Borrowed temporary dropped on nightly but works on stable

[djc]

I tried this code:

                writeln!(
                    cfg.process.stdout().lock(),
                    "{} ({})",
                    toolchain.name(),
                    match source {
                        ActiveSource::Default => &"default" as &dyn fmt::Display,
                        _ => &source.to_reason(),
                    }
                )?;

This compiles on stable (1.91). However, current nightly fails:

error[E0716]: temporary value dropped while borrowed
    --> src/cli/rustup_mode.rs:1225:31
     |
1223 | /                     match source {
1224 | |                         ActiveSource::Default => &"default" as &dyn fmt::Display,
1225 | |                         _ => &source.to_reason(),
     | |                               ^^^^^^^^^^^^^^^^^-
     | |                               |                |
     | |                               |                temporary value is freed at the end of this statement
     | |                               creates a temporary value which is freed while still in use
1226 | |                     }
     | |_____________________- borrow later used here
     |
     = note: consider using a `let` binding to create a longer lived value

I tried to make a minimal reproduction but this playground fails on stable, too.

Meta

nightly:

rustc 1.93.0-nightly (b15a874aa 2025-11-02)
binary: rustc
commit-hash: b15a874aafe7eab9ea3ac2c1d59c7b03e1425027
commit-date: 2025-11-02
host: aarch64-apple-darwin
release: 1.93.0-nightly
LLVM version: 21.1.3

rustc --version --verbose:

rustc 1.91.0 (f8297e351 2025-10-28)
binary: rustc
commit-hash: f8297e351a40c1439a467bbbb6879088047f50b3
commit-date: 2025-10-28
host: aarch64-apple-darwin
release: 1.91.0
LLVM version: 21.1.2

[Noratrieb]

@dianne

[theemathas]

Minimal reproduction: https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=47622718fd479b14b67f431c8515f951

There has to be at least two things being formatted in the macro for this weirdness to happen, and they must not be one of the things that get optimized by the compiler into part of the format string.

This code is supposed to produce this warning in stable, but for some reason the warning didn't fire. The warning was added in #147056.

The code currently compiles in stable due to a bug, that is intended to be fixed in version 1.92, making the code not compile.

[djc]

@theemathas thanks for taking the time. Fixing rustup in

• cli: avoid dropped temporary rustup#4584

[dianne]

Oops, it looks like I missed temporary scopes from match arms when writing the future-compatibility warning. The lint fires if you change the match to an if expression. Unfortunately, unless the fix for #145784 is pushed back to 1.93, I'm not sure much can be done, since it is indeed intentional breakage.

[theemathas]

Lang-nominating to reconfirm whether to continue with this intended breakage in 1.92, given that the FCW for it in 1.91 was incomplete.