std: Depend on `backtrace` crate from crates.io

This commit removes all in-tree support for generating backtraces in
favor of depending on the `backtrace` crate on crates.io. This resolves
a very longstanding piece of duplication where the standard library has
long contained the ability to generate a backtrace on panics, but the
code was later extracted and duplicated on crates.io with the
`backtrace` crate. Since that fork each implementation has seen various
improvements one way or another, but typically `backtrace`-the-crate has
lagged behind libstd in one way or another.

The goal here is to remove this duplication of a fairly critical piece
of code and ensure that there's only one source of truth for generating
backtraces between the standard library and the crate on crates.io.
Recently I've been working to bring the `backtrace` crate on crates.io
up to speed with the support in the standard library which includes:

* Support for `StackWalkEx` on MSVC to recover inline frames with
  debuginfo.
* Using `libbacktrace` by default on MinGW targets.
* Supporting `libbacktrace` on OSX as an option.
* Ensuring all the requisite support in `backtrace`-the-crate compiles
  with `#![no_std]`.
* Updating the `libbacktrace` implementation in `backtrace`-the-crate to
  initialize the global state with the correct filename where necessary.

After reviewing the code in libstd the `backtrace` crate should be at
exact feature parity with libstd today. The backtraces generated should
have the same symbols and same number of frames in general, and there's
not known divergence from libstd currently.

Note that one major difference between libstd's backtrace support and
the `backtrace` crate is that on OSX the crates.io crate enables the
`coresymbolication` feature by default. This feature, however, uses
private internal APIs that aren't published for OSX. While they provide
more accurate backtraces this isn't appropriate for libstd distributed
as a binary, so libstd's dependency on the `backtrace` crate explicitly
disables this feature and forces OSX to use `libbacktrace` as a
symbolication strategy.

The long-term goal of this refactoring is to eventually move us towards
a world where we can drop `libbacktrace` entirely and simply use Gimli
and the surrounding crates for backtrace support. That's still aways off
but hopefully will much more easily enabled by having the source of
truth for backtraces live in crates.io!

Procedurally if we go forward with this I'd like to transfer the
`backtrace-rs` crate to the rust-lang GitHub organization as well, but I
figured I'd hold off on that until we get closer to merging.

2 files changed, 0 insertions, 118 deletions

diff --git a/src/libstd/sys/cloudabi/backtrace.rs b/src/libstd/sys/cloudabi/backtrace.rs
deleted file mode 100644
index 17719a29b6e..00000000000
--- a/src/libstd/sys/cloudabi/backtrace.rs
+++ /dev/null
@@ -1,116 +0,0 @@
-use crate::error::Error;
-use crate::ffi::CStr;
-use crate::fmt;
-use crate::intrinsics;
-use crate::io;
-use crate::sys_common::backtrace::Frame;
-
-use unwind as uw;
-
-pub struct BacktraceContext;
-
-struct Context<'a> {
-    idx: usize,
-    frames: &'a mut [Frame],
-}
-
-#[derive(Debug)]
-struct UnwindError(uw::_Unwind_Reason_Code);
-
-impl Error for UnwindError {
-    fn description(&self) -> &'static str {
-        "unexpected return value while unwinding"
-    }
-}
-
-impl fmt::Display for UnwindError {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(f, "{}: {:?}", self.description(), self.0)
-    }
-}
-
-#[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 };
-    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))
-        }
-        _ => Err(io::Error::new(
-            io::ErrorKind::Other,
-            UnwindError(result_unwind),
-        )),
-    }
-}
-
-extern "C" 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<'_>) };
-    if cx.idx >= cx.frames.len() {
-        return uw::_URC_NORMAL_STOP;
-    }
-
-    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 _;
-    }
-
-    let symaddr = unsafe { uw::_Unwind_FindEnclosingFunction(ip) };
-    cx.frames[cx.idx] = Frame {
-        symbol_addr: symaddr as *mut u8,
-        exact_position: ip as *mut u8,
-        inline_context: 0,
-    };
-    cx.idx += 1;
-
-    uw::_URC_NO_REASON
-}
-
-pub fn foreach_symbol_fileline<F>(_: Frame, _: F, _: &BacktraceContext) -> io::Result<bool>
-where
-    F: FnMut(&[u8], u32) -> io::Result<()>,
-{
-    // No way to obtain this information on CloudABI.
-    Ok(false)
-}
-
-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 as *mut _, &mut info) == 0 || info.dli_sname.is_null() {
-                None
-            } else {
-                CStr::from_ptr(info.dli_sname).to_str().ok()
-            };
-        callback(symname)
-    }
-}
-
-#[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 "C" {
-    fn dladdr(addr: *const libc::c_void, info: *mut Dl_info) -> libc::c_int;
-}
diff --git a/src/libstd/sys/cloudabi/mod.rs b/src/libstd/sys/cloudabi/mod.rs
index 3f8e67a7af8..c3b8bbd0426 100644
--- a/src/libstd/sys/cloudabi/mod.rs
+++ b/src/libstd/sys/cloudabi/mod.rs
@@ -4,8 +4,6 @@ use crate::mem;
 #[path = "../unix/alloc.rs"]
 pub mod alloc;
 pub mod args;
-#[cfg(feature = "backtrace")]
-pub mod backtrace;
 #[path = "../unix/cmath.rs"]
 pub mod cmath;
 pub mod condvar;