abi: unsized field in union - assert to delay bug by davidtwco · Pull Request #...

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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 #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.