
-mod imp;
+mod imp {
+    pub use sys_common::gnu::libbacktrace::{foreach_symbol_fileline, resolve_symname};
+}
diff --git a/src/libstd/sys/unix/backtrace/tracing/backtrace_fn.rs b/src/libstd/sys/unix/backtrace/tracing/backtrace_fn.rs
index ca2e70b5003..ecd32aa9462 100644
--- a/src/libstd/sys/unix/backtrace/tracing/backtrace_fn.rs
+++ b/src/libstd/sys/unix/backtrace/tracing/backtrace_fn.rs
@@ -18,39 +18,31 @@
 /// simple to use it should be used only on iOS devices as the only viable
 /// option.
 
-use io::prelude::*;
 use io;
 use libc;
-use mem;
-use sys::mutex::Mutex;
+use sys::backtrace::BacktraceContext;
+use sys_common::backtrace::Frame;
 
-use super::super::printing::print;
-
-#[inline(never)]
-pub fn write(w: &mut Write) -> io::Result<()> {
-    extern {
-        fn backtrace(buf: *mut *mut libc::c_void,
-                     sz: libc::c_int) -> libc::c_int;
+#[inline(never)] // if we know this is a function call, we can skip it when
+                 // tracing
+pub fn unwind_backtrace(frames: &mut [Frame])
+    -> io::Result<(usize, BacktraceContext)>
+{
+    const FRAME_LEN: usize = 100;
+    assert!(FRAME_LEN >= frames.len());
+    let mut raw_frames = [::ptr::null_mut(); FRAME_LEN];
+    let nb_frames = unsafe {
+        backtrace(raw_frames.as_mut_ptr(), raw_frames.len() as libc::c_int)
+    } as usize;
+    for (from, to) in raw_frames.iter().zip(frames.iter_mut()).take(nb_frames) {
+        *to = Frame {
+            exact_position: *from,
+            symbol_addr: *from,
+        };
     }
+    Ok((nb_frames as usize, BacktraceContext))
+}
 
-    // while it doesn't requires lock for work as everything is
-    // local, it still displays much nicer backtraces when a
-    // couple of threads panic simultaneously
-    static LOCK: Mutex = Mutex::new();
-    unsafe {
-        LOCK.lock();
-
-        writeln!(w, "stack backtrace:")?;
-        // 100 lines should be enough
-        const SIZE: usize = 100;
-        let mut buf: [*mut libc::c_void; SIZE] = mem::zeroed();
-        let cnt = backtrace(buf.as_mut_ptr(), SIZE as libc::c_int) as usize;
-
-        // skipping the first one as it is write itself
-        for i in 1..cnt {
-            print(w, i as isize, buf[i], buf[i])?
-        }
-        LOCK.unlock();
-    }
-    Ok(())
+extern {
+    fn backtrace(buf: *mut *mut libc::c_void, sz: libc::c_int) -> libc::c_int;
 }
diff --git a/src/libstd/sys/unix/backtrace/tracing/gcc_s.rs b/src/libstd/sys/unix/backtrace/tracing/gcc_s.rs
index c1b45620ab0..cfeabaddda9 100644
--- a/src/libstd/sys/unix/backtrace/tracing/gcc_s.rs
+++ b/src/libstd/sys/unix/backtrace/tracing/gcc_s.rs
@@ -8,102 +8,99 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+use error::Error;
 use io;
-use io::prelude::*;
 use libc;
-use mem;
-use sys_common::mutex::Mutex;
+use sys::backtrace::BacktraceContext;
+use sys_common::backtrace::Frame;
 
-use super::super::printing::print;
 use unwind as uw;
 
-#[inline(never)] // if we know this is a function call, we can skip it when
-                 // tracing
-pub fn write(w: &mut Write) -> io::Result<()> {
-    struct Context<'a> {
-        idx: isize,
-        writer: &'a mut (Write+'a),
-        last_error: Option<io::Error>,
-    }
+struct Context<'a> {
+    idx: usize,
+    frames: &'a mut [Frame],
+}
 
