Pass --cfg=bootstrap for proc macros built by stage0

[jyn514]

Cargo has a bug where it ignores RUSTFLAGS when building proc macro
crates (rust-lang/cargo#4423).
However, sometimes rustc_macro needs to have conditional
compilation when there are breaking changes to the libproc_macro API
(see for example #83363). Previously, this wasn't possible, because the
crate couldn't tell the difference between stage 0 and stage 1.

Another alternative is to unconditionally build rustc_macros with the
master libstd instead of the beta one (i.e. use --sysroot stage0-sysroot), but that led to strange and maddening errors:

error[E0460]: found possibly newer version of crate `std` which `synstructure` depends on
 --> compiler/rustc_macros/src/lib.rs:5:5
  |
5 | use synstructure::decl_derive;
  |     ^^^^^^^^^^^^
  |
  = note: perhaps that crate needs to be recompiled?
  = note: the following crate versions were found:
          crate `std`: /home/joshua/rustc2/build/x86_64-unknown-linux-gnu/stage0-sysroot/lib/rustlib/x86_64-unknown-linux-gnu/lib/libstd-b3602c301b71cc3d.rmeta
          crate `synstructure`: /home/joshua/rustc2/build/x86_64-unknown-linux-gnu/stage0-rustc/release/deps/libsynstructure-74ee66863479e972.rmeta
error[E0460]: found possibly newer version of crate `std` which `proc_macro2` depends on
  --> /home/joshua/.local/lib/cargo/registry/src/github.com-1ecc6299db9ec823/tracing-attributes-0.1.13/src/lib.rs:90:5
   |
90 | use proc_macro2::TokenStream;
   |     ^^^^^^^^^^^
   |
   = note: perhaps that crate needs to be recompiled?
   = note: the following crate versions were found:
           crate `std`: /home/joshua/rustc2/build/x86_64-unknown-linux-gnu/stage0-sysroot/lib/rustlib/x86_64-unknown-linux-gnu/lib/libstd-b3602c301b71cc3d.rmeta
           crate `proc_macro2`: /home/joshua/rustc2/build/x86_64-unknown-linux-gnu/stage0-rustc/release/deps/libproc_macro2-a83c1f01610c129e.rlib

r? @Mark-Simulacrum cc @jhpratt

[Mark-Simulacrum]

I.. wouldn't call this a bug :) More like 'unexpected behavior'. rust-lang/cargo#4423 seems like the more appropriate issue to cite, though.

[jyn514]

Oops, yes, that's the one I meant to link to. The issue title isn't quite right though - we're neither cross-compiling nor building a build script.

[jyn514]

Hmm, this doesn't actually work - when I pass --all-targets to x.py, cargo doesn't pass --crate-type proc-macro :/ I'm not sure how to fix this.

#83363 (comment)

[Mark-Simulacrum]

I unfortunately likely won't have too much time to help debug that problem, but I suspect it's either the same problem as noted in the comments (i.e., target-specific magic) or perhaps a separate Cargo bug that could be fixed. I imagine it should be reproducible outside rustbuild though, in which case you could file a new issue on rust-lang/cargo :)

[jyn514]

I suspect it's either the same problem as noted in the comments (i.e., target-specific magic) or perhaps a separate Cargo bug that could be fixed.

I don't think this is actually a bug - passing --crate-type=proc-macro breaks unit tests:

$ RUSTFLAGS=--crate-type=proc-macro cargo test -q
error: test failed, to rerun pass '--lib'

Caused by:
  could not execute process `/home/joshua/.local/lib/cargo/target/debug/deps/libproc_macro_example-65ddfb6f3c001ad1.so --quiet` (never executed)

I guess I could special case --crate-name rustc_macros instead? But that seems awfully fragile :/

[jyn514]

This should be ready to go; I tested by rebasing #83363 on top of this and running x.py check --stage 1 and x.py check --all-targets.

[Mark-Simulacrum]

@bors r+ rollup

[bors]

📌 Commit dc30258 has been approved by Mark-Simulacrum