[Bug rust/30330] GDB 13.1 no longer prints length of Rust slice wrappers

["tromey at sourceware dot org" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=30330

--- Comment #7 from Tom Tromey <tromey at sourceware dot org> ---
I found this comment in
rust/compiler/rustc_codegen_llvm/src/debuginfo/metadata.rs:

/// Create debuginfo for `[T]` and `str`. These are unsized.
///
/// NOTE: We currently emit just emit the debuginfo for the element type here
/// (i.e. `T` for slices and `u8` for `str`), so that we end up with
/// `*const T` for the `data_ptr` field of the corresponding fat-pointer
/// debuginfo of `&[T]`.
///
/// It would be preferable and more accurate if we emitted a DIArray of T
/// without an upper bound instead. That is, LLVM already supports emitting
/// debuginfo of arrays of unknown size. But GDB currently seems to end up
/// in an infinite loop when confronted with such a type.


I don't recall seeing a report about this infinite loop.  I'm not exactly
sure how to reproduce it.

Anyway I tend to think the rustc debuginfo here is just bad.  I'm reluctant
to try to work around it.  It would be better if the DWARF accurately
described what is really being emitted, including some information about
where the array bounds can be found dynamically.

To implement a workaround here, I guess I'd need to know exactly which field(s)
might have to change from 'T' to 'T[]' to be correct.  I don't know how to
find this.

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