Comment 4 for bug 1974265

The simplified PoC:

$ echo _RYAbB2_ | ./binutils/cxxfilt
Segmentation fault

Root Cause Analysis:

1. In `rust_demangle_callback`, "_R" at beginning causes `rdm.version == 0`[1][2], so `demangle_path` in the else-branch is called[3] with `rdm.sym == "YAbB2_"`, with `rdm.next` field initialized to 0.

2. Next char 'Y' causes `demangle_type` to be called[4] in a switch-case branch, with `rdm.next == 1`.

3. Next char 'A' causes `demangle_type` to be called first[5] to consume char 'b', then `demangle_const` is called[6] with `rdm.next == 3`.

4. Next char 'B' is consumed[7], and "2_" is parsed as `integer_62`[8] to be value 3, and this vlaue is assigned to `rdm.next`, then `demangle_const` is called recursively[9]. However, currently the situation is `rdm.next == 3` again, which is exactly same as last call to `demangle_const`, and this causes infinite recursion.

[1] https://github.com/bminor/binutils-gdb/blob/626d0e40e55c35a4f143b70def498734e8ed3c2a/libiberty/rust-demangle.c#L1356
[2] https://github.com/bminor/binutils-gdb/blob/626d0e40e55c35a4f143b70def498734e8ed3c2a/libiberty/rust-demangle.c#L1362
[3] https://github.com/bminor/binutils-gdb/blob/626d0e40e55c35a4f143b70def498734e8ed3c2a/libiberty/rust-demangle.c#L1453
[4] https://github.com/bminor/binutils-gdb/blob/626d0e40e55c35a4f143b70def498734e8ed3c2a/libiberty/rust-demangle.c#L759
[5] https://github.com/bminor/binutils-gdb/blob/626d0e40e55c35a4f143b70def498734e8ed3c2a/libiberty/rust-demangle.c#L934
[6] https://github.com/bminor/binutils-gdb/blob/626d0e40e55c35a4f143b70def498734e8ed3c2a/libiberty/rust-demangle.c#L938
[7] https://github.com/bminor/binutils-gdb/blob/626d0e40e55c35a4f143b70def498734e8ed3c2a/libiberty/rust-demangle.c#L1151
[8] https://github.com/bminor/binutils-gdb/blob/626d0e40e55c35a4f143b70def498734e8ed3c2a/libiberty/rust-demangle.c#L1153
[9] https://github.com/bminor/binutils-gdb/blob/626d0e40e55c35a4f143b70def498734e8ed3c2a/libiberty/rust-demangle.c#L1158

Possible Fix:

The infinite recursion check added in this [commit](https://github.com/bminor/binutils-gdb/commit/f10f8617a302f45dae721eae0cd659911f03d864) should also be applied to `demangle_const`