[Bug rust/29735] Can't have multiple breakpoints - crash otherwise

["tpzker at thepuzzlemaker dot info" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=29735

The Puzzlemaker <tpzker at thepuzzlemaker dot info> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |tpzker at thepuzzlemaker dot info

--- Comment #7 from The Puzzlemaker <tpzker at thepuzzlemaker dot info> ---
Created attachment 15198
  --> https://sourceware.org/bugzilla/attachment.cgi?id=15198&action=edit
Patch for this issue, or an issue similar to it

I experienced this in 13.2. I believe it's related to the null pointer
optimization, or at least the case I encountered was.

```
use std::num::NonZeroUsize;
use std::hint::black_box;

pub fn main() {
    foo(black_box(NonZeroUsize::new(5)), black_box(NonZeroUsize::new(5)))
}

#[inline(never)]
pub fn foo(x: Option<NonZeroUsize>, y: Option<NonZeroUsize>) {
    black_box(x.unwrap().checked_add(y.unwrap().into()));
}
```

Put this in `tmp.rs`.
Compile with `rustc tmp.rs -Copt-level=1 -Cdebuginfo=full`
Run `gdb tmp::foo`
`break foo`
`run`
`info args`

Observe assertion fail:
```
x = core::option::Option<core::num::nonzero::NonZeroUsize>::Some(
../../gdb/gdbtypes.h:985: internal-error: field: Assertion `idx >= 0 && idx <
num_fields ()' failed.
A problem internal to GDB has been detected,
further debugging may prove unreliable.
```

This issue is related to this portion of code in `rust_lang::print_enum`:
```
 461   type = resolve_dynamic_type (type, view, value_address (val));
 ...   [snip]
 472   int variant_fieldno = rust_enum_variant (type);
 473   val = value_field (val, variant_fieldno);
 474   struct type *variant_type = type->field (variant_fieldno).type ();
```
https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=gdb/rust-lang.c;h=74808ad0716e4d67f6af859f05ebff2b55d5e276;hb=662243de0e14a4945555a480dca33c0e677976eb#l461
The type from `resolve_dynamic_type` makes only the variant field of the active
discriminant in active. However, the type of `val` has *all* variant fields
enabled, thus making this index meaningless for that. I created a patch for
this, which is somewhat hacky--any maintainers here, feel free to change it up
a bit; I am a Rust developer by nature and don't know much about idiomatic C/++
and in general, what the GDB codebase looks like.

Please let me know if you'd like me to test/update this patch to the v14
codebase.

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