improve offset_from docs #82751
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improve offset_from docs #82751
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r? @dtolnay (rust-highfive has picked a reviewer for you, use r? to override) |
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Cc @rust-lang/lang I think this is already an established guarantee that "two pointers are within some allocated object of Rust type |
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| /// For instance, no known 64-bit platform can ever serve a request | ||
| /// for 2<sup>63</sup> bytes due to page-table limitations or splitting the address space. | ||
| /// However, some 32-bit and 16-bit platforms may successfully serve a request for | ||
| /// more than `isize::MAX` bytes with things like Physical Address | ||
| /// Extension. As such, memory acquired directly from allocators or memory | ||
| /// mapped files *may* be too large to handle with this function. | ||
| /// (Note that [`offset`] and [`add`] also have a similar limitation and hence cannot be used on |
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Is the link for offset defined somewhere?
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Yeah, it already exists and is further up.
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@bors r+ rollup |
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📌 Commit ebe0407 has been approved by |
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Rollup of 13 pull requests Successful merges: - rust-lang#77916 (Change built-in kernel targets to be os = none throughout) - rust-lang#82130 (Make some Option, Result methods unstably const) - rust-lang#82292 (Prevent specialized ZipImpl from calling `__iterator_get_unchecked` twice with the same index) - rust-lang#82402 (Remove RefCell around `module_trait_cache`) - rust-lang#82592 (Improve transmute docs with further clarifications) - rust-lang#82651 (Cleanup rustdoc warnings) - rust-lang#82720 (Fix diagnostic suggests adding type `[type error]`) - rust-lang#82751 (improve offset_from docs) - rust-lang#82793 (Move some tests to more suitable subdirs) - rust-lang#82803 (rustdoc: Add an unstable option to print all unversioned files) - rust-lang#82808 (Sync rustc_codegen_cranelift) - rust-lang#82822 (Fix typo) - rust-lang#82837 (tweak MaybeUninit docs) Failed merges: r? `@ghost` `@rustbot` modify labels: rollup
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@thomcc pointed out that the current docs leave it kind of unclear how one can satisfy the "no wrapping around
isizeor the address space" requirement ofoffset_from, so make the docs clearer about that.FWIW, I don't think I entirely agree with that second paragraph about large objects (that I left mostly unchanged here). LLVM, to my knowledge, fundamentally assumes that all allocations fit into an
isize::MAX. So in that sense creating a larger allocation is simply UB. I would expect a guarantee that Rust heap allocation methods will never return allocations larger thanisize::MAX(or rather, Rust heap allocation methods should require that theLayoutis no larger thanisize::MAX). However, I cannot find any such requirement documented currently. Large allocations are not mentioned at all in the allocator docs, which is quite surprising -- even if we say that such allocations are not insta-UB (which I think is incompatible with LLVM), they are still extremely footgunny sinceptr::offset/ptr::adddo not support offsetting by more thanisize::MAXbytes.Furthermore, the allocator docs don't even say anything about allocations wrapping around the address space. But that is certainly something allocators must ensure never happens; we cannot expect clients to defend against this.
Cc @rust-lang/wg-allocators