How to use backtrace after stabilisation?

[Morganamilo]

I can't quite figure out how to use backtrace now that it's stable.

The backtrace method is gone from the error type, however looking at the docs it says:

If using the nightly channel, or stable with features = ["backtrace"], a backtrace is captured and printed with the error if the underlying error type does not already provide its own. In order to see backtraces, they must be enabled through the environment variables described in std::backtrace:

This seems to imply it prints automatically now but it does not. Searching the docs for backtrace doesn't give any results.

[ArekPiekarz]

Which compiler version do you use? Backtrace is stabilized, but still only on nightly - it will arrive on the stable branch in Rust 1.65.

[Morganamilo]

I'm using nightly, where it's currently stable.

[ArekPiekarz]

I guess one would need to use the Provider API.

[Morganamilo]

I don't get what this has to do with me printing a backtrace.

[dtolnay]

It should still work as documented:

fn main() {
    let error = anyhow::Error::msg("huh");
    println!("{:?}", error);
}

$ RUST_LIB_BACKTRACE=1 cargo run

huh

Stack backtrace:
   0: anyhow::error::<impl anyhow::Error>::msg
             at anyhow-1.0.64/src/error.rs:83:36
   1: repro::main
             at ./src/main.rs:2:17
   2: core::ops::function::FnOnce::call_once
             at /rustc/289279de116707f28cf9c18e4bbb8c6ec84ad75b/library/core/src/ops/function.rs:248:5
   …
  13: std::rt::lang_start_internal
             at /rustc/289279de116707f28cf9c18e4bbb8c6ec84ad75b/library/std/src/rt.rs:148:20
  14: std::rt::lang_start
             at /rustc/289279de116707f28cf9c18e4bbb8c6ec84ad75b/library/std/src/rt.rs:165:17
   …
  18: _start

or using println!("{:#?}", error.backtrace()):

Backtrace [
    { fn: "anyhow::error::<impl anyhow::Error>::msg", file: "anyhow-1.0.64/src/error.rs", line: 83 },
    { fn: "testing::main", file: "./src/main.rs", line: 2 },
    { fn: "core::ops::function::FnOnce::call_once", file: "/rustc/289279de116707f28cf9c18e4bbb8c6ec84ad75b/library/core/src/ops/function.rs", line: 248 },
    …
    { fn: "std::rt::lang_start_internal", file: "/rustc/289279de116707f28cf9c18e4bbb8c6ec84ad75b/library/std/src/rt.rs", line: 148 },
    { fn: "std::rt::lang_start", file: "/rustc/289279de116707f28cf9c18e4bbb8c6ec84ad75b/library/std/src/rt.rs", line: 165 },
    …
    { fn: "_start" },
]

[dtolnay]

Be mindful that a specific anyhow version is specific to one particular iteration of the unstable backtrace API, so for the backtrace API that's used by rustc nightly-2022-08-13 or newer, you would need to be using anyhow 1.0.63 or newer.

[Morganamilo]

Hey so I'm not home right now so I can't quite test this. But my code doesn't just println the error. I manually unwind the context to print it how I want. I would like some way to also print the backtrace manually.

[dtolnay]

Then #262 (comment) is the answer to that. For example using https://doc.rust-lang.org/nightly/core/any/fn.request_ref.html:

#![feature(provide_any)]

use std::any;
use std::backtrace::Backtrace;

fn main() {
    let error = anyhow::Error::msg("huh");
    println!("{:#?}", error.backtrace());
    for cause in error.chain().skip(1) {
        if let Some(backtrace) = any::request_ref::<Backtrace>(cause) {
            println!("{:#?}", backtrace);
        }
    }
}