Use backtrace via generic member access

build.rs

@@ -15,15 +15,17 @@ compile_error! {
 // type. If the current toolchain is able to compile it, we go ahead and use
 // backtrace in anyhow.
 const PROBE: &str = r#"
-    #![feature(backtrace)]
-    #![allow(dead_code)]
+    #![feature(error_generic_member_access, provide_any)]
 
+    use std::any::Demand;
     use std::backtrace::{Backtrace, BacktraceStatus};
     use std::error::Error;
     use std::fmt::{self, Display};
 
     #[derive(Debug)]
-    struct E;
+    struct E {
+        backtrace: Backtrace,
+    }
 
     impl Display for E {
         fn fmt(&self, _formatter: &mut fmt::Formatter) -> fmt::Result {
@@ -32,14 +34,20 @@ const PROBE: &str = r#"
     }
 
     impl Error for E {
-        fn backtrace(&self) -> Option<&Backtrace> {
-            let backtrace = Backtrace::capture();
-            match backtrace.status() {
-                BacktraceStatus::Captured | BacktraceStatus::Disabled | _ => {}
-            }
-            unimplemented!()
+        fn provide<'a>(&'a self, req: &mut Demand<'a>) {
+            req.provide_ref(&self.backtrace);
         }
     }
+
+    const _: fn() = || {
+        let backtrace: Backtrace = Backtrace::capture();
+        let status: BacktraceStatus = backtrace.status();
+        match status {
+            BacktraceStatus::Captured | BacktraceStatus::Disabled | _ => {}
+        }
+    };
+
+    const _: fn(&dyn Error) -> Option<&Backtrace> = |err| err.request_ref::<Backtrace>();
 "#;
 
 fn main() {

src/backtrace.rs

@@ -38,7 +38,7 @@ macro_rules! backtrace {
 #[cfg(backtrace)]
 macro_rules! backtrace_if_absent {
     ($err:expr) => {
-        match $err.backtrace() {
+        match ($err as &dyn std::error::Error).request_ref::<std::backtrace::Backtrace>() {
             Some(_) => None,
             None => backtrace!(),
         }

src/context.rs

@@ -4,7 +4,7 @@ use core::convert::Infallible;
 use core::fmt::{self, Debug, Display, Write};
 
 #[cfg(backtrace)]
-use std::backtrace::Backtrace;
+use std::any::Demand;
 
 mod ext {
     use super::*;
@@ -24,7 +24,7 @@ mod ext {
         where
             C: Display + Send + Sync + 'static,
         {
-            let backtrace = backtrace_if_absent!(self);
+            let backtrace = backtrace_if_absent!(&self);
             Error::from_context(context, self, backtrace)
         }
     }
@@ -123,28 +123,29 @@ where
     C: Display,
     E: StdError + 'static,
 {
-    #[cfg(backtrace)]
-    fn backtrace(&self) -> Option<&Backtrace> {
-        self.error.backtrace()
-    }
-
     fn source(&self) -> Option<&(dyn StdError + 'static)> {
         Some(&self.error)
     }
+
+    #[cfg(backtrace)]
+    fn provide<'a>(&'a self, req: &mut Demand<'a>) {
+        self.error.provide(req);
+    }
 }
 
 impl<C> StdError for ContextError<C, Error>
 where
     C: Display,
 {
-    #[cfg(backtrace)]
-    fn backtrace(&self) -> Option<&Backtrace> {
-        Some(self.error.backtrace())
-    }
-
     fn source(&self) -> Option<&(dyn StdError + 'static)> {
         Some(unsafe { crate::ErrorImpl::error(self.error.inner.by_ref()) })
     }
+
+    #[cfg(backtrace)]
+    fn provide<'a>(&'a self, req: &mut Demand<'a>) {
+        req.provide_ref(self.error.backtrace());
+        self.error.provide(req);
+    }
 }
 
 struct Quoted<C>(C);

src/error.rs

@@ -5,6 +5,8 @@ use crate::ptr::Mut;
 use crate::ptr::{Own, Ref};
 use crate::{Error, StdError};
 use alloc::boxed::Box;
+#[cfg(backtrace)]
+use core::any::Demand;
 use core::any::TypeId;
 use core::fmt::{self, Debug, Display};
 use core::mem::ManuallyDrop;
@@ -31,7 +33,7 @@ impl Error {
     where
         E: StdError + Send + Sync + 'static,
     {
-        let backtrace = backtrace_if_absent!(error);
+        let backtrace = backtrace_if_absent!(&error);
         Error::from_std(error, backtrace)
     }
 