-    // When using libbacktrace, we use some necessary global state, so we
-    // need to prevent more than one thread from entering this block. This
-    // is semi-reasonable in terms of printing anyway, and we know that all
-    // I/O done here is blocking I/O, not green I/O, so we don't have to
-    // worry about this being a native vs green mutex.
-    static LOCK: Mutex = Mutex::new();
-    unsafe {
-        LOCK.lock();
+#[derive(Debug)]
+struct UnwindError(uw::_Unwind_Reason_Code);
 
-        writeln!(w, "stack backtrace:")?;
+impl Error for UnwindError {
+    fn description(&self) -> &'static str {
+        "unexpected return value while unwinding"
+    }
+}
 
-        let mut cx = Context { writer: w, last_error: None, idx: 0 };
-        let ret = match {
-            uw::_Unwind_Backtrace(trace_fn,
-                                  &mut cx as *mut Context as *mut libc::c_void)
-        } {
-            uw::_URC_NO_REASON => {
-                match cx.last_error {
-                    Some(err) => Err(err),
-                    None => Ok(())
-                }
-            }
-            _ => Ok(()),
-        };
-        LOCK.unlock();
-        return ret
+impl ::fmt::Display for UnwindError {
+    fn fmt(&self, f: &mut ::fmt::Formatter) -> ::fmt::Result {
+        write!(f, "{}: {:?}", self.description(), self.0)
     }
+}
 
-    extern fn trace_fn(ctx: *mut uw::_Unwind_Context,
-                       arg: *mut libc::c_void) -> uw::_Unwind_Reason_Code {
-        let cx: &mut Context = unsafe { mem::transmute(arg) };
-        let mut ip_before_insn = 0;
-        let mut ip = unsafe {
-            uw::_Unwind_GetIPInfo(ctx, &mut ip_before_insn) as *mut libc::c_void
-        };
-        if !ip.is_null() && ip_before_insn == 0 {
-            // this is a non-signaling frame, so `ip` refers to the address
-            // after the calling instruction. account for that.
-            ip = (ip as usize - 1) as *mut _;
+#[inline(never)] // if we know this is a function call, we can skip it when
+                 // tracing
+pub fn unwind_backtrace(frames: &mut [Frame])
+    -> io::Result<(usize, BacktraceContext)>
+{
+    let mut cx = Context {
+        idx: 0,
+        frames: frames,
+    };
+    let result_unwind = unsafe {
+        uw::_Unwind_Backtrace(trace_fn,
+                              &mut cx as *mut Context
+                              as *mut libc::c_void)
+    };
+    // See libunwind:src/unwind/Backtrace.c for the return values.
+    // No, there is no doc.
+    match result_unwind {
+        // These return codes seem to be benign and need to be ignored for backtraces
+        // to show up properly on all tested platforms.
+        uw::_URC_END_OF_STACK | uw::_URC_FATAL_PHASE1_ERROR | uw::_URC_FAILURE => {
+            Ok((cx.idx, BacktraceContext))
         }
-
-        // dladdr() on osx gets whiny when we use FindEnclosingFunction, and
-        // it appears to work fine without it, so we only use
-        // FindEnclosingFunction on non-osx platforms. In doing so, we get a
-        // slightly more accurate stack trace in the process.
-        //
-        // This is often because panic involves the last instruction of a
-        // function being "call std::rt::begin_unwind", with no ret
-        // instructions after it. This means that the return instruction
-        // pointer points *outside* of the calling function, and by
-        // unwinding it we go back to the original function.
-        let symaddr = if cfg!(target_os = "macos") || cfg!(target_os = "ios") {
-            ip
-        } else {
-            unsafe { uw::_Unwind_FindEnclosingFunction(ip) }
-        };
-
-        // Don't print out the first few frames (they're not user frames)
-        cx.idx += 1;
-        if cx.idx <= 0 { return uw::_URC_NO_REASON }
-        // Don't print ginormous backtraces
-        if cx.idx > 100 {
-            match write!(cx.writer, " ... <frames omitted>\n") {
-                Ok(()) => {}
-                Err(e) => { cx.last_error = Some(e); }
-            }
-            return uw::_URC_FAILURE
+        _ => {
+            Err(io::Error::new(io::ErrorKind::Other,
+                               UnwindError(result_unwind)))
         }
+    }
+}
 
