gdb pretty-printers not working for struct fields with gdb 10.x

[mullr]

Hi! I'm trying to get pretty printing to work smoothly for rust in gdb. For a long time I was pretty sure nothing was working at all, but after liberal amounts of print(...) debugging in the associated python scripts, it appears that the pretty printing only kicks in when directly printing out one of the special structs, and not when they appear as members of something else.

For example, if I have a Vec and print that directly from gdb, I see the result of the pretty printers. But if I have a struct TrivialNewtype(Vec) and try to print that from gdb, I see all the guts of the Vec (the pretty printer is not active).

I made a small repro: https://github.com/mullr/gdb-pretty-test

For convenience, the code is:

struct TrivialNewtype(Vec<u8>);

fn test() {
    let bare_vec = vec![1u8, 2, 3];
    let newtype_vec = TrivialNewtype(vec![1u8, 2, 3]);
    let placeholder = 12;
}

fn main() {
    test()
}

I am building and debugging like this:

cargo build
rust-gdb target/debug/gdb-pretty-test -ex 'b test' -ex 'r' -ex 'n' -ex 'n' -ex 'p bare_vec' -ex 'p newtype_vec'

I expected that when I print newtype_vec, the pretty printer for Vec would be used in a nested way.

Instead, this was printed:

Breakpoint 1, gdb_pretty_test::test () at src/main.rs:5
5           let bare_vec = vec![1u8, 2, 3];
6           let newtype_vec = TrivialNewtype(vec![1u8, 2, 3]);
7           let placeholder = 12;
$1 = Vec(size=3) = {1, 2, 3}
$2 = gdb_pretty_test::TrivialNewtype (alloc::vec::Vec<u8, alloc::alloc::Global> {buf: alloc::raw_vec::RawVec<u8, alloc::alloc::Global> {ptr: core::ptr::unique::Unique<u8> {pointer: 0x55555559abc0, _marker: core::marker::PhantomData<u8>}, cap: 3, alloc: alloc::alloc::Global}, len: 3})

Meta

rustc --version --verbose:

rustc 1.52.1 (9bc8c42bb 2021-05-09)
binary: rustc
commit-hash: 9bc8c42bb2f19e745a63f3445f1ac248fb015e53
commit-date: 2021-05-09
host: x86_64-unknown-linux-gnu
release: 1.52.1
LLVM version: 12.0.0

This happens for nightly as well (rustc 1.54.0-nightly (5c02926 2021-05-11) (from rustc 1.54.0-nightly (ca82264 2021-05-09))

gdb --version:

GNU gdb (Ubuntu 10.1-2ubuntu2) 10.1.90.20210411-git
Copyright (C) 2021 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

[mullr]

I tried this with a fresh ubuntu 20.04 VM, (gdb 9.2) and it's working there:

Breakpoint 1, gdb_pretty_test::test () at src/main.rs:5
5           let bare_vec = vec![1u8, 2, 3];
6           let newtype_vec = TrivialNewtype(vec![1u8, 2, 3]);
7           let placeholder = 12;
$1 = Vec(size=3) = {1, 2, 3}
$2 = gdb_pretty_test::TrivialNewtype (Vec(size=3) = {1, 2, 3})

Ubuntu 20.10 (gdb 9.2) works as well.

With a fresh Ubuntu 21.04 VM (gdb 10.1), the problem occurs. So it seems possible this is related to the gdb version.

[mullr]

gdb 10.2 is broken as well, but the problem goes away on a manual downgrade to gdb 9.2. So this is pretty clearly an issue with gdb 10+.

[mullr]

I bisected through gdb's history and found that this is the offending commit:
bminor/binutils-gdb@5f56f7c

[mullr]

I've posted a question on this subject to the gdb mailing list; I'll update back here with any progress.

[mullr]

Confirmed that this is a bug in gdb.

[tromey]

I fixed it for gdb 11. There most likely won't be another release of gdb 10. https://sourceware.org/pipermail/gdb-patches/2021-May/178895.html

[tromey]

We're working up to the gdb 12 release now. Given that this isn't a bug in rust, and because gdb 11 fixes the problem, I'm going to close this bug.