DebugMap::finish panics if a value returns fmt::Error

[Marwes]

use std::fmt;

struct ErrorFmt;

impl fmt::Debug for ErrorFmt {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        Err(fmt::Error)
    }
}

struct A {
    field: ErrorFmt,
}

impl fmt::Debug for A {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_map()
            .entry(&"field", &ErrorFmt)
            .finish()
    }
}

fn main() {
    println!("{:?}", A { field: ErrorFmt });
}

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=4376349fa408130a65c262873bf24055

thread 'main' panicked at 'attempted to finish a map with a partial entry', src/libcore/fmt/builders.rs:924:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.

I think the assert should just be removed so that the error gets propagated up?

[Centril]

cc @KodrAus

[KodrAus]

Ah we could probably shift those assertions so they're only checked so long as there isn't an error captured (by moving them within the and_then call inside DebugMap). They don't make any sense if you're already short-circuiting formatting.