Cannot build a no_std crate that uses a proc macro that uses std

[ernoc]

Given:

• A crates index where all crates are no_std compatible
• A crate we want to build for bare metal which uses a proc macro that depends on some other crate that needs std (e.g.: prost-types)

rules_rust will pull in the proc macro's dependencies and then propagate the std feature to all crates in the index, meaning no we can no longer build our crate for bare metal.

[bmclarnon]

Here's a repro for the issue (tested with bazel 7.2.1):

• WORKSPACE:

  # Copyright 2024 Google LLC.
  # SPDX-License-Identifier: Apache-2.0
  
  load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
  
  http_archive(
      name = "rules_rust",
      integrity = "sha256-35cwdTOaqqu4y+aXgIUU2C2PAKMz4+uyJ7/UMIGCmFs=",
      url = "https://github.com/bazelbuild/rules_rust/releases/download/0.47.1/rules_rust-v0.47.1.tar.gz",
  )
  load("@rules_rust//rust:repositories.bzl", "rules_rust_dependencies", "rust_register_toolchains")
  rules_rust_dependencies()
  rust_register_toolchains(
      edition = "2021",
      versions = ["1.78.0"],
      extra_target_triples = ["x86_64-unknown-none"],
  )
  
  # rust_library requires a C++ toolchain supporting x86_64-unknown-none.
  http_archive(
      name = "aspect_gcc_toolchain",
      sha256 = "3341394b1376fb96a87ac3ca01c582f7f18e7dc5e16e8cf40880a31dd7ac0e1e",
      strip_prefix = "gcc-toolchain-0.4.2",
      url = "https://github.com/aspect-build/gcc-toolchain/archive/refs/tags/0.4.2.tar.gz",
  )
  load("@aspect_gcc_toolchain//toolchain:repositories.bzl", "gcc_toolchain_dependencies")
  gcc_toolchain_dependencies()
  load("@aspect_gcc_toolchain//toolchain:defs.bzl", "ARCHS", "gcc_register_toolchain")
  gcc_register_toolchain(
      name = "gcc_toolchain_x86_64_unknown_none",
      extra_ldflags = ["-nostdlib"],
      target_arch = ARCHS.x86_64,
      target_compatible_with = ["@platforms//cpu:x86_64", "@platforms//os:none"],
  )
  
  load("@rules_rust//crate_universe:defs.bzl", "crate", "crates_repository")
  
  # anyhow builds: bazel build --platforms=//:x86_64-unknown-none --@rules_rust//:no_std=alloc @anyhow_crate_index//:anyhow
  crates_repository(
      name = "anyhow_crate_index",
      cargo_lockfile = "//:anyhow.lock",
      lockfile = "//:anyhow-lock.json",
      packages = {
          "anyhow": crate.spec(default_features = False, version = "1.0"),
      },
      rust_version = "1.78.0",
      supported_platform_triples = ["x86_64-unknown-linux-gnu", "x86_64-unknown-none"],
  )
  load("@anyhow_crate_index//:defs.bzl", anyhow_crate_repositories = "crate_repositories")
  anyhow_crate_repositories()
  
  # prost-types builds: bazel build --platforms=//:x86_64-unknown-none --@rules_rust//:no_std=alloc @broken_crate_index//:prost-types
  # anyhow now fails to build: bazel build --platforms=//:x86_64-unknown-none --@rules_rust//:no_std=alloc @broken_crate_index//:anyhow
  crates_repository(
      name = "broken_crate_index",
      cargo_lockfile = "//:broken.lock",
      lockfile = "//:broken-lock.json",
      packages = {
          "anyhow": crate.spec(default_features = False, version = "1.0"),
          "prost-types": crate.spec(default_features = False, version = "0.12"),
      },
      rust_version = "1.78.0",
      supported_platform_triples = ["x86_64-unknown-linux-gnu", "x86_64-unknown-none"],
  )
  load("@broken_crate_index//:defs.bzl", broken_crate_repositories = "crate_repositories")
  broken_crate_repositories()

• BUILD:

  # Copyright 2024 Google LLC.
  # SPDX-License-Identifier: Apache-2.0
  platform(
      name = "x86_64-unknown-none",
      constraint_values = [
          "@platforms//cpu:x86_64",
          "@platforms//os:none",
      ],
  )

Repro steps:

$ touch {anyhow,broken}{.lock,-lock.json} && CARGO_BAZEL_REPIN=1 bazel sync
$ bazel build --platforms=//:x86_64-unknown-none --@rules_rust//:no_std=alloc @anyhow_crate_index//:anyhow @broken_crate_index//:prost-types  # succeeds
$ bazel build --platforms=//:x86_64-unknown-none --@rules_rust//:no_std=alloc @broken_crate_index//:anyhow  # fails
...
error[E0463]: can't find crate for `std`
   --> external/broken_crate_index__anyhow-1.0.86/src/lib.rs:251:1
    |
251 | extern crate std;
    | ^^^^^^^^^^^^^^^^^ can't find crate
    |
    = note: the `x86_64-unknown-none` target may not support the standard library

prost-types depends on prost-derive, which is a proc macro (i.e. provides compile-time codegen rules). prost-derive depends on anyhow with the std feature -- which is okay because the usage only happens at compile time. But crates_repository merges features across all dependencies -- including those reached via proc macros -- resulting in the std feature being added to @broken_crate_index//:anyhow (compare the anyhow entries in anyhow-lock.json and broken-lock.json).

This is similar to https://github.com/bazelbuild/rules_rust/issues/205, but in addition to building proc macros for the host platform, I believe dependencies of proc macros (or at least their features) also need to be isolated between the host and target platforms.