Pass cfg to build script

[dtolnay]

#1543 #1559 broke proc-macro2's documentation build — see dtolnay/proc-macro2#309.

As far as I can tell the difference is that cargo rustdoc -- --cfg mycfg sets mycfg during both the compilation of the build.rs and the invocation of rustdoc, whereas cargo rustdoc --config 'build.rustdocflags=["--cfg", "mycfg"]' sets it only during rustdoc and not the compilation of build.rs.

As far as I can tell there is now no way to set cfg for a build script build on docs.rs.

@willcrichton @jyn514

[willcrichton]

Thanks for the report @dtolnay!

@jyn514 what do you think is the right fix here? We could add cfg as a new field to [package.metadata.docs.rs] if it's a special case. Or I could duplicate the rustdoc-args field between --config 'build.rustdocflags=<flags>' and rustdoc -- <flags>. Not sure if that would also cause duplicate flag CLI errors.

[jyn514]

As far as I can tell there is now no way to set cfg for a build script build on docs.rs.

Can't you set it with RUSTFLAGS? There's a lever for modifying it in the metadata with rustc-args.

Hmm, I guess that applies it to all dependencies, too ... is that a problem for your use-case?

[dtolnay]

Nope, cfg from rustc-args does not get applied either. That was already in the build that regressed. See https://github.com/dtolnay/proc-macro2/blob/1.0.33/Cargo.toml#L19-L20 and https://docs.rs/crate/proc-macro2/1.0.33/builds/473000.

[Nemo157]

As far as I can tell the difference is that cargo rustdoc -- --cfg mycfg sets mycfg during both the compilation of the build.rs and the invocation of rustdoc

This seems like it would be incorrect behavior, and doesn't match what I see testing it locally. I also tried building the proc-macro2==1.0.33 docs with my script that behaves similarly to how docs.rs used to, which failed similarly to that build. That ended up running the command:

env DOCS_RS=1 RUSTCFLAGS='--cfg procmacro2_semver_exempt' cargo rustdoc --locked --target x86_64-unknown-linux-gnu -- -Z unstable-options --cfg procmacro2_semver_exempt --cfg doc_cfg --extern-html-root-url quote=https://docs.rs/quote/1.0.10 --extern-html-root-url unicode_xid=https://docs.rs/unicode-xid/0.2.2

[jyn514]

RUSTCFLAGS='--cfg procmacro2_semver_exempt'

Shouldn't that be RUSTFLAGS?

[Nemo157]

Yep, but using RUSTFLAGS doesn't fix it, the --cfg doesn't appear to get passed to the build script build.

[Nemo157]

I'm really confused, copying the exact command run from the previous successful build while trying to build proc-macro2==1.0.32 locally also fails in the same way, even when overriding to the nightly that was used.

"cargo" "+nightly-2021-10-26" "rustdoc" "--lib" "-Zrustdoc-map" "-Z" "unstable-options" "--config" "build.rustflags=[\"--cfg\", \"procmacro2_semver_exempt\"]" "-Zunstable-options" "--config=doc.extern-map.registries.crates-io=\"https://docs.rs/{pkg_name}/{version}/x86_64-unknown-linux-gnu\"" "-j2" "--" "--cfg" "procmacro2_semver_exempt" "--cfg" "doc_cfg" "-Z" "unstable-options" "--emit=invocation-specific" "--resource-suffix" "-20211026-1.58.0-nightly-e269e6bf4" "--static-root-path" "/" "--cap-lints" "warn" "--disable-per-crate-search"

[Nemo157]

Ok, found the reason I was having that fail too, --config build.rustflags gets overridden by target.x86_64-unknown-linux-gnu.rustflags in my global config instead of being merged together. That shouldn't affect docs.rs since it shouldn't have a global .cargo/config.

After trying a bunch of changes I finally found the difference, the --target x86_64-unknown-linux-gnu changes whether build.rustflags gets passed to the build-script or not. That seems like a cargo bug to me.

[jyn514]

Ah, ok - that's caused by #1559 then, not #1543.

[jyn514]

cc rust-lang/cargo#7677

[Nemo157]

And for the specific failure here: rust-lang/cargo#4423. I've always assumed build.rustflags was global arguments to apply to all builds, but it looks like it actually means "the args for the implicit host target, but only for host-only builds like proc-macros and build-scripts when the final artifact is the implicit host target"; and there's no way to explicitly set flags for "host-only non-artifact builds".

[Nemo157]

Ah, following links a little more I found https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#host-config which might be able to do what we need; -Zhost-config --config=host.rustflags=....

[Nemo157]

To clarify what happened and how I think we should fix this:

Prior to #1559:

• compiling for x86_64-unknown-linux-gnu would apply the specified rustc-args to all builds, including while compiling the build-scripts
• compiling for other targets would apply the specified rustc-args to only the builds that get linked into the final artifact, so not including the build-scripts

After #1559:

• all (non-proc-macro) builds are now treated as cross-compiles
• compiling for all targets would apply the specified rustc-args to only the artifact builds, not including the build-scripts

I think we should update to apply the specified rustc-args to all builds no matter what the --target is. I attempted to use -Zhost-config to implement this, but ran into rust-lang/cargo#10206; so I think we're blocked on fixing this till that is fixed in cargo.

[jyn514]

rust-lang/cargo#10206 has been fixed.