Make proc_macro_span optional with nightly

[andreeaflorescu]

From the documentation of proc_macro_span I was understanding that if you don't allow unstable features in your nightly build, then the build of the crate should still work:
"Enabled when building with nightly, unless -Z allow-feature in RUSTFLAGS disallows unstable features."

I am using the following configuration in my .cargo/config.toml:

[unstable]
allow-features = []

And the following rust nightly: nightly-2023-09-25.

When building a dummy crate that just has proc-macro2 as a dependency, the following happens:

 cargo build --verbose
       Dirty proc-macro2 v1.0.67: the config settings changed
   Compiling proc-macro2 v1.0.67
       Dirty unicode-ident v1.0.12: the config settings changed
   Compiling unicode-ident v1.0.12
     Running `/Users/fandree/.rustup/toolchains/nightly-2023-09-25-aarch64-apple-darwin/bin/rustc --crate-name build_script_build --edition=2021 /Users/fandree/.cargo/registry/src/index.crates.io-6f17d22bba15001f/proc-macro2-1.0.67/build.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --diagnostic-width=312 -Zallow-features= --crate-type bin --emit=dep-info,link -C embed-bitcode=no --cfg 'feature="default"' --cfg 'feature="proc-macro"' -C metadata=643d0f678a6ab910 -C extra-filename=-643d0f678a6ab910 --out-dir /Users/fandree/sources/rust-crates/test-crate/target/debug/build/proc-macro2-643d0f678a6ab910 -L dependency=/Users/fandree/sources/rust-crates/test-crate/target/debug/deps --cap-lints allow`
     Running `/Users/fandree/.rustup/toolchains/nightly-2023-09-25-aarch64-apple-darwin/bin/rustc --crate-name unicode_ident --edition=2018 /Users/fandree/.cargo/registry/src/index.crates.io-6f17d22bba15001f/unicode-ident-1.0.12/src/lib.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --diagnostic-width=312 -Zallow-features= --crate-type lib --emit=dep-info,metadata,link -C embed-bitcode=no -C debuginfo=2 -C split-debuginfo=unpacked -C metadata=19688c2ad055990d -C extra-filename=-19688c2ad055990d --out-dir /Users/fandree/sources/rust-crates/test-crate/target/debug/deps -L dependency=/Users/fandree/sources/rust-crates/test-crate/target/debug/deps --cap-lints allow`
     Running `/Users/fandree/sources/rust-crates/test-crate/target/debug/build/proc-macro2-643d0f678a6ab910/build-script-build`
     Running `/Users/fandree/.rustup/toolchains/nightly-2023-09-25-aarch64-apple-darwin/bin/rustc --crate-name proc_macro2 --edition=2021 /Users/fandree/.cargo/registry/src/index.crates.io-6f17d22bba15001f/proc-macro2-1.0.67/src/lib.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --diagnostic-width=312 -Zallow-features= --crate-type lib --emit=dep-info,metadata,link -C embed-bitcode=no -C debuginfo=2 -C split-debuginfo=unpacked --cfg 'feature="default"' --cfg 'feature="proc-macro"' -C metadata=aef881fdd2157960 -C extra-filename=-aef881fdd2157960 --out-dir /Users/fandree/sources/rust-crates/test-crate/target/debug/deps -L dependency=/Users/fandree/sources/rust-crates/test-crate/target/debug/deps --extern unicode_ident=/Users/fandree/sources/rust-crates/test-crate/target/debug/deps/libunicode_ident-19688c2ad055990d.rmeta --cap-lints allow --cfg wrap_proc_macro --cfg proc_macro_span`
error[E0725]: the feature `proc_macro_span` is not in the list of allowed features
  --> /Users/fandree/.cargo/registry/src/index.crates.io-6f17d22bba15001f/proc-macro2-1.0.67/src/lib.rs:90:59
   |
90 | #![cfg_attr(any(proc_macro_span, super_unstable), feature(proc_macro_span))]

The --cfg proc_macro_span was added even though we also pass the -Zallow-features= flag. Is this the expected behavior?

I would like to be able to build proc-macro2 with nightly without having to enable the unstable feature. Is this possible?

[dtolnay]

I suspect this is a Cargo bug. Cargo is not propagating that -Zallow-features= in the CARGO_ENCODED_RUSTFLAGS seen by the build script.

I tested this by creating a 2-line .cargo/config.toml as you showed, and a build script containing:

// build.rs

fn main() {
    std::process::Command::new("env").status().unwrap();
    std::process::exit(1);
}

The output of env printed by cargo build shows that CARGO_ENCODED_RUSTFLAGS is empty. I believe it should contain -Zallow-features=. The whole point of Cargo providing CARGO_ENCODED_RUSTFLAGS to build scripts is so that the build script can make rustc invocations using the same flags that Cargo would use for its own rustc invocations.

[andreeaflorescu]

Indeed, it looks like the flag is not passed when using [unstable], but it is passed when using:

[build]
rustflags = ["-Zallow-features="]

I am not very sure if I should open an issue in cargo though, as it might be expected based on this 🤷‍♀️ rust-lang/cargo#12437 (comment).

Out of curiosity, what is the reasoning for always enabling proc_macro_span? Considering it's unstable, my expectation would be for it to be enabled only by using a non-default feature, and not the other way around.