Change:
--> $DIR/issue-79690.rs:29:1 | LL | const G: Fat = unsafe { Transmute { t: FOO }.u }; <del> | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered read of part of a pointer at .1.<deref>.size.foo </del> <ins> | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered (potentially part of) a pointer at .1.<deref>.size.foo, but expected plain (non-pointer) bytes </ins> | = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.

Discussion:
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glacier ""
Error: Parsing glacier command in failed: ...'cier https' ''... Please let know if you're having trouble with this bot.
Compiler error on but ICE on label +regression-from-stable-to-stable
cc
A previous (now-fixed) ICE with the same error message is
Thanks for the report! Would be good to get a better backtrace; this one seems to be missing many frames and there are no line numbers.
, I updated the backtrace in my first post. I used rustup to get the latest trunk version, and the one I have been using does not have line numbers. If there is a flag or configuration for rustup to get rustc binaries with line numbers, I would be happy to change the settings on my testing machine.
I think getting line numbers requires a debug build of the compiler, so it's not quite as easy as installing something via rustup.
Labeling .
Here's a backtrace with debug info: <details</details
Unfortunately, this bottoms out ; more digging will be needed to determine the true underlying cause of the error.
Here is the actual backtrace of the underlying CTFE/Miri error: <details</detailsIt points to this line: can handle pointer values just fine, so I think what is happening is that we are reading parts of a pointer value here, which indeed is not supported in CTFE.
So... I guess the fix is to make this kind of error also be permitted during validation and turn it into a proper validation error?
Permitting it at the top-level is no good, since then we don't know "where in the value" it arose. We need a version of that catches this error and turns it into a validation error with the current "path".
Oh... right, we should fix this in general, and not just wrap in .

Maybe we should not have any automatic way to bubble up errors and require all sites to use ?
I think we should wrap it but put the wrapped thing into a helper function sow e can also use it for the other places. So basically, you want to use a different error type for validation, one that excludes all the other errors? I am open to refactorings here.
Let's start out with wrapping, and then I can experiment with the refactoring so we can have a look at the usability effect of it Instructions: wrap the call at in a macro invocation and capture the error. Emit a validation message about the fact that reading parts of a pointer is not possible during CTFE. Don't forget to add a test (the failing example from this issue works great as a test) and run the test with
There are quite a few calls in that will all have this problem, so ideally they should all be treated this way. To avoid code duplication, it will probably make sense to move this into a helper function.
Reopening this to track that is still called unguarded a few times in , so e.g. the issue can still arise with .