Cannot build a no_std crate that uses a proc macro that uses std about rules_rust HOT 1 OPEN

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 #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.

from rules_rust.