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"a temporary value which is freed while still in use" in nightly but not on stable #105891

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"a temporary value which is freed while still in use" in nightly but not on stable#105891
Labels
C-bugCategory: This is a bug.P-mediumMedium priorityT-compilerRelevant to the compiler team, which will review and decide on the PR/issue.regression-from-stable-to-stablePerformance or correctness regression from one stable version to another.
Milestone
1.67.0

Description

@poliorcetics
poliorcetics
opened on Dec 19, 2022

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/github.com-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>

Activity

  1. poliorcetics
    added
    C-bugCategory: This is a bug.
    regression-untriagedUntriaged performance or correctness regression.
    on Dec 19, 2022
  2. rustbot
    added
    regression-from-stable-to-nightlyPerformance or correctness regression from stable to nightly.
    I-prioritizeIssue needs a team member to assess the impact. Will be replaced by P-{low,medium,high,critical}
    and removed
    regression-untriagedUntriaged performance or correctness regression.
    on Dec 19, 2022

  3. poliorcetics commented on Dec 19, 2022

    @poliorcetics
    poliorcetics
    on Dec 19, 2022
    ContributorAuthor

    This also fails in beta, (Rust 1.67 beta)

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

  4. rustbot
    added
    regression-from-stable-to-betaPerformance or correctness regression from stable to beta.
    on Dec 19, 2022

  5. zachs18 commented on Dec 19, 2022

    @zachs18
    zachs18
    on Dec 19, 2022 · Hidden as duplicate

  6. albertlarsan68 commented on Dec 19, 2022

    @albertlarsan68
    albertlarsan68
    on Dec 19, 2022
    Member

    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

  7. rustbot
    removed
    regression-from-stable-to-nightlyPerformance or correctness regression from stable to nightly.
    on Dec 19, 2022

  8. est31 commented on Dec 19, 2022

    @est31
    est31
    on Dec 19, 2022
    Member

    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.

  9. jruderman commented on Dec 19, 2022

    @jruderman
    jruderman
    on Dec 19, 2022
    Contributor

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

  10. est31 commented on Dec 19, 2022

    @est31
    est31
    on Dec 19, 2022
    Member

    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.

  11. apiraino commented on Dec 21, 2022

    @apiraino
    apiraino
    on Dec 21, 2022
    Contributor

    WG-prioritization assigning priority (Zulip discussion).

    @rustbot label -I-prioritize +P-medium

  12. rustbot
    added
    P-mediumMedium priority
    and removed
    I-prioritizeIssue needs a team member to assess the impact. Will be replaced by P-{low,medium,high,critical}
    on Dec 21, 2022
  13. apiraino
    added
    T-compilerRelevant to the compiler team, which will review and decide on the PR/issue.
    on Dec 21, 2022
  14. Mark-Simulacrum
    added
    regression-from-stable-to-stablePerformance or correctness regression from one stable version to another.
    and removed
    regression-from-stable-to-betaPerformance or correctness regression from stable to beta.
    on Mar 3, 2023
  15. Mark-Simulacrum
    added this to the 1.67.0 milestone on Mar 3, 2023

  16. poliorcetics commented on Aug 7, 2026

    @poliorcetics
    poliorcetics
    on Aug 7, 2026
    ContributorAuthor

    Lint is now (using est31's reduced example above)

     --> src/main.rs:5:24
  |
5 |     let _res = is_not(&"windows".to_string()) && is_not(&"macos".to_string());
  |                        ^^^^^^^^^^^^^^^^^^^^^-    ------ borrow later used here
  |                        |                    |
  |                        |                    temporary value is freed at the end of this statement
  |                        creates a temporary value which is freed while still in use
  |
help: consider using a `let` binding to create a longer lived value
  |
5 ~     let binding = "windows".to_string();
6 ~     let _res = is_not(&binding) && is_not(&"macos".to_string());
  |

    which I feel is a lot clearer and fixes the issue

  17. poliorcetics
    closed this as completedon Aug 7, 2026
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