-        // Once we hit an error, stop trying to print more frames
-        if cx.last_error.is_some() { return uw::_URC_FAILURE }
+extern fn trace_fn(ctx: *mut uw::_Unwind_Context,
+                   arg: *mut libc::c_void) -> uw::_Unwind_Reason_Code {
+    let cx = unsafe { &mut *(arg as *mut Context) };
+    let mut ip_before_insn = 0;
+    let mut ip = unsafe {
+        uw::_Unwind_GetIPInfo(ctx, &mut ip_before_insn) as *mut libc::c_void
+    };
+    if !ip.is_null() && ip_before_insn == 0 {
+        // this is a non-signaling frame, so `ip` refers to the address
+        // after the calling instruction. account for that.
+        ip = (ip as usize - 1) as *mut _;
+    }
 
-        match print(cx.writer, cx.idx, ip, symaddr) {
-            Ok(()) => {}
-            Err(e) => { cx.last_error = Some(e); }
-        }
+    // dladdr() on osx gets whiny when we use FindEnclosingFunction, and
+    // it appears to work fine without it, so we only use
+    // FindEnclosingFunction on non-osx platforms. In doing so, we get a
+    // slightly more accurate stack trace in the process.
+    //
+    // This is often because panic involves the last instruction of a
+    // function being "call std::rt::begin_unwind", with no ret
+    // instructions after it. This means that the return instruction
+    // pointer points *outside* of the calling function, and by
+    // unwinding it we go back to the original function.
+    let symaddr = if cfg!(target_os = "macos") || cfg!(target_os = "ios") {
+        ip
+    } else {
+        unsafe { uw::_Unwind_FindEnclosingFunction(ip) }
+    };
 
-        // keep going
-        uw::_URC_NO_REASON
+    if cx.idx < cx.frames.len() {
+        cx.frames[cx.idx] = Frame {
+            symbol_addr: symaddr,
+            exact_position: ip,
+        };
+        cx.idx += 1;
     }
+
+    uw::_URC_NO_REASON
 }
diff --git a/src/libstd/sys/windows/backtrace_gnu.rs b/src/libstd/sys/windows/backtrace/backtrace_gnu.rs
index f0d29dd4178..f0d29dd4178 100644
--- a/src/libstd/sys/windows/backtrace_gnu.rs
+++ b/src/libstd/sys/windows/backtrace/backtrace_gnu.rs
diff --git a/src/libstd/sys/windows/backtrace.rs b/src/libstd/sys/windows/backtrace/mod.rs
index 94aaf439f3d..3c3fd8d3e4a 100644
--- a/src/libstd/sys/windows/backtrace.rs
+++ b/src/libstd/sys/windows/backtrace/mod.rs
@@ -24,37 +24,87 @@
 
 #![allow(deprecated)] // dynamic_lib
 
-use io::prelude::*;
-
 use io;
 use libc::c_void;
 use mem;
 use ptr;
 use sys::c;
 use sys::dynamic_lib::DynamicLibrary;
-use sys::mutex::Mutex;
+use sys_common::backtrace::Frame;
 
 macro_rules! sym {
     ($lib:expr, $e:expr, $t:ident) => (
-        match $lib.symbol($e) {
-            Ok(f) => $crate::mem::transmute::<usize, $t>(f),
-            Err(..) => return Ok(())
-        }
+        $lib.symbol($e).map(|f| unsafe {
+            $crate::mem::transmute::<usize, $t>(f)
+        })
     )
 }
 
