start PR over again

library/core/src/backtrace.rs

@@ -0,0 +1,46 @@
+//! hi
+#![unstable(feature = "core_backtrace", issue = "74465")]
+use crate::fmt;
+
+// perma(?)-unstable
+#[unstable(feature = "core_backtrace", issue = "74465")]
+///
+pub trait RawBacktraceImpl: fmt::Debug + fmt::Display + 'static {
+    ///
+    unsafe fn drop_and_free(self: *mut Self);
+}
+
+#[unstable(feature = "core_backtrace", issue = "74465")]
+///
+pub struct Backtrace {
+    inner: *mut dyn RawBacktraceImpl,
+}
+
+#[unstable(feature = "core_backtrace", issue = "74465")]
+unsafe impl Send for Backtrace {}
+
+#[unstable(feature = "core_backtrace", issue = "74465")]
+unsafe impl Sync for Backtrace {}
+
+#[unstable(feature = "core_backtrace", issue = "74465")]
+impl Drop for Backtrace {
+    fn drop(&mut self) {
+        unsafe { RawBacktraceImpl::drop_and_free(self.inner) }
+    }
+}
+
+#[unstable(feature = "core_backtrace", issue = "74465")]
+impl fmt::Debug for Backtrace {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let imp: &dyn RawBacktraceImpl = unsafe { &*self.inner };
+        fmt::Debug::fmt(imp, fmt)
+    }
+}
+
+#[unstable(feature = "core_backtrace", issue = "74465")]
+impl fmt::Display for Backtrace {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let imp: &dyn RawBacktraceImpl = unsafe { &*self.inner };
+        fmt::Display::fmt(imp, fmt)
+    }
+}

library/core/src/lib.rs

@@ -241,6 +241,7 @@ pub mod ops;
 pub mod any;
 pub mod array;
 pub mod ascii;
+pub mod backtrace;
 pub mod cell;
 pub mod char;
 pub mod ffi;

library/std/src/backtrace.rs

@@ -104,16 +104,20 @@ use crate::sync::Once;
 use crate::sys_common::backtrace::{lock, output_filename};
 use crate::vec::Vec;
 
+pub use core::backtrace::Backtrace;
+
 /// A captured OS thread stack backtrace.
 ///
 /// This type represents a stack backtrace for an OS thread captured at a
 /// previous point in time. In some instances the `Backtrace` type may
 /// internally be empty due to configuration. For more information see
 /// `Backtrace::capture`.
-pub struct Backtrace {
+struct BacktraceImpl {
     inner: Inner,
 }
 
+impl core::backtrace::RawBacktraceImpl for BacktraceImpl {}
+
 /// The current status of a backtrace, indicating whether it was captured or
 /// whether it is empty for some other reason.
 #[non_exhaustive]
@@ -173,7 +177,7 @@ enum BytesOrWide {
     Wide(Vec<u16>),
 }
 
-impl fmt::Debug for Backtrace {
+impl fmt::Debug for BacktraceImpl {
     fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
         let capture = match &self.inner {
             Inner::Unsupported => return fmt.write_str("<unsupported>"),
@@ -372,7 +376,7 @@ impl<'a> Backtrace {
     }
 }
 
-impl fmt::Display for Backtrace {
+impl fmt::Display for BacktraceImpl {
     fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
         let capture = match &self.inner {
             Inner::Unsupported => return fmt.write_str("unsupported backtrace"),

library/std/src/lib.rs

@@ -249,6 +249,7 @@
 #![feature(const_raw_ptr_deref)]
 #![feature(const_ipv4)]
 #![feature(container_error_extra)]
+#![feature(core_backtrace)]
 #![feature(core_intrinsics)]
 #![feature(custom_test_frameworks)]
 #![feature(decl_macro)]