Cfg missing during build script build

[dtolnay]

Crate name: proc-macro2
Old working build: https://docs.rs/crate/proc-macro2/1.0.4/builds/186665
Build failure link: https://docs.rs/crate/proc-macro2/1.0.6/builds/192719

Additional details:
With the switch to rustwide, it seems that --cfg entries from our [package.metadata.docs.rs] are no longer seen by our build script (but still present when building the crate), which causes the build to fail.

Notice how in the working 1.0.4 build the rustc invocation that builds build.rs has --cfg procmacro2_semver_exempt. From tracing through the types in the failing 1.0.6 build, I believe that failure could only happen if --cfg procmacro2_semver_exempt is set for the lib.rs rustc invocation but not for the build.rs rustc invocation.

Relevant part of Cargo.toml: https://github.com/alexcrichton/proc-macro2/blob/1.0.6/Cargo.toml#L18-L20

Diff between working build current failing build: dtolnay/proc-macro2@1.0.4...1.0.6
(no relevant changes on our end).

[dtolnay]

FYI @pietroalbini -- this was causing the failure you saw in #23 (comment).

[jyn514]

@dtolnay it looks like this error started occurring after the switch to rustwide (#407). It's probably due to rust-lang/cargo#4423 - as you can see in the build logs --target is set in the latest (broken) build, but not in the old one.

[elichai]

@dtolnay I don't think this is an issue with docs.rs.
dtolnay/proc-macro2#207

[jyn514]

This was fixed in #491, 1.0.7 is live: https://docs.rs/crate/proc-macro2/1.0.7