Read traceback data from external debug info

[daurnimator]

Part of #2300

Currently on top of #3005

To play with external debug info I've been using:

const std = @import("std");

pub fn main() void {
    std.debug.dumpCurrentStackTrace(@returnAddress());
}

and compiling+running with:

./bin/zig build-exe ../play.zig
objcopy --only-keep-debug play play.debug
objcopy --strip-all play play
objcopy --add-gnu-debuglink=play.debug play
./play

[daurnimator]

Weirdly, this diff fixes the open issue 2 above:

diff --git a/std/debug.zig b/std/debug.zig
index 0cb3d86f2..a6909ef9c 100644
--- a/std/debug.zig
+++ b/std/debug.zig
@@ -1218,14 +1218,19 @@ fn openSelfDebugInfoPosix(allocator: *mem.Allocator) !DwarfInfo {
         );
     };
     errdefer os.munmap(self_exe_mmap);
-    var self_exe_mmap_seekable = io.SliceSeekableInStream.init(self_exe_mmap);
+
+    const S = struct {
+        var self_exe_mmap_seekable: io.SliceSeekableInStream = undefined;
+    };
+
+    S.self_exe_mmap_seekable = io.SliceSeekableInStream.init(self_exe_mmap);
 
     return openElfDebugInfo(
         allocator,
         // TODO https://github.com/ziglang/zig/issues/764
-        @ptrCast(*DwarfSeekableStream, &self_exe_mmap_seekable.seekable_stream),
+        @ptrCast(*DwarfSeekableStream, &S.self_exe_mmap_seekable.seekable_stream),
         // TODO https://github.com/ziglang/zig/issues/764
-        @ptrCast(*DwarfInStream, &self_exe_mmap_seekable.stream),
+        @ptrCast(*DwarfInStream, &S.self_exe_mmap_seekable.stream),
     );
 }
 

Is this a copy elision bug?

[andrewrk]

please re-open when you have something ready for review