cargo build - error[E0658] - proc_macro::Literal::byte_character(byte)

[Rahix]

Originally reported in #535.

Due to a change in proc-macro2 version v1.0.80, the toolchain version we use is no longer supported (see dtolnay/proc-macro2#451). To reproduce:

cargo generate --git https://github.com/rahix/avr-hal-template -r bb141bd00f6b9687e986db8aeb66a3f3f50cca12 -n proc-macro-regression
cd proc-macro-regression/
cargo build

which end with the following build error:

   Compiling proc-macro2 v1.0.81
error[E0658]: use of unstable library feature 'proc_macro_byte_character'
   --> /home/rahix/.cargo/registry/src/index.crates.io-6f17d22bba15001f/proc-macro2-1.0.81/src/wrapper.rs:871:21
    |
871 |                     proc_macro::Literal::byte_character(byte)
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: see issue #115268 <https://github.com/rust-lang/rust/issues/115268> for more information
    = help: add `#![feature(proc_macro_byte_character)]` to the crate attributes to enable
    = note: this compiler was built on 2024-03-21; consider upgrading it if it is out of date

error[E0658]: use of unstable library feature 'proc_macro_c_str_literals'
   --> /home/rahix/.cargo/registry/src/index.crates.io-6f17d22bba15001f/proc-macro2-1.0.81/src/wrapper.rs:898:21
    |
898 |                     proc_macro::Literal::c_string(string)
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: see issue #119750 <https://github.com/rust-lang/rust/issues/119750> for more information
    = help: add `#![feature(proc_macro_c_str_literals)]` to the crate attributes to enable
    = note: this compiler was built on 2024-03-21; consider upgrading it if it is out of date

For more information about this error, try `rustc --explain E0658`.
error: could not compile `proc-macro2` (lib) due to 2 previous errors

---

The solution to this issue comes in two parts:

• We need to upgrade to a newer rust toolchain version again.
• We need to pin proc-macro2 to v1.0.79 somewhere to ensure this won't hit us again. I am unsure where the right place for this is:
  • avr-device? Pulled in by everyone who might care about this, but feels like the wrong level for such a fix.
  • avr-hal-template? Gives users control over the fix, if it ever needs changing. Downside is that it does not reach existing users, only new ones...
  • avr-hal/arduino-hal/atmega-hal? Should allow reaching existing users more easily, but still not really the right level for the fix...

[Rahix]

The avr-hal-template is updated to pin the dependency version. New projects generated from the template will no longer run into this problem. Unclear whether this is enough or whether we should do more...

avr-hal itself also needs fixing for the examples.

For documentation, the dependency can be pinned using these lines in Cargo.toml:

# The latest releases of `proc-macro2` do not support the rust toolchain that
# we use.  Thus, we must fix this dependency to an older version where our
# toolchain is still supported.  See https://github.com/Rahix/avr-hal/issues/537
[build-dependencies.proc-macro2]
version = "=1.0.79"

[Rahix]

Examples in this repository now also have the dependency pin (#544).

[Rahix]

So avr-device will need to deal with this for its own CI, too. So maybe the solution is to simply pin proc-macro2 in avr-device-macros so it will solve the problem for each and every downstream crate (See Rahix/avr-device#156).

What does everyone else think about this? Are there any problems with doing this?