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The Wire · Showcase

RUST REVERTS DBG! MACRO AFTER REGRESSION CASCADE

By RepoJournal · Filed · About Rust

The `dbg!` macro is going back to square one after three separate regressions forced the compiler team to abandon an experimental tearing optimization.

Rust is reverting the `dbg!` macro to its original implementation across all branches [1]. The macro had been modified in rust-lang/rust#149869, but the change triggered a cascade of regressions that proved too costly to fix forward. Two regressions were caught and patched [1], but a third emerged in #155902 with a proposed fix still pending, making a full revert the safer path. This is the second revert on this change; the team already backed it out on beta to protect the 1.95 release. The decision reflects a pragmatic choice: experimental optimizations to core macros need ironclad testing before shipping, and three bites at the apple is enough.

Elsewhere on the compiler desk, a six-PR rollup landed that spans FreeBSD aarch64 support, miri subtree updates, and xray instrumentation for bare-metal targets [2]. The miri subtree was refreshed independently as well [3]. Privacy checking in the compiler got a structural upgrade [4], moving from fixed-point iteration to a queue-based approach that's more efficient when visibility updates propagate. On the stdarch side, intrinsic test generation got cleaner: the team removed manual indentation logic and now pipe generated sources through rustfmt [ref:11, ref:12], simplifying the test generator significantly. These are quiet wins that improve maintainability without user-facing changes.

Action items

References

  1. [1] Revert tearing changes to `dbg!` ↗ rust-lang/rust
  2. [2] Rollup of 6 pull requests ↗ rust-lang/rust
  3. [3] miri subtree update ↗ rust-lang/rust
  4. [4] Privacy: try use queue instead of fixed-point iteration ↗ rust-lang/rust
  5. [5] intrinsic-test: remove all indentation logic rust-lang/stdarch
  6. [6] intrinsic-test: run rustfmt on generated sources ↗ rust-lang/stdarch

FAQ

What changed in Rust on May 20, 2026?
The `dbg!` macro is going back to square one after three separate regressions forced the compiler team to abandon an experimental tearing optimization.
What should Rust teams do about it?
Watch for dbg! behavior changes in nightly; the revert lands immediately • If you use `aarch64-unknown-none` with xray profiling, the infrastructure is now in place • Monitor PR #155915 on the remaining dbg! regression; this will determine if the revert holds
Which Rust repositories shipped on May 20, 2026?
rust-lang/rust, rust-lang/stdarch

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