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

["jistone at redhat dot com" <sourceware-bugzilla@sourceware.org>]

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

            Bug ID: 30330
           Summary: GDB 13.1 no longer prints length of Rust slice
                    wrappers
           Product: gdb
           Version: 13.1
            Status: NEW
          Severity: normal
          Priority: P2
         Component: rust
          Assignee: unassigned at sourceware dot org
          Reporter: jistone at redhat dot com
                CC: tromey at sourceware dot org
  Target Milestone: ---

GDB 13.1 has regressed in the Rust debuginfo testsuite here:
https://github.com/rust-lang/rust/blob/a73288371e3fa0a610fbc11e7e8418017bdfde42/tests/debuginfo/unsized.rs#L7-L9

I have reduced it to the following test case compiled with Rust 1.68.2, rustc
-g unsized.rs

    #![allow(unused)]

    #[derive(Debug)]
    struct Foo<T: ?Sized> {
        value: T,
    }

    fn main() {
        let foo: Foo<[u8; 4]> = Foo { value: *b"abc\0" };
        let a: &Foo<[u8]> = &foo;
        dbg!(a);
    }

With GDB 12.1 after stepping to the dbg line, I get:

    (gdb) p a
    $1 = &unsized::Foo<[u8]> {data_ptr: 0x7fffffffd488, length: 4}

With GDB 13.1 on the same executable, I get:

    (gdb) p a
    $1 = &unsized::Foo<[u8]> 0x7fffffffd488

I bisected the change to this commit:
https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=506ec52e8805d8edd538d6bd11750489a8c8bbee

commit 506ec52e8805d8edd538d6bd11750489a8c8bbee
Author: Tom Tromey <tom@tromey.com>
Date:   Fri Mar 25 13:36:53 2022 -0600

    Reimplement Rust slice printing

    The current nightly Rust compiler (aka 1.61) added better DWARF
    representation for unsized types.  Fixing this is PR rust/21466; but
    the code is actually the same as what is required to make slice
    printing more useful, which is PR rust/23871.  This patch implements
    this.  I tested this against various Rust compilers: 1.48, current
    stable, current beta, and current nightly.

    Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=21466
    Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=23871

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