non-pub function no longer compiled in debug profile, causing link errors on thumbv7em-none-eabihf with defmt

[jonathanpallant]

Code

I tried this code:

$ git clone https://github.com/ferrous-systems/rust-exercises.git
$ cd rust-exercises
$ git checkout v1.17.0
$ cd nrf52_code/radio_app
$ rustup target add thumbv7em-none-eabihf --toolchain=nightly
$ cargo install flip-link (or change `.cargo/config.toml` to not use flip-link - either is fine)
$ cargo +nightly build

I expected to see this happen:

The binaries build like, they do on stable.

Instead, this happened:

  = note: rust-lld: error: /Users/jonathan/Documents/clients/training/open-rust-embedded-2024-10-02/rust-exercises-v1.17.0/nrf52-code/radio-app/target/thumbv7em-none-eabihf/debug/build/defmt-36a2ab5d209daca3/out/defmt.x:7: symbol not found: __defmt_default_panic
          rust-lld: error: /Users/jonathan/Documents/clients/training/open-rust-embedded-2024-10-02/rust-exercises-v1.17.0/nrf52-code/radio-app/target/thumbv7em-none-eabihf/debug/build/defmt-36a2ab5d209daca3/out/defmt.x:7: symbol not found: __defmt_default_panic
          rust-lld: error: /Users/jonathan/Documents/clients/training/open-rust-embedded-2024-10-02/rust-exercises-v1.17.0/nrf52-code/radio-app/target/thumbv7em-none-eabihf/debug/build/defmt-36a2ab5d209daca3/out/defmt.x:7: symbol not found: __defmt_default_panic
          rust-lld: error: /Users/jonathan/Documents/clients/training/open-rust-embedded-2024-10-02/rust-exercises-v1.17.0/nrf52-code/radio-app/target/thumbv7em-none-eabihf/debug/build/defmt-36a2ab5d209daca3/out/defmt.x:7: symbol not found: __defmt_default_panic

That symbol is defined here: https://github.com/knurling-rs/defmt/blob/b7a89f56059fc66ac3c7cc3445d092829599e699/defmt/src/lib.rs#L365

Bisect

@Dirbaio said:

searched nightlies: from nightly-2024-07-30 to nightly-2024-10-02
regressed nightly: nightly-2024-08-01
searched commit range: f8060d2...28a58f2
regressed commit: 0b5eb7b

That's the LLVM 19 upgrade.

[Dirbaio]

it regressed both with and without --release.

[jonathanpallant]

I also ran a bisect:

searched nightlies: from nightly-2024-07-01 to nightly-2024-10-02
regressed nightly: nightly-2024-08-01
searched commit range: f8060d2...28a58f2
regressed commit: 0b5eb7b

[apiraino]

WG-prioritization assigning priority (Zulip discussion).

I can also reproduce on current beta so tagging accordingly

@rustbot label -I-prioritize +P-medium +regression-from-stable-to-beta

[Dirbaio]

minimized, zero deps -> https://github.com/Dirbaio/defmt-repro

[dirbaio@mars defmt-repro]$ rustc --version
rustc 1.83.0-nightly (06bb8364a 2024-10-01)
[dirbaio@mars defmt-repro]$ cargo build
   Compiling radio_app v0.0.0 (/home/dirbaio/defmt-repro)
error: linking with `rust-lld` failed: exit status: 1
  |
  = note: LC_ALL="C" PATH="/home/dirb(snip)47a" "--gc-sections" "-Tlol.x"
  = note: rust-lld: error: lol.x:1: symbol not found: __defmt_default_panic
          rust-lld: error: lol.x:1: symbol not found: __defmt_default_panic
          rust-lld: error: lol.x:1: symbol not found: __defmt_default_panic

error: could not compile `radio_app` (bin "radio_app") due to 1 previous error
[dirbaio@mars defmt-repro]$ rustc +stable --version
rustc 1.81.0 (eeb90cda1 2024-09-04)
[dirbaio@mars defmt-repro]$ cargo +stable build
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.06s

[Dirbaio]

maybe related: llvm/llvm-project@ebb326a