Implement lint for black_boxing ZSTs #150037
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The job Click to see the possible cause of the failure (guessed by this bot) |
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| lint_builtin_black_box_zst_call = `black_box` on zero-sized callable `{$ty}` has no effect on call opacity | ||
| .label = zero-sized callable passed here | ||
| lint_builtin_black_box_zst_help = coerce to a function pointer and call `black_box` on that pointer instead |
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It'd be cool if we were actually able to suggest a code fix here! But, I don't think that's necessary for this PR, just a nice future improvement.
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Like? any example or idea?
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guess the error message already says to coerce the object to a pointer, but we could also show a suggested code snippet. I think that would also let rust-analyzer make the change automatically. Given an error message like:
error: `black_box` on zero-sized callable `fn(u32, u32) -> u32 {add}` has no effect on call opacity
--> $DIR/lint-black-box-zst-call.rs:10:18
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LL | let add_bb = black_box(add);
| ^^^^^^^^^^---^
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| zero-sized callable passed here
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= note: zero-sized callable values have no runtime representation, so the call still targets the original function directly
= help: coerce to a function pointer and call `black_box` on that pointer instead
The suggestion would be something like:
let add_ptr: fn(u32, u32) -> u32 = add;
let add_bb = black_box(add_ptr);
We have some other errors and warnings that make suggestions like this, so I'd try to look at what they do and see if you can reuse that infrastructure.
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@bors r+ |
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Lints are under the purview of t-lang. This is insta-stable as far as I can see. Has there been a t-lang FCP? |
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@bors r- |
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Ah, good call! Thanks for stepping in here. |
Thanks for flagging this. I am happy to follow whatever process is needed. |
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r? eholk |
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Nominated for @rust-lang/lang since this adds a new lint. |
In the PR lint for black_boxing ZSTs is implemented as discussed in #137658
This PR implements a new lint which warns when
std::hint::black_boxis called with a Zero-Sized Type (ZST).Why the lint is needed?
The compiler is currently "silently failing" to do what the user asked. A warning closes the gap between what the user thinks is happening and what is actually happening. Users reach for black_box specifically to prevent optimizations. When passed a ZST black_box effectively does nothing and the user receives no feedback. Creating confusion and flaws.
Changes:
Added
#[rustc_diagnostic_item = "black_box"]tolibrary/core/src/hint.rs.Implemented a
LateLintPassto detect calls toblack_boxwhere the argument's layout is a ZST.Closes: #137658