@@ -530,7 +532,7 @@ where
 {
     #[cold]
     fn from(error: E) -> Self {
-        let backtrace = backtrace_if_absent!(error);
+        let backtrace = backtrace_if_absent!(&error);
         Error::from_std(error, backtrace)
     }
 }
@@ -886,13 +888,21 @@ impl ErrorImpl {
             .as_ref()
             .or_else(|| {
                 #[cfg(backtrace)]
-                return Self::error(this).backtrace();
-                #[cfg(all(not(backtrace), feature = "backtrace"))]
+                return Self::error(this).request_ref::<Backtrace>();
+                #[cfg(not(backtrace))]
                 return (vtable(this.ptr).object_backtrace)(this);
             })
             .expect("backtrace capture failed")
     }
 
+    #[cfg(backtrace)]
+    unsafe fn provide<'a>(this: Ref<'a, Self>, req: &mut Demand<'a>) {
+        if let Some(backtrace) = &this.deref().backtrace {
+            req.provide_ref(backtrace);
+        }
+        Self::error(this).provide(req);
+    }
+
     #[cold]
     pub(crate) unsafe fn chain(this: Ref<Self>) -> Chain {
         Chain::new(Self::error(this))
@@ -903,14 +913,14 @@ impl<E> StdError for ErrorImpl<E>
 where
     E: StdError,
 {
-    #[cfg(backtrace)]
-    fn backtrace(&self) -> Option<&Backtrace> {
-        Some(unsafe { ErrorImpl::backtrace(self.erase()) })
-    }
-
     fn source(&self) -> Option<&(dyn StdError + 'static)> {
         unsafe { ErrorImpl::error(self.erase()).source() }
     }
+
+    #[cfg(backtrace)]
+    fn provide<'a>(&'a self, req: &mut Demand<'a>) {
+        unsafe { ErrorImpl::provide(self.erase(), req) }
+    }
 }
 
 impl<E> Debug for ErrorImpl<E>

src/kind.rs

@@ -110,7 +110,7 @@ impl BoxedKind for Box<dyn StdError + Send + Sync> {}
 impl Boxed {
     #[cold]
     pub fn new(self, error: Box<dyn StdError + Send + Sync>) -> Error {
-        let backtrace = backtrace_if_absent!(error);
+        let backtrace = backtrace_if_absent!(&*error);
         Error::from_boxed(error, backtrace)
     }
 }

src/lib.rs

@@ -211,7 +211,7 @@
 //! non-Anyhow error type inside a function that returns Anyhow's error type.
 
 #![doc(html_root_url = "https://docs.rs/anyhow/1.0.62")]
-#![cfg_attr(backtrace, feature(backtrace))]
+#![cfg_attr(backtrace, feature(error_generic_member_access, provide_any))]
 #![cfg_attr(doc_cfg, feature(doc_cfg))]
 #![cfg_attr(not(feature = "std"), no_std)]
 #![deny(dead_code, unused_imports, unused_mut)]

src/wrapper.rs

@@ -1,6 +1,9 @@
 use crate::StdError;
 use core::fmt::{self, Debug, Display};
 
+#[cfg(backtrace)]
+use std::any::Demand;
+
 #[repr(transparent)]
 pub struct MessageError<M>(pub M);
 
@@ -67,12 +70,12 @@ impl Display for BoxedError {
 
 #[cfg(feature = "std")]
 impl StdError for BoxedError {
-    #[cfg(backtrace)]
-    fn backtrace(&self) -> Option<&crate::backtrace::Backtrace> {
-        self.0.backtrace()
-    }
-
     fn source(&self) -> Option<&(dyn StdError + 'static)> {
         self.0.source()
     }
+
+    #[cfg(backtrace)]
+    fn provide<'a>(&'a self, req: &mut Demand<'a>) {
+        self.0.provide(req);
+    }
 }