-#[cfg(target_env = "msvc")]
-#[path = "printing/msvc.rs"]
-mod printing;
-
-#[cfg(target_env = "gnu")]
-#[path = "printing/gnu.rs"]
 mod printing;
 
 #[cfg(target_env = "gnu")]
 #[path = "backtrace_gnu.rs"]
 pub mod gnu;
 
+pub use self::printing::{resolve_symname, foreach_symbol_fileline};
+
+pub fn unwind_backtrace(frames: &mut [Frame])
+    -> io::Result<(usize, BacktraceContext)>
+{
+    let dbghelp = DynamicLibrary::open("dbghelp.dll")?;
+
+    // Fetch the symbols necessary from dbghelp.dll
+    let SymInitialize = sym!(dbghelp, "SymInitialize", SymInitializeFn)?;
+    let SymCleanup = sym!(dbghelp, "SymCleanup", SymCleanupFn)?;
+    let StackWalk64 = sym!(dbghelp, "StackWalk64", StackWalk64Fn)?;
+
+    // Allocate necessary structures for doing the stack walk
+    let process = unsafe { c::GetCurrentProcess() };
+    let thread = unsafe { c::GetCurrentThread() };
+    let mut context: c::CONTEXT = unsafe { mem::zeroed() };
+    unsafe { c::RtlCaptureContext(&mut context) };
+    let mut frame: c::STACKFRAME64 = unsafe { mem::zeroed() };
+    let image = init_frame(&mut frame, &context);
+
+    let backtrace_context = BacktraceContext {
+        handle: process,
+        SymCleanup: SymCleanup,
+        dbghelp: dbghelp,
+    };
+
+    // Initialize this process's symbols
+    let ret = unsafe { SymInitialize(process, ptr::null_mut(), c::TRUE) };
+    if ret != c::TRUE {
+        return Ok((0, backtrace_context))
+    }
+
+    // And now that we're done with all the setup, do the stack walking!
+    // Start from -1 to avoid printing this stack frame, which will
+    // always be exactly the same.
+    let mut i = 0;
+    unsafe {
+        while i < frames.len() &&
+              StackWalk64(image, process, thread, &mut frame, &mut context,
+                          ptr::null_mut(),
+                          ptr::null_mut(),
+                          ptr::null_mut(),
+                          ptr::null_mut()) == c::TRUE
+        {
+            let addr = frame.AddrPC.Offset;
+            if addr == frame.AddrReturn.Offset || addr == 0 ||
+               frame.AddrReturn.Offset == 0 { break }
+
+            frames[i] = Frame {
+                symbol_addr: (addr - 1) as *const c_void,
+                exact_position: (addr - 1) as *const c_void,
+            };
+            i += 1;
+        }
+    }
+
+    Ok((i, backtrace_context))
+}
+
 type SymInitializeFn =
     unsafe extern "system" fn(c::HANDLE, *mut c_void,
                               c::BOOL) -> c::BOOL;
@@ -68,8 +118,8 @@ type StackWalk64Fn =
                               *mut c_void, *mut c_void) -> c::BOOL;
 
 #[cfg(target_arch = "x86")]
-pub fn init_frame(frame: &mut c::STACKFRAME64,
-                  ctx: &c::CONTEXT) -> c::DWORD {
+fn init_frame(frame: &mut c::STACKFRAME64,
+              ctx: &c::CONTEXT) -> c::DWORD {
     frame.AddrPC.Offset = ctx.Eip as u64;
     frame.AddrPC.Mode = c::ADDRESS_MODE::AddrModeFlat;
     frame.AddrStack.Offset = ctx.Esp as u64;
@@ -80,8 +130,8 @@ pub fn init_frame(frame: &mut c::STACKFRAME64,
 }
 
