Start working on proof of concept for exposing Backtrace in core

compiler/rustc_hir/src/lang_items.rs

@@ -271,6 +271,10 @@ language_item_table! {
     // libstd panic entry point. Necessary for const eval to be able to catch it
     BeginPanic,              sym::begin_panic,         begin_panic_fn,             Target::Fn;
 
+    BacktraceEnabled,        sym::backtrace_enabled,   backtrace_enabled,          Target::Fn;
+    BacktraceCreate,         sym::backtrace_create,    backtrace_create,           Target::Fn;
+    BacktraceStatus,         sym::backtrace_status,    backtrace_status,           Target::Fn;
+
     ExchangeMalloc,          sym::exchange_malloc,     exchange_malloc_fn,         Target::Fn;
     BoxFree,                 sym::box_free,            box_free_fn,                Target::Fn;
     DropInPlace,             sym::drop_in_place,       drop_in_place_fn,           Target::Fn;

compiler/rustc_hir/src/weak_lang_items.rs

@@ -48,4 +48,7 @@ weak_lang_items! {
     eh_personality,     EhPersonality,      rust_eh_personality;
     eh_catch_typeinfo,  EhCatchTypeinfo,    rust_eh_catch_typeinfo;
     oom,                Oom,                rust_oom;
+    backtrace_create,   BacktraceCreate,    rust_backtrace_create;
+    backtrace_enabled,  BacktraceEnabled,   rust_backtrace_enabled;
+    backtrace_status,   BacktraceStatus,    rust_backtrace_status;
 }

compiler/rustc_span/src/symbol.rs

@@ -289,6 +289,9 @@ symbols! {
         automatically_derived,
         avx512_target_feature,
         await_macro,
+        backtrace_create,
+        backtrace_enabled,
+        backtrace_status,
         bang,
         begin_panic,
         bench,
@@ -931,6 +934,9 @@ symbols! {
         rust_2015_preview,
         rust_2018_preview,
         rust_2021_preview,
+        rust_backtrace_create,
+        rust_backtrace_enabled,
+        rust_backtrace_status,
         rust_begin_unwind,
         rust_eh_catch_typeinfo,
         rust_eh_personality,

