Allow crate_universe to use external rust_host_tools (#3418)

This allows users of `rules_rust` to define their own `rust_host_tools`
modules and use it with `crate_universe` to control the versions of
rustc and cargo used by it.

For example:

```python
rust_host_tools = use_extension("@rules_rust//rust:extensions.bzl", "rust_host_tools")
rust_host_tools.host_tools(
    name = "my_rust_host_tools",
    version = "1.86.0",
)
use_repo(
    rust_host_tools,
    "my_rust_host_tools",
)

crate = use_extension("@rules_rust//crate_universe:extensions.bzl", "crate")

crate.from_cargo(
    name = "crate_index",
    manifests = ["//:Cargo.toml"],
    cargo_lockfile = "//:Cargo.lock",
    lockfile = "//:cargo-bazel-lock.json",
    host_tools = "@my_rust_host_tools",
)

use_repo(
    crate,
    "crate_index",
)
```

Author: UebelAndre

MODULE.bazel

@@ -56,14 +56,9 @@ rust_host_tools = use_extension("//rust:extensions.bzl", "rust_host_tools")
 rust_host_tools.host_tools(
     name = "rust_host_tools",
 )
-rust_host_tools.host_tools(
-    name = "rust_host_tools_nightly",
-    version = "nightly",
-)
 use_repo(
     rust_host_tools,
     "rust_host_tools",
-    "rust_host_tools_nightly",
 )
 
 rust_test = use_extension("//test:test_extensions.bzl", "rust_test", dev_dependency = True)

crate_universe/extensions.bzl

@@ -195,7 +195,7 @@ You could use the syntax specified in the above document to place it in `Cargo.t
 
 This method has the following consequences:
 * if you use shared dependency tree with your project these binary dependencies will interfere with yours (may conflict)
-* you have to use  nightly `host_tools_repo` to generate dependencies because
+* you have to use  nightly `host_tools` to generate dependencies because
 
 Alternatively you can specify this in a separate `repo` with `cargo.from_specs` syntax:
 
@@ -207,7 +207,7 @@ bindeps.annotation(crate = "cargo-machete", gen_all_binaries = True)
 
 bindeps.from_specs(
   name = "bindeps",
-  host_tools_repo = "rust_host_tools_nightly",
+  host_tools = "@rust_host_tools_nightly",
 )
 
 use_repo(bindeps, "bindeps")
@@ -835,21 +835,14 @@ def _get_host_cargo_rustc(module_ctx, host_triple, host_tools_repo):
     """A helper function to get the path to the host cargo and rustc binaries.
 
     Args:
-        module_ctx: The module extension's context.
-        host_triple: The platform triple for the machine executing this extension.
-        host_tools_repo: The `rust_host_tools` repository to use.
+        module_ctx (module_ctx): The module extension's context.
+        host_triple (triple): The platform triple for the machine executing this extension.
+        host_tools_repo (Label): The `rust_host_tools` repository to use.
     Returns:
-        A tuple of path to cargo, path to rustc.
+        tuple[Path, Path]: A tuple of path to cargo, path to rustc.
     """
     binary_ext = system_to_binary_ext(host_triple.system)
-
-    # For Bazel 7 and below, `module_ctx.path` will also watch files so to avoid
-    # volatility in lock files caused by referencing host specific files, direct
-    # references are avoided. Note that `BUILD.bazel` is not used as it also
-    # contains host specific data.
-    root = module_ctx.path(Label("@{rust_host_tools}//:WORKSPACE.bazel".format(
-        rust_host_tools = host_tools_repo,
-    )))
+    root = module_ctx.path(host_tools_repo)
 
     cargo_path = root.dirname.get_child("bin/cargo{}".format(binary_ext))
     rustc_path = root.dirname.get_child("bin/rustc{}".format(binary_ext))
@@ -972,7 +965,7 @@ def _crate_impl(module_ctx):
                 for m in cfg.manifests:
                     module_ctx.watch(m)
 
-            cargo_path, rustc_path = _get_host_cargo_rustc(module_ctx, host_triple, cfg.host_tools_repo)
+            cargo_path, rustc_path = _get_host_cargo_rustc(module_ctx, host_triple, cfg.host_tools)
             cargo_bazel_fn = new_cargo_bazel_fn(
                 repository_ctx = module_ctx,
                 cargo_bazel_path = generator,
@@ -1037,9 +1030,9 @@ _FROM_COMMON_ATTRS = {
     "cargo_lockfile": CRATES_VENDOR_ATTRS["cargo_lockfile"],
     "generate_binaries": CRATES_VENDOR_ATTRS["generate_binaries"],
     "generate_build_scripts": CRATES_VENDOR_ATTRS["generate_build_scripts"],
-    "host_tools_repo": attr.string(
-        doc = "The name of the `rust_host_tools` repository to use.",
-        default = "rust_host_tools",
+    "host_tools": attr.label(
+        doc = "The `rust_host_tools` repository to use.",
+        default = "@rust_host_tools",
     ),
     "isolated": attr.bool(
         doc = (

examples/crate_universe/MODULE.bazel

@@ -263,13 +263,23 @@ use_repo(
 # C A R G O   B I N D E P S
 ###############################################################################
 
+rust_host_tools = use_extension("@rules_rust//rust:extensions.bzl", "rust_host_tools")
+rust_host_tools.host_tools(
+    name = "rust_host_tools_nightly",
+    version = "nightly",
+)
+use_repo(
+    rust_host_tools,
+    "rust_host_tools_nightly",
+)
+
 # https://bazelbuild.github.io/rules_rust/crate_universe_bzlmod.html
 crate_index_cargo_bindeps = use_extension("@rules_rust//crate_universe:extensions.bzl", "crate")
 crate_index_cargo_bindeps.from_cargo(
     name = "crate_index_cargo_bindeps",
     cargo_lockfile = "//cargo_bindeps:Cargo.lock",
     generate_binaries = True,
-    host_tools_repo = "rust_host_tools_nightly",
+    host_tools = "@rust_host_tools_nightly",
     manifests = ["//cargo_bindeps:Cargo.toml"],
 )
 use_repo(

rust/repositories.bzl

@@ -526,9 +526,14 @@ def _rust_toolchain_tools_repository_impl(ctx):
         )
         sha256s.update(rustc_dev_sha256)
 
-    ctx.file("WORKSPACE.bazel", "")
+    ctx.file("WORKSPACE.bazel", """workspace(name = "{}")""".format(
+        ctx.name,
+    ))
     ctx.file("BUILD.bazel", "\n".join(build_components))
 
+    # Used to locate `rust_host_tools` repositories.
+    ctx.file(ctx.name, "")
+
     repro = {"name": ctx.name}
     for key in _RUST_TOOLCHAIN_REPOSITORY_ATTRS:
         repro[key] = getattr(ctx.attr, key)