r? @jackh726

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There's another possible implementation, where we issue E0161 for unsized box expressions even when unsized_locals are disabled. That approach keeps the checking in rustc_mir, but it currently emits duplicate errors for cases like rust fn foo(x: Box<[i32]>) { box *x; }

Given that the implementation in this PR gives a clearer error message, I think it's probably the better option.

Thanks! Going to @bors rollup=never in case there are any subtle changes here that need to be bisected.

@bors r+

@bors r+ rollup=never

📌 Commit c75a930 has been approved by jackh726

I wonder if it's worth a new obligation cause code if we want to remove box syntax eventually anyway 🤔

Those are cheap :)

⌛ Testing commit c75a930 with merge 1e3d632...

☀️ Test successful - checks-actions
Approved by: jackh726
Pushing 1e3d632 to master...

Thanks for the review!

I wonder if it's worth a new obligation cause code if we want to remove box syntax eventually anyway thinking

I don't think this will complicate it, since check_box can be removed in its entirety when the syntax goes away. The cost should only be paid by code that uses box expressions.