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1.100-beta regression: `box` patterns have been removed (feature `box_patterns`)

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Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
65/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Active
Tech stack
rust
Domain
compilers

Research direction

The regression is reported at src/element/simplify.rs:63, in a box pattern matching a negated operand. Read that match arm and check the compiler guidance in the issue; then run the project’s tests or crater reproduction to verify the pattern compiles on beta and the simplification still works. The payload does not name a specific test file.

Written by the indexing model from the issue text.

Description

I-prioritize needs-triage regression-from-stable-to-beta T-lang

Found in crater run: https://crater-reports.s3.amazonaws.com/beta-1.100/1.99.0-beta.8/gh/vic1707.xprs/log.txt

Code:

/* match arm in match block */
            #[cfg(NIGHTLY)]
            // (-a) + a => 0
            BinOp {
                op: Plus,
                lhs: Element::UnOp(box UnOp { op: Minus, operand }),
                rhs,
            } if operand == rhs => Number(0.0),

Warning on stable:

[INFO] [stderr] warning: box pattern syntax is experimental
[INFO] [stderr]   --> src/element/simplify.rs:63:36
[INFO] [stderr]    |
[INFO] [stderr] 63 |                 lhs: Element::UnOp(box UnOp { op: Minus, operand }),
[INFO] [stderr]    |                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[INFO] [stderr]    |
[INFO] [stderr]    = note: see issue #29641 <https://github.com/rust-lang/rust/issues/29641> for more information
[INFO] [stderr]    = warning: unstable syntax can change at any point in the future, causing a hard error!
[INFO] [stderr]    = note: for more information, see issue #154045 <https://github.com/rust-lang/rust/issues/154045>

Error on beta:

[INFO] [stdout] error: `box` patterns have been removed (feature `box_patterns`)
[INFO] [stdout]   --> src/element/simplify.rs:63:36
[INFO] [stdout]    |
[INFO] [stdout] 63 |                 lhs: Element::UnOp(box UnOp { op: Minus, operand }),
[INFO] [stdout]    |                                    ^^^
[INFO] [stdout]    |
[INFO] [stdout]    = help: enable feature `deref_patterns` instead and...
[INFO] [stdout] help: ...if possible just remove keyword `box`...
[INFO] [stdout]    |
[INFO] [stdout] 63 -                 lhs: Element::UnOp(box UnOp { op: Minus, operand }),
[INFO] [stdout] 63 +                 lhs: Element::UnOp(UnOp { op: Minus, operand }),
[INFO] [stdout]    |
[INFO] [stdout] help: ...otherwise replace it with an invocation of macro `deref`
[INFO] [stdout]    |
[INFO] [stdout] 63 -                 lhs: Element::UnOp(box UnOp { op: Minus, operand }),
[INFO] [stdout] 63 +                 lhs: Element::UnOp(deref!(UnOp { op: Minus, operand })),
[INFO] [stdout]    |
Dominant language
Rust
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Merged PRs (30d)
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