"a temporary value which is freed while still in use" in nightly but not on stable

[poliorcetics]

Code

I tried this code:

// Do: cargo add obfstr

fn main() {
    let product = "macos";
    let is_not = |s| !product.eq_ignore_ascii_case(s);

    let _res = is_not(obfstr::obfstr!("windows")) && is_not(obfstr::obfstr!("macos"));
}

I expected to see this happen: the code compiles without problems on stable and nightly

Instead, this happened: compilation failed but only on nightly

RUSTFLAGS="-Z macro-backtrace" cg +nightly build -q

error[E0716]: temporary value dropped while borrowed
   --> /Users/alexis/.local/share/cargo/registry/src/mygithub.libinneed.workers.dev-1ecc6299db9ec823/obfstr-0.4.1/src/bytes.rs:93:16
    |
54  |   macro_rules! obfstr {
    |   ------------------- in this expansion of `obfstr::obfstr!` (#1)
...
66  |           $crate::unsafe_as_str($crate::obfbytes!($s.as_bytes()))
    |                                 -------------------------------- in this macro invocation (#2)
...
72  |   macro_rules! obfbytes {
    |   --------------------- in this expansion of `$crate::obfbytes!` (#2)
...
86  |           &$crate::__obfbytes!($s)
    |            ----------------------- in this macro invocation (#3)
...
92  |   macro_rules! __obfbytes {
    |   ----------------------- in this expansion of `$crate::__obfbytes!` (#3)
93  |       ($s:expr) => {{
    |  ___________________^
94  | |         const _OBFBYTES_STRING: &[u8] = $s;
95  | |         const _OBFBYTES_LEN: usize = _OBFBYTES_STRING.len();
96  | |         const _OBFBYTES_KEYSTREAM: [u8; _OBFBYTES_LEN] = $crate::bytes::keystream::<_OBFBYTES_LEN>($crate::__entropy!("key", stringify!($...
...   |
103 | |             &_OBFBYTES_KEYSTREAM)
104 | |     }};
    | |_____^ creates a temporary value which is freed while still in use
    |
   ::: src/main.rs:5:49
    |
5   |       let _res = is_not(obfstr::obfstr!("windows")) && is_not(obfstr::obfstr!("macos"));
    |                         ---------------------------    ------ borrow later used here
    |                         |                         |
    |                         |                         temporary value is freed at the end of this statement
    |                         in this macro invocation (#1)
    |
    = 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 `pg2` due to previous error

Version it worked on

It most recently worked on: Rust 1.66

Version with regression

rustc --version --verbose:

rustc 1.68.0-nightly (d0dc9efff 2022-12-18)
binary: rustc
commit-hash: d0dc9efff14ac0a1eeceffd1e605e37eeb8362a0
commit-date: 2022-12-18
host: aarch64-apple-darwin
release: 1.68.0-nightly
LLVM version: 15.0.6

Backtrace

Backtrace

<backtrace>

[poliorcetics]

This also fails in beta, (Rust 1.67 beta)

@rustbot label +regression-from-stable-to-beta

[zachs18]

I'm guessing this is related to #103293 (merged 2022-12-04). I'm bisecting to find out.

Bisect results: "Regression in nightly-2022-12-05" (commit bisect in progress).

[albertlarsan68]

searched nightlies: from nightly-2022-10-29 to nightly-2022-12-08 (around where beta branched)
regressed nightly: nightly-2022-12-05
searched commit range: 2341517...53e4b9d
regressed commit: 19c250a (#103293, cc @est31 and @nagisa)

bisected with cargo-bisect-rustc v0.6.5

Host triple: x86_64-pc-windows-msvc
Reproduce with:

cargo bisect-rustc --preserve --access github --start 1.66.0 --regress=error --end 2022-12-08 -- check

@rustbot label -regression-from-stable-to-nightly

[est31]

This also fails on stable if you have a three-chain. E.g.:

let _res = is_not(obfstr::obfstr!("linux")) && is_not(obfstr::obfstr!("windows")) && is_not(obfstr::obfstr!("macos"));

gives:

error[E0716]: temporary value dropped while borrowed
 --> src/main.rs:5:59
  |
5 |     let _res = is_not(obfstr::obfstr!("linux")) && is_not(obfstr::obfstr!("windows")) && is_not(obfstr::obfstr!("macos"));
  |                                                           ^^^^^^^^^^^^^^^^^^^^^^^^^^-    ------ borrow later used here
  |                                                           |                         |
  |                                                           |                         temporary value is freed at the end of this statement
  |                                                           creates a temporary which is freed while still in use
  |
  = note: consider using a `let` binding to create a longer lived value
  = note: this error originates in the macro `$crate::__obfbytes` which comes from the expansion of the macro `obfstr::obfstr` (in Nightly builds, run with -Z macro-backtrace for more info)

I need to investigate more on what obfstr is doing, but it's likely that this is an expected breakage. With #103293, it was known that it is going to cause some breakage. crater runs showed only one regression at build time.

A workaround is easy, you can add &str:

fn main() {
    let product = "macos";
    let is_not = |s: &str| !product.eq_ignore_ascii_case(s);

    let _res = is_not(obfstr::obfstr!("windows")) && is_not(obfstr::obfstr!("macos"));
}

That way, your code will also work with three-chains.

[jruderman]

I guess adding &str changes the closure to borrow instead of move? Should the compiler suggest adding &str?

[est31]

Hmm this is basically the same regression as the one that broke spaad_internal.

Reducing the example from above to not need obfstr:

fn main() {
    let product = "macos";
    let is_not = |s| !product.eq_ignore_ascii_case(s);

    let _res = is_not(&"windows".to_string()) && is_not(&"macos".to_string());
}

Should the compiler suggest adding &str?

The best would be to fix the underlying closure lifetime (or type?) inference issue that got surfaced (not caused) by my PR in these instances. If that doesn't work, a suggestion would definitely improve things I think. I don't really know how those parts of the compiler work tho.

[apiraino]

WG-prioritization assigning priority (Zulip discussion).

@rustbot label -I-prioritize +P-medium