Backtracing after stack overflow does not work on macOS

I'm trying to get a backtrace from a SIGSEGV caused by stack overflow (hitting guard page). It seems that this is not working on macOS.

My reproduction case:

use backtrace::Backtrace;
use std::{mem, ptr};

#[inline(never)]
fn f(x: i32) -> i32 {
    if x == 0 || x == 1 {
        1
    } else {
        f(x - 1) + f(x - 2)
    }
}

fn main() {
    unsafe {
        let mut handler: libc::sigaction = mem::zeroed();
        handler.sa_flags = libc::SA_ONSTACK;
        handler.sa_sigaction = trap_handler as usize;
        libc::sigemptyset(&mut handler.sa_mask);
        assert_eq!(libc::sigaction(libc::SIGSEGV, &handler, ptr::null_mut()), 0);

        // Backtracing from a normal SIGSEGV works
        //println!("Before invalid write");
        //ptr::write_volatile(0 as *mut u32, 0);
        //println!("After invalid write");

        // Backtracing from a stack overflow crashes
        println!("Before stack overflow");
        println!("{}", f(0xfffffff));
        println!("After stack overflow");
    }
}

unsafe extern "C" fn trap_handler(
    _: libc::c_int
) {
    println!("Backtrace begin");
    let backtrace = Backtrace::new_unresolved();
    println!("Backtrace result: {:?}", backtrace);
}

Output:

% ./target/release/backtrace-stackoverflow-bug
Before stack overflow
Backtrace begin
zsh: segmentation fault  ./target/release/backtrace-stackoverflow-bug

Rust version:

rustc 1.46.0-nightly (16957bd4d 2020-06-30)
binary: rustc
commit-hash: 16957bd4d3a5377263f76ed74c572aad8e4b7e59
commit-date: 2020-06-30
host: x86_64-apple-darwin
release: 1.46.0-nightly
LLVM version: 10.0

Start with the macOS reproduction and the call to Backtrace::new_unresolved() inside trap_handler, comparing it with the normal SIGSEGV case shown in the issue. Investigate why stack-overflow handling fails after “Backtrace begin”; done means the supplied reproduction produces a backtrace instead of crashing.