`backtrace` brings in `std` even with `std` feature disabled

I reproduced this with a minimal #![no_std] executable that does nothing but return an exit code:

main.rs

#![no_std]
#![no_main]

#[no_mangle]
fn main() -> i32 {
    42
}

#[panic_handler]
fn panic(_info: &core::panic::PanicInfo) -> ! {
    loop {}
}

cargo run works as expected (exits with code 42) with the following Cargo.toml, which includes the backtrace dependency (with default-features = false so it doesn't have the std feature):

Cargo.toml

[package]
name = "example"
version = "0.1.0"
edition = "2021"

[dependencies]
backtrace = { version = "0.3.69", default-features = false }

[profile.dev]
panic = "abort"

[profile.release]
panic = "abort"

At least in macOS, I needed to add compiler flags for this to run; here is my .cargo/config.toml

.cargo/config.toml

[target.'cfg(target_os = "macos")']
rustflags = [
  "-C",
  "link-arg=-e", # -e _main sets the entrypoint symbol since we're #[no_main]
  "-C",
  "link-arg=_main",
  "-C",
  "link-arg=-lSystem", # all macOS apps need -lSystem
  "-C", # Note: force-unwind-tables is not needed to reproduce
  "force-unwind-tables",
]

At this point, if I actually try to use backtrace - doing nothing else but adding use to main.rs like so:

use backtrace::trace_unsynchronized;

...now cargo check fails, saying that backtrace is bringing in the std crate which introduces a conflicting panic_impl implementation:

warning: unused import: `backtrace::trace_unsynchronized`
 --> src/main.rs:4:5
  |
4 | use backtrace::trace_unsynchronized;
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(unused_imports)]` on by default

error[E0152]: found duplicate lang item `panic_impl`
  --> src/main.rs:12:1
   |
12 | fn panic(_info: &core::panic::PanicInfo) -> ! {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: the lang item is first defined in crate `std` (which `backtrace` depends on)
   = note: first definition in `std` loaded from /Users/rtfeldman/.rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/aarch64-apple-darwin/lib/libstd-1990072ee01a7130.rlib
   = note: second definition in the local crate (`example`)

For more information about this error, try `rustc --explain E0152`.

Note that the error[E0152]: found duplicate lang item 'panic_impl' explicitly says the lang item is first defined in crate 'std' (which 'backtrace' depends on), and that everything worked until I added the (unused) use backtrace::trace_unsynchronized; declaration. Also, I know the std feature is off, because if I try to import something that needs it (e.g. backtrace::Backtrace), I get an error.

This is with stable rustc 1.68.0 (2c8cc3432 2023-03-06) and cargo 1.68.0 (115f34552 2023-02-26).

Unfortunately, this seems to make it impossible to use backtrace in a no_std executable - unless I'm missing something!

Start with the minimal no_std main.rs and Cargo.toml in the report, then inspect how importing backtrace::trace_unsynchronized enables the dependency graph despite default-features = false. Reproduce with the stated Rust and Cargo versions; done means cargo check succeeds with the import without a duplicate panic_impl from std.