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Borrow checker false dropped while still borrowed #38915
Description
The following does not compile:
use std::ffi::OsString;
use std::io::{Result as IoResult};
use std::path::Path;
fn main() {
let paths: Vec<OsString> = vec!["filename".into()];
// Switching these two lines appeases our lord and savior the borrow checker
let f = std::fs::rename;
let dests = paths.clone();
// ---------------------------
let pairs = paths.iter().zip(dests.iter());
// ----- borrow occurs here
foo(f, pairs);
}
// `dests` does not live long enough.
// ^ `dests` dropped here while still borrowed
fn foo<P, I>(f: fn(P, P) -> IoResult<()>, pairs: I)
where P: AsRef<Path>,
I: Iterator<Item=(P, P)>,
{
for (src, dest) in pairs {
let _ = f(src, dest);
}
}EDIT: simplified code.
Reacted by Dmytro Meleshko
Activity
durka commentedon Jan 9, 2017
That's bizarre. Somehow the borrow checker thinks that
fcontains a borrow ofdests. Removing the call tofooalso eliminates the error.letheed commentedon Jan 9, 2017
Exactly. I have a hard time figuring out what is happening here; I'm not familiar with the compiler's internals.
philipc commentedon Jan 9, 2017
Simplified a bit more:
Rufflewind commentedon Oct 30, 2017
Looking at @philipc's example, I believe the problem is that the type of the local variable
fis actually the specialized type (since Rust does not support generics as first-class values). If you try to write out the specialized type, you get:for some unknown
'a. The problem is that the presence of'aaffects not just the argument but also the function pointer itself, so the function pointer cannot outlive'aeither.If we could somehow specialize
for<T> fn(T)asfor<'a> fn(&'a i32)(is this sound?), then this could be worked around by giving the local variablefthe correct type annotation.letheed commentedon Nov 27, 2017
@Rufflewind That was a long time ago but I remember trying to use
for<'a>because I something like that, but to no avail.memoryruins commentedon Jan 30, 2019
Triage: now compiles on 2018 edition.
2015 errors:
rustc: 1.32.0Auto merge of #59114 - matthewjasper:enable-migate-2015, r=<try>
Auto merge of #59114 - matthewjasper:enable-migate-2015, r=pnkfelix
Auto merge of #59114 - matthewjasper:enable-migate-2015, r=pnkfelix