Printing backtrace will leak PDB file handles

[andylizi]

backtrace-rs uses StackWalkEx on Windows, which calls SymLoadModuleEx in turn. The symbol modules are never unloaded afterward, keeping the PDB files open until the process exits.

Normally this wouldn't be an issue, since most programs exit on panic anyway. But it's causing rust-lang/rust-analyzer#9932, where when a proc macro panics in rust-analyzer, it locks the PDB file and making subsequent build attempts fail:

note: LINK : fatal error LNK1201: error writing to program database 'target\debug\deps***_derive-fd4461955b1f812b.pdb';
check for insufficient disk space, invalid path, or insufficient privilege

Steps to reproduce

// Run with `RUST_BACKTRACE=1`
fn main() {
    let _ = std::panic::catch_unwind(|| panic!());
    loop { std::thread::park(); }
}

[alexcrichton]

One thing you might be able to try is to call SymCleanup perhaps? I think you'd need to call SymInitialize at some point afterwards though since this crate thinks that the symbols have already been initialized. I don't know precisely how this works internaly in dbghelp.dll though.