 #[cfg(target_arch = "x86_64")]
-pub fn init_frame(frame: &mut c::STACKFRAME64,
-                  ctx: &c::CONTEXT) -> c::DWORD {
+fn init_frame(frame: &mut c::STACKFRAME64,
+              ctx: &c::CONTEXT) -> c::DWORD {
     frame.AddrPC.Offset = ctx.Rip as u64;
     frame.AddrPC.Mode = c::ADDRESS_MODE::AddrModeFlat;
     frame.AddrStack.Offset = ctx.Rsp as u64;
@@ -91,73 +141,16 @@ pub fn init_frame(frame: &mut c::STACKFRAME64,
     c::IMAGE_FILE_MACHINE_AMD64
 }
 
-struct Cleanup {
+pub struct BacktraceContext {
     handle: c::HANDLE,
     SymCleanup: SymCleanupFn,
+    // Only used in printing for msvc and not gnu
+    #[allow(dead_code)]
+    dbghelp: DynamicLibrary,
 }
 
-impl Drop for Cleanup {
+impl Drop for BacktraceContext {
     fn drop(&mut self) {
         unsafe { (self.SymCleanup)(self.handle); }
     }
 }
-
-pub fn write(w: &mut Write) -> io::Result<()> {
-    // According to windows documentation, all dbghelp functions are
-    // single-threaded.
-    static LOCK: Mutex = Mutex::new();
-    unsafe {
-        LOCK.lock();
-        let res = _write(w);
-        LOCK.unlock();
-        return res
-    }
-}
-
-unsafe fn _write(w: &mut Write) -> io::Result<()> {
-    let dbghelp = match DynamicLibrary::open("dbghelp.dll") {
-        Ok(lib) => lib,
-        Err(..) => return Ok(()),
-    };
-
-    // Fetch the symbols necessary from dbghelp.dll
-    let SymInitialize = sym!(dbghelp, "SymInitialize", SymInitializeFn);
-    let SymCleanup = sym!(dbghelp, "SymCleanup", SymCleanupFn);
-    let StackWalk64 = sym!(dbghelp, "StackWalk64", StackWalk64Fn);
-
-    // Allocate necessary structures for doing the stack walk
-    let process = c::GetCurrentProcess();
-    let thread = c::GetCurrentThread();
-    let mut context: c::CONTEXT = mem::zeroed();
-    c::RtlCaptureContext(&mut context);
-    let mut frame: c::STACKFRAME64 = mem::zeroed();
-    let image = init_frame(&mut frame, &context);
-
-    // Initialize this process's symbols
-    let ret = SymInitialize(process, ptr::null_mut(), c::TRUE);
-    if ret != c::TRUE { return Ok(()) }
-    let _c = Cleanup { handle: process, SymCleanup: SymCleanup };
-
-    // And now that we're done with all the setup, do the stack walking!
-    // Start from -1 to avoid printing this stack frame, which will
-    // always be exactly the same.
-    let mut i = -1;
-    write!(w, "stack backtrace:\n")?;
-    while StackWalk64(image, process, thread, &mut frame, &mut context,
-                      ptr::null_mut(),
-                      ptr::null_mut(),
-                      ptr::null_mut(),
-                      ptr::null_mut()) == c::TRUE {
-        let addr = frame.AddrPC.Offset;
-        if addr == frame.AddrReturn.Offset || addr == 0 ||
-           frame.AddrReturn.Offset == 0 { break }
-
-        i += 1;
-
-        if i >= 0 {
-            printing::print(w, i, addr - 1, process, &dbghelp)?;
-        }
-    }
-
-    Ok(())
-}
diff --git a/src/libstd/sys/unix/backtrace/printing/gnu.rs b/src/libstd/sys/windows/backtrace/printing/mod.rs
index fb06fbedaf5..3e566f6e2bd 100644
--- a/src/libstd/sys/unix/backtrace/printing/gnu.rs
+++ b/src/libstd/sys/windows/backtrace/printing/mod.rs
@@ -8,4 +8,13 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-pub use sys_common::gnu::libbacktrace::print;
