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abi: unsized field in union - assert to delay bug Fixes #113279. > Unions cannot have unsized fields, and as such, layout computation for unions asserts that each union field is sized (as this would normally have halted compilation earlier). > > However, if a generator ends up with an unsized local - a circumstance in which an error will always have been emitted earlier, for example, if attempting to dereference a `&str` - then the generator transform will produce a union with an unsized field. > > Since https://github.com/rust-lang/rust/pull/110107, later passes will be run, such as constant propagation, and can attempt layout computation on the generator, which will result in layout computation of `str` in the context of it being a field of a union - and so the aforementioned assertion would cause an ICE. > > It didn't seem appropriate to try and detect this case in the MIR body and skip this specific pass; tainting the MIR body or delaying a bug from the generator transform (or elsewhere) wouldn't prevent this either (as neither would prevent the later pass from running); and tainting when the deref of `&str` is reported, if that's possible, would unnecessarily prevent potential other errors from being reported later in compilation, and is very tailored to this specific case of getting a unsized type in a generator. > > Given that this circumstance can only happen when an error should have already been reported, the correct fix appears to be just changing the assert to a delayed bug. This will still assert if there is some circumstance where this occurs and no error has been reported, but it won't crash the compiler in this instance. While debugging this, I noticed a translation ICE in a delayed bug, so I fixed that too: > During borrowck, the `MultiSpan` from a buffered diagnostic is cloned and used to emit a delayed bug indicating a diagnostic was buffered - when the buffered diagnostic is translated, then the cloned `MultiSpan` may contain labels which can only render with the diagnostic's arguments, but the delayed bug being emitted won't have those arguments. Adds a function which clones `MultiSpan` without also cloning the contained labels, and use this function when creating the buffered diagnostic delayed bug. |
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