proc_macro2 1.0.33 causing breakages with rust-1.57-nightly

[maxburke]

Since 1.0.33 was published many of our builds started failing with the following error:

error[E0658]: use of unstable library feature 'proc_macro_is_available'
  --> /root/.cargo/registry/src/github.com-1ecc6299db9ec823/proc-macro2-1.0.33/src/detection.rs:28:21
   |
28 |     let available = proc_macro::is_available();
   |                     ^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: see issue #71436 <https://github.com/rust-lang/rust/issues/71436> for more information
   = help: add `#![feature(proc_macro_is_available)]` to the crate attributes to enable
For more information about this error, try `rustc --explain E0658`.
error: could not compile `proc-macro2` due to previous error
warning: build failed, waiting for other jobs to finish...
error: failed to compile `cargo-deny v0.11.0`, intermediate artifacts can be found at `/tmp/cargo-installKTra1Y`

This is the toolchain version we are on:

$ rustc --version
rustc 1.57.0-nightly (f03eb6bef 2021-10-02)

We've been stepping back to earlier versions of the package where we can but for some things (like cargo-deny above) we're having trouble with.

[dtolnay]

rustc 1.57.0-nightly (f03eb6bef 2021-10-02) is an old nightly toolchain. This crate in general does not support arbitrary old nightly toolchains.

[maxburke]

I understand that supporting "arbitrary old nightly toolchains" is burdensome as a package maintainer. We need to use a nightly toolchain because some of our critical dependencies rely on features not yet stabilized, we would be on stable versions if not for that.

But it also really sucks to have your whole build system grind to a halt because of a patch version update.

[dtolnay]

This is up to you. The expectation when using the nightly toolchain is that you pull in the new one every 'night'. If you are pinning the toolchain to an old nightly, then you need to pin your crates too.

[kaleidawave]

Having this issue developing with rustc 1.55.0 (c8dfcfe04 2021-09-06), so have pinned crate version to 1.0.32. Given proc_macro::is_available(); was stabilised at 1.57, shouldn't the rust-version in cargo toml be 1.57? (I am a bit of noob on versioning so I am probably wrong ...?)

[dtolnay]

The rust-version in Cargo.toml is accurate. This crate supports every stable compiler since the number shown there. As stated above though, this crate does not support arbitrary old nightly toolchains.