"temporary value dropped while borrowed" occurs with let-chains

[aznhe21]

I tried this code:

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=02cc60332ebe5829791b841b96fa8c5b

fn let_chains(entry: std::io::Result<std::fs::DirEntry>) {
    if let Ok(entry) = entry
        && let Some(s) = entry.file_name().to_str()
        && s.contains("")
    {
        println!("will fail to compile");
    }
}

fn multiple_ifs(entry: std::io::Result<std::fs::DirEntry>) {
    if let Ok(entry) = entry {
        if let Some(s) = entry.file_name().to_str() {
            if s.contains("") {
                println!("will compile successfully");
            }
        }
    }
}

I expected to see this happen: both compiles successfully

Instead, this happened: let_chains fails to compile

error[E0716]: temporary value dropped while borrowed
 --> src/lib.rs:3:26
  |
3 |         && let Some(s) = entry.file_name().to_str()
  |                          ^^^^^^^^^^^^^^^^^        - temporary value is freed at the end of this statement
  |                          |
  |                          creates a temporary which is freed while still in use
4 |         && s.contains("")
  |            -------------- borrow later used here
  |
  = note: consider using a `let` binding to create a longer lived value

For more information about this error, try `rustc --explain E0716`.
error: could not compile `rust2` due to previous error

Meta

rustc --version --verbose:

rustc 1.65.0-nightly (d394408fb 2022-08-07)
binary: rustc
commit-hash: d394408fb38c4de61f765a3ed5189d2731a1da91
commit-date: 2022-08-07
host: x86_64-unknown-linux-gnu
release: 1.65.0-nightly
LLVM version: 14.0.6

[Noratrieb]

This may have something to do with #99852, as there is something about let_chain drops here as well, but I don't for sure.

[c410-f3r]

The same behaviour happens with an older compiler version. Here goes an example that compiles with let_chains.

fn let_chains(entry: std::io::Result<std::fs::DirEntry>) {
    if let Ok(entry) = entry
        && let file_name = entry.file_name()
        && let Some(s) = file_name.to_str()
        && s.contains("")
    {
        println!("Ok");
    }
}

It is not up to me to decide if this behaviour is a blocker that will revert stabilization but in case of such event, I won't have enough free time to investigate and fix stuff in the near future.

[c410-f3r]

Personally and despite the differences, I think that the let_chains version is "correct" and the nested version is surprisingly holding a reference of an owned structure (file_name()) without an explicit bind.

[Noratrieb]

if let drops the scrutinee after the statement. Playground

[c410-f3r]

cc @est31 who have been dealing with dropping stuff related to let else

[est31]

Some things that might be helpful to know:

• The drop order changes for let else in Let-else: break out scopes when a let-else pattern fails to match #99518 also had borrow checker impact and made an example compile (see the PR's thread). I'm not a borrow checker expert, so I might be wrong, but this too might be fixed by drop order changes alone.
• In Add let else drop order tests #99291 I'm studying drop order impacts of conditional statements, motivated by let else, but the test that PR adds also includes let chains. There I observed that let chains generally behave drop order wise like if let when there is a let involved, and if when it's a bool. I have only studied them regarding the body contained inside though. This table provides a summary of drop orders (disclaimer: it's outdated for let else): Tracking Issue for RFC 3137: let-else statements #87335 (comment)

[est31]

I've locally amended src/test/ui/let-else/let-else-drop-order-nested.rs of #99291 to add if let chains:

fn if_let_chain_let(v: u32) {
    macro_rules! if_let_chain {
        ($arg:pat, $exp:expr) => {
            if true && let $arg = $exp && ({
                print!("postfix,");
                true
            },).0 {
                print!("body,");
            } else {
                print!("else,");
            }
        };
    }
    nestings!(if_let_chain, "if let chain (let)", v);
    print!("\n  ref &:        ");
    if_let_chain!(0, &Droppy(v).0);

    print!("\n  ref mut &mut: ");
    if_let_chain!(0, &mut Droppy(v).0);

    println!("");
}

And this is the output:

v is 0 (match), construct is if let chain (let)
  vanilla:      drop,postfix,body,
  &:            drop,postfix,body,
  &mut:         drop,postfix,body,
  move:         drop,postfix,body,
  tuple:        drop,postfix,body,
  array:        drop,postfix,body,
  nested:       drop,inner,postfix,body,
  fn(this):     drop,postfix,body,
  fn(&self):    drop,postfix,body,
  ref &:        drop,postfix,body,
  ref mut &mut: drop,postfix,body,
[...]
v is 1 (mismatch), construct is if let chain (let)
  vanilla:      drop,else,
  &:            drop,else,
  &mut:         drop,else,
  move:         drop,else,
  tuple:        drop,else,
  array:        drop,else,
  nested:       drop,inner,else,
  fn(this):     drop,else,
  fn(&self):    drop,else,
  ref &:        drop,else,
  ref mut &mut: drop,else,

Which actually runs counter to what I wrote above, because it predrops. It seems that it depends on whether the let in the chain is preceded by a && or not: if you change the line with if true && let $arg = $exp && ({ to if let $arg = $exp && ({, removing the true &&, you get this:

v is 0 (match), construct is if let chain (let)
  vanilla:      postfix,body,drop,
  &:            postfix,body,drop,
  &mut:         postfix,body,drop,
  move:         postfix,body,drop,
  tuple:        postfix,body,drop,
  array:        postfix,body,drop,
  nested:       postfix,body,drop,inner,
  fn(this):     postfix,body,drop,
  fn(&self):    postfix,body,drop,
  ref &:        postfix,body,drop,
  ref mut &mut: postfix,body,drop,
[...]
v is 1 (mismatch), construct is if let chain (let)
  vanilla:      else,drop,
  &:            else,drop,
  &mut:         else,drop,
  move:         else,drop,
  tuple:        else,drop,
  array:        else,drop,
  nested:       else,drop,inner,
  fn(this):     else,drop,
  fn(&self):    else,drop,
  ref &:        else,drop,
  ref mut &mut: else,drop,

So that rule that I wrote above is not correct, but the underlying behaviour is more complicated. Hmmm...

[c410-f3r]

Thanks @est31

[nathanwhit]

@rustbot claim

[pnkfelix]

@wesleywiser pointed this out to me.

It may be motivation to revert the stablization of let-chains on beta. Lets try to ensure we talk about this at Thursday triage

@rustbot label I-compiler-nominated

[pnkfelix]

False alarm; @wesleywiser pointed out to me that let-chains is already been marked unstable (on beta and nightly).

@rustbot label: -I-compiler-nominated