+#[cfg(target_env = "msvc")]
+#[path = "msvc.rs"]
+mod printing;
+
+#[cfg(target_env = "gnu")]
+mod printing {
+    pub use sys_common::gnu::libbacktrace::{foreach_symbol_fileline, resolve_symname};
+}
+
+pub use self::printing::{foreach_symbol_fileline, resolve_symname};
diff --git a/src/libstd/sys/windows/backtrace/printing/msvc.rs b/src/libstd/sys/windows/backtrace/printing/msvc.rs
new file mode 100644
index 00000000000..3107d784324
--- /dev/null
+++ b/src/libstd/sys/windows/backtrace/printing/msvc.rs
@@ -0,0 +1,83 @@
+// Copyright 2014 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.
+
+use ffi::CStr;
+use io;
+use libc::{c_ulong, c_int, c_char};
+use mem;
+use sys::c;
+use sys::backtrace::BacktraceContext;
+use sys_common::backtrace::Frame;
+
+type SymFromAddrFn =
+    unsafe extern "system" fn(c::HANDLE, u64, *mut u64,
+                              *mut c::SYMBOL_INFO) -> c::BOOL;
+type SymGetLineFromAddr64Fn =
+    unsafe extern "system" fn(c::HANDLE, u64, *mut u32,
+                              *mut c::IMAGEHLP_LINE64) -> c::BOOL;
+
+/// Converts a pointer to symbol to its string value.
+pub fn resolve_symname<F>(frame: Frame,
+                          callback: F,
+                          context: &BacktraceContext) -> io::Result<()>
+    where F: FnOnce(Option<&str>) -> io::Result<()>
+{
+    let SymFromAddr = sym!(&context.dbghelp, "SymFromAddr", SymFromAddrFn)?;
+
+    unsafe {
+        let mut info: c::SYMBOL_INFO = mem::zeroed();
+        info.MaxNameLen = c::MAX_SYM_NAME as c_ulong;
+        // the struct size in C.  the value is different to
+        // `size_of::<SYMBOL_INFO>() - MAX_SYM_NAME + 1` (== 81)
+        // due to struct alignment.
+        info.SizeOfStruct = 88;
+
+        let mut displacement = 0u64;
+        let ret = SymFromAddr(context.handle,
+                              frame.symbol_addr as u64,
+                              &mut displacement,
+                              &mut info);
+
+        let symname = if ret == c::TRUE {
+            let ptr = info.Name.as_ptr() as *const c_char;
+            CStr::from_ptr(ptr).to_str().ok()
+        } else {
+            None
+        };
+        callback(symname)
+    }
+}
+
+pub fn foreach_symbol_fileline<F>(frame: Frame,
+                                  mut f: F,
+                                  context: &BacktraceContext)
+    -> io::Result<bool>
+    where F: FnMut(&[u8], c_int) -> io::Result<()>
+{
+    let SymGetLineFromAddr64 = sym!(&context.dbghelp,
+                                    "SymGetLineFromAddr64",
+                                    SymGetLineFromAddr64Fn)?;
+
+    unsafe {
+        let mut line: c::IMAGEHLP_LINE64 = mem::zeroed();
+        line.SizeOfStruct = ::mem::size_of::<c::IMAGEHLP_LINE64>() as u32;
+
+        let mut displacement = 0u32;
+        let ret = SymGetLineFromAddr64(context.handle,
+                                       frame.exact_position as u64,
+                                       &mut displacement,
+                                       &mut line);
+        if ret == c::TRUE {
+            let name = CStr::from_ptr(line.Filename).to_bytes();
+            f(name, line.LineNumber as c_int)?;
+        }
+        Ok(false)
+    }
+}