library/core/src/backtrace.rs

@@ -0,0 +1,200 @@
+//! hi
+#![unstable(feature = "backtrace", issue = "74465")]
+use crate::{fmt, ptr};
+
+/// The current status of a backtrace, indicating whether it was captured or
+/// whether it is empty for some other reason.
+#[non_exhaustive]
+#[derive(Debug, PartialEq, Eq)]
+pub enum BacktraceStatus {
+    /// Capturing a backtrace is not supported, likely because it's not
+    /// implemented for the current platform.
+    Unsupported,
+    /// Capturing a backtrace has been disabled through either the
+    /// `RUST_LIB_BACKTRACE` or `RUST_BACKTRACE` environment variables.
+    Disabled,
+    /// A backtrace has been captured and the `Backtrace` should print
+    /// reasonable information when rendered.
+    Captured,
+}
+
+// perma(?)-unstable
+#[unstable(feature = "backtrace", issue = "74465")]
+///
+pub trait RawBacktrace: fmt::Debug + fmt::Display + 'static {
+    ///
+    unsafe fn drop_and_free(self: *mut Self);
+}
+
+struct UnsupportedBacktrace;
+
+impl UnsupportedBacktrace  {
+    #[allow(dead_code)]
+    const fn create() -> Backtrace {
+        // don't add members to Self
+        let _ = Self {};
+
+        Backtrace {
+            inner: ptr::NonNull::<Self>::dangling().as_ptr(),
+        }
+    }
+}
+
+impl RawBacktrace for UnsupportedBacktrace {
+    unsafe fn drop_and_free(self: *mut Self) {}
+}
+
+impl fmt::Display for UnsupportedBacktrace {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt.write_str("unsupported backtrace")
+    }
+}
+
+impl fmt::Debug for UnsupportedBacktrace {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt.write_str("<unsupported>")
+    }
+}
+struct DisabledBacktrace;
+
+impl DisabledBacktrace {
+    const fn create() -> Backtrace {
+        // don't add members to Self
+        let _ = Self {};
+
+        Backtrace {
+            inner: ptr::NonNull::<Self>::dangling().as_ptr(),
+        }
+    }
+}
+
+impl RawBacktrace for DisabledBacktrace {
+    unsafe fn drop_and_free(self: *mut Self) {}
+}
+
+impl fmt::Display for DisabledBacktrace {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt.write_str("disabled backtrace")
+    }
+}
+
+impl fmt::Debug for DisabledBacktrace {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt.write_str("<disabled>")
+    }
+}
+
+#[unstable(feature = "backtrace", issue = "74465")]
+///
+pub struct Backtrace {
+    ///
+    inner: *mut dyn RawBacktrace,
+}
+
+/// Global implementation of backtrace functionality. Called to create
+/// `RawBacktrace` trait objects.
+extern "Rust" {
+    #[lang = "backtrace_create"]
+    fn backtrace_create(ip: usize) -> *mut dyn RawBacktrace;
+
+    #[lang = "backtrace_enabled"]
+    fn backtrace_enabled() -> bool;
+
+    #[lang = "backtrace_status"]
+    fn backtrace_status(raw: *mut dyn RawBacktrace) -> BacktraceStatus;
+}
+
+impl Backtrace {
+    fn create(ip: usize) -> Backtrace {
+        let inner = unsafe { backtrace_create(ip) };
+        Backtrace { inner }
+    }
+
+    /// Returns whether backtrace captures are enabled through environment
+    /// variables.
+    fn enabled() -> bool {
+        unsafe { backtrace_enabled() }
+    }
+
+    /// Capture a stack backtrace of the current thread.
+    ///
+    /// This function will capture a stack backtrace of the current OS thread of
+    /// execution, returning a `Backtrace` type which can be later used to print
+    /// the entire stack trace or render it to a string.
+    ///
+    /// This function will be a noop if the `RUST_BACKTRACE` or
+    /// `RUST_LIB_BACKTRACE` backtrace variables are both not set. If either
+    /// environment variable is set and enabled then this function will actually
+    /// capture a backtrace. Capturing a backtrace can be both memory intensive
+    /// and slow, so these environment variables allow liberally using
+    /// `Backtrace::capture` and only incurring a slowdown when the environment
+    /// variables are set.
+    ///
+    /// To forcibly capture a backtrace regardless of environment variables, use
+    /// the `Backtrace::force_capture` function.
+    #[inline(never)] // want to make sure there's a frame here to remove
+    pub fn capture() -> Backtrace {
+        if !Backtrace::enabled() {
+            return Backtrace::disabled();
+        }
+
+        Self::create(Backtrace::capture as usize)
+    }
+
+    /// Forcibly captures a full backtrace, regardless of environment variable
+    /// configuration.
+    ///
+    /// This function behaves the same as `capture` except that it ignores the
+    /// values of the `RUST_BACKTRACE` and `RUST_LIB_BACKTRACE` environment
+    /// variables, always capturing a backtrace.
+    ///
+    /// Note that capturing a backtrace can be an expensive operation on some
+    /// platforms, so this should be used with caution in performance-sensitive
+    /// parts of code.
+    #[inline(never)] // want to make sure there's a frame here to remove
+    pub fn force_capture() -> Backtrace {
+        Self::create(Backtrace::force_capture as usize)
+    }
+
+    /// Forcibly captures a disabled backtrace, regardless of environment
+    /// variable configuration.
+    pub const fn disabled() -> Backtrace {
+        DisabledBacktrace::create()
+    }
+
+    /// Returns the status of this backtrace, indicating whether this backtrace
+    /// request was unsupported, disabled, or a stack trace was actually
+    /// captured.
+    pub fn status(&self) -> BacktraceStatus {
+        unsafe { backtrace_status(self.inner) }
+    }
+}
+
+#[unstable(feature = "backtrace", issue = "74465")]
+unsafe impl Send for Backtrace {}
+
+#[unstable(feature = "backtrace", issue = "74465")]
+unsafe impl Sync for Backtrace {}
+
+#[unstable(feature = "backtrace", issue = "74465")]
+impl Drop for Backtrace {
+    fn drop(&mut self) {
+        unsafe { RawBacktrace::drop_and_free(self.inner) }
+    }
+}
+
+#[unstable(feature = "backtrace", issue = "74465")]
+impl fmt::Debug for Backtrace {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let imp: &dyn RawBacktrace = unsafe { &*self.inner };
+        fmt::Debug::fmt(imp, fmt)
+    }
+}
+
+#[unstable(feature = "backtrace", issue = "74465")]
+impl fmt::Display for Backtrace {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let imp: &dyn RawBacktrace = unsafe { &*self.inner };
+        fmt::Display::fmt(imp, fmt)
+    }
+}

library/core/src/lib.rs

@@ -116,6 +116,7 @@
 #![feature(fundamental)]
 #![feature(intrinsics)]
 #![feature(lang_items)]
+#![feature(linkage)]
 #![feature(link_llvm_intrinsics)]
 #![feature(llvm_asm)]
 #![feature(negative_impls)]
@@ -241,6 +242,7 @@ pub mod ops;
 pub mod any;
 pub mod array;
 pub mod ascii;
+pub mod backtrace;
 pub mod cell;
 pub mod char;
 pub mod ffi;