Support vec![const { ... }; n] syntax
#133412
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Sorry no, this is indeed a libs-api thing, as this is insta-stable and needs an FCP to merge. Assigning to dtolnay who seconded the ACP. |
library/alloc/src/vec/mod.rs
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| // SAFETY: If `value` is the result of some const expression, we can make as many | ||
| // copies as needed. |
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This is an interesting argument that I have not seen before. I can't see anything immediately wrong with it, but paging in @rust-lang/opsem for extra scrutiny.
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This should be the case with the current const capabilities, but I'm not 100% sure it holds with every possible extension, namely leaking const allocation to runtime. I think the rules around const object unification would make this a valid execution, but I don't have all the relevant info in cache.
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We do intent that consts can still be implemented as many copies of the same thing for codegen. So whatever we do with const allocation should support that.
But it may be good to put a comment in the source here explaining that this is the standard library doing privileged reasoning, and not a stable guarantee.
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I have updated the safety comment. Please let me know if there is anything missing or inaccurate.
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| // Avoid calling the destructor of `elem`. |
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This raises the interesting question whether the destructor should be executed at all for vec![const { ... }; 0]. For 0-length arrays, our current behavior here is rather strange: #79580.
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I tested the array of const expression case, the destructor is not called: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=f9ffe1eb972bc415b0e6bf6bf48c18c1. This is consistent with the current implementation.
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Makes sense. Would be good to have a test.
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☔ The latest upstream changes (presumably #133533) made this pull request unmergeable. Please resolve the merge conflicts. |
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ACP: rust-lang/libs-team#484.