diff --git a/src/libstd/sys/windows/printing/gnu.rs b/src/libstd/sys/windows/printing/gnu.rs
deleted file mode 100644
index be2d5273c07..00000000000
--- a/src/libstd/sys/windows/printing/gnu.rs
+++ /dev/null
@@ -1,26 +0,0 @@
-// Copyright 2014 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.
-
-use io::prelude::*;
-use io;
-use libc::c_void;
-use sys::c;
-use sys::dynamic_lib::DynamicLibrary;
-use sys_common::gnu::libbacktrace;
-
-pub fn print(w: &mut Write,
-             i: isize,
-             addr: u64,
-             _process: c::HANDLE,
-             _dbghelp: &DynamicLibrary)
-              -> io::Result<()> {
-    let addr = addr as usize as *mut c_void;
-    libbacktrace::print(w, i, addr, addr)
-}
diff --git a/src/libstd/sys/windows/printing/msvc.rs b/src/libstd/sys/windows/printing/msvc.rs
deleted file mode 100644
index 9c29ac4082a..00000000000
--- a/src/libstd/sys/windows/printing/msvc.rs
+++ /dev/null
@@ -1,73 +0,0 @@
-// Copyright 2014 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.
-
-use ffi::CStr;
-use io::prelude::*;
-use io;
-use libc::{c_ulong, c_int, c_char, c_void};
-use mem;
-use sys::c;
-use sys::dynamic_lib::DynamicLibrary;
-use sys_common::backtrace::{output, output_fileline};
-
-type SymFromAddrFn =
-    unsafe extern "system" fn(c::HANDLE, u64, *mut u64,
-                              *mut c::SYMBOL_INFO) -> c::BOOL;
-type SymGetLineFromAddr64Fn =
-    unsafe extern "system" fn(c::HANDLE, u64, *mut u32,
-                              *mut c::IMAGEHLP_LINE64) -> c::BOOL;
-
-pub fn print(w: &mut Write,
-             i: isize,
-             addr: u64,
-             process: c::HANDLE,
-             dbghelp: &DynamicLibrary)
-              -> io::Result<()> {
-    unsafe {
-        let SymFromAddr = sym!(dbghelp, "SymFromAddr", SymFromAddrFn);
-        let SymGetLineFromAddr64 = sym!(dbghelp,
-                                        "SymGetLineFromAddr64",
-                                        SymGetLineFromAddr64Fn);
-
-        let mut info: c::SYMBOL_INFO = mem::zeroed();
-        info.MaxNameLen = c::MAX_SYM_NAME as c_ulong;
-        // the struct size in C.  the value is different to
-        // `size_of::<SYMBOL_INFO>() - MAX_SYM_NAME + 1` (== 81)
-        // due to struct alignment.
-        info.SizeOfStruct = 88;
-
-        let mut displacement = 0u64;
-        let ret = SymFromAddr(process, addr, &mut displacement, &mut info);
-
-        let name = if ret == c::TRUE {
-            let ptr = info.Name.as_ptr() as *const c_char;
-            Some(CStr::from_ptr(ptr).to_bytes())
-        } else {
-            None
-        };
-
-        output(w, i, addr as usize as *mut c_void, name)?;
-
-        // Now find out the filename and line number
-        let mut line: c::IMAGEHLP_LINE64 = mem::zeroed();
-        line.SizeOfStruct = ::mem::size_of::<c::IMAGEHLP_LINE64>() as u32;
-
-        let mut displacement = 0u32;
-        let ret = SymGetLineFromAddr64(process, addr, &mut displacement, &mut line);
-        if ret == c::TRUE {
-            output_fileline(w,
-                            CStr::from_ptr(line.Filename).to_bytes(),
-                            line.LineNumber as c_int,
-                            false)
-        } else {
-            Ok(())
-        }
-    }
-}