require CTFE to detect UB #60
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RalfJung
commented
Oct 31, 2020
| This can change from compiler version to compiler version: CTFE code that causes UB could build fine with one compiler and fail to build with another. | ||
| (This is in accordance with the general policy that unsound code is not subject to strict stability guarantees.) | ||
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| [UB]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html | ||
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| # Reference-level explanation | ||
| [reference-level-explanation]: #reference-level-explanation |
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As usual I have no idea how to split the RFC intention across "guide-level" and "reference-level" explanation.^^ That separation makes very little sense to me, for most of the RFCs that I end up writing...
oli-obk
approved these changes
Nov 1, 2020
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non-detected UB, CTFE continues in a reasonable way, instead of producing an arbitrary result.
yes, we should not permit CTFE to explicitly create bogus values on UB when, in order to do that, it would need to detect the UB and choose to create a bogus value. If UB is detected, interpretation must be aborted. If UB is not detected, CTFE continues in a reasonable way.
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This is what @oli-obk and me agreed on here, except I went one step further and now require that for non-detected UB, CTFE continues in a reasonable way, instead of producing an arbitrary result. Once we keep the unoptimized MIR around, that is cheap to do. For example, this means that when the code violates alignment, CTFE must either raise an error or continue as if the pointer was actually well-aligned (and perform the appropriate load from the given region of memory). The previous wording would have allowed CTFE to return
Undefwhen loading from an unaligned pointer -- I cannot see a good reason why we would permit CTFE to do that.