Read the Rust version from rust-toolchain.toml in rust.toolchain

MODULE.bazel

@@ -15,6 +15,7 @@ bazel_dep(name = "platforms", version = "1.1.0")
 bazel_dep(name = "rules_cc", version = "0.2.4")
 bazel_dep(name = "rules_license", version = "1.0.0")
 bazel_dep(name = "rules_shell", version = "0.6.1")
+bazel_dep(name = "toml.bzl", version = "0.4.1")
 bazel_dep(name = "apple_support", version = "1.24.1")
 
 internal_deps = use_extension("//rust/private:internal_extensions.bzl", "i")

docs/src/index.md

@@ -42,6 +42,16 @@ rust.toolchain(
 )
 ```
 
+If the repository has a [`rust-toolchain.toml`](https://rust-lang.github.io/rustup/overrides.html#the-toolchain-file) for rustup and cargo, `rust_toolchain_file` reads the version from it instead, so it is only written once. Its `channel` must be an exact release (`1.85.0`, `nightly-2025-01-01`), as rules_rust downloads a fixed release rather than asking rustup what `stable` is today:
+
+```python
+rust = use_extension("@rules_rust//rust:extensions.bzl", "rust")
+rust.toolchain(
+    edition = "2021",
+    rust_toolchain_file = "//:rust-toolchain.toml",
+)
+```
+
 By default, a `stable` and `nightly` toolchain will be registered if no `toolchain` method is called (and thus no specific versions are registered). However, if only 1 version is passed and it is from the `nightly` or `beta` release channels (i.e. __not__ `stable`), then the following build setting flag must be present, either on the command line or set in the project's `.bazelrc` file:
 
 ```text

examples/hello_world_no_cargo/MODULE.bazel

@@ -9,7 +9,10 @@ local_path_override(
 )
 
 rust = use_extension("@rules_rust//rust:extensions.bzl", "rust")
-rust.toolchain(edition = "2021")
+rust.toolchain(
+    edition = "2021",
+    rust_toolchain_file = "//:rust-toolchain.toml",
+)
 use_repo(rust, "rust_toolchains")
 
 register_toolchains("@rust_toolchains//:all")

examples/hello_world_no_cargo/rust-toolchain.toml

@@ -0,0 +1,3 @@
+# What rustup and cargo read; MODULE.bazel points rules_rust at it too.
+[toolchain]
+channel = "1.98.0"

rust/extensions.bzl

@@ -16,6 +16,7 @@ load(
     "DEFAULT_NIGHTLY_VERSION",
     "DEFAULT_STATIC_RUST_URL_TEMPLATES",
 )
+load("//rust/private:rust_toolchain_file.bzl", "parse_rust_toolchain_file")
 
 _RUST_TOOLCHAIN_VERSIONS = [
     rust_common.default_version,
@@ -120,7 +121,23 @@ def _rust_impl(module_ctx):
             fail("Cannot define both extra_rustc_flags and extra_rustc_flags_triples")
         if toolchain.extra_exec_rustc_flags and toolchain.extra_exec_rustc_flags_triples:
             fail("Cannot define both extra_exec_rustc_flags and extra_exec_rustc_flags_triples")
-        if len(toolchain.versions) == 0:
+
+        versions = toolchain.versions
+        extra_target_triples = toolchain.extra_target_triples
+        dev_components = toolchain.dev_components
+        if toolchain.rust_toolchain_file:
+            if toolchain.versions != _RUST_TOOLCHAIN_VERSIONS:
+                fail("Cannot define both versions and rust_toolchain_file")
+            toolchain_file = parse_rust_toolchain_file(module_ctx.read(module_ctx.path(toolchain.rust_toolchain_file)))
+            versions = toolchain_file.versions
+            extra_target_triples = extra_target_triples + [
+                triple
+                for triple in toolchain_file.extra_target_triples
+                if triple not in extra_target_triples
+            ]
+            dev_components = dev_components or toolchain_file.dev_components
+
+        if len(versions) == 0:
             # If the root module has asked for rules_rust to not register default
             # toolchains, an empty repository named `rust_toolchains` is created
             # so that the `register_toolchains()` in MODULES.bazel is still
@@ -132,7 +149,7 @@ def _rust_impl(module_ctx):
 
             rust_register_toolchains(
                 hub_name = "rust_toolchains",
-                dev_components = toolchain.dev_components,
+                dev_components = dev_components,
                 edition = toolchain.edition,
                 extra_rustc_flags = extra_rustc_flags,
                 extra_exec_rustc_flags = extra_exec_rustc_flags,
@@ -141,11 +158,11 @@ def _rust_impl(module_ctx):
                 rustfmt_version = toolchain.rustfmt_version,
                 rust_analyzer_version = toolchain.rust_analyzer_version,
                 sha256s = toolchain.sha256s,
-                extra_target_triples = toolchain.extra_target_triples,
+                extra_target_triples = extra_target_triples,
                 opt_level = toolchain.opt_level if toolchain.opt_level else None,
                 strip_level = toolchain.strip_level if toolchain.strip_level else None,
                 urls = toolchain.urls,
-                versions = toolchain.versions,
+                versions = versions,
                 compact_windows_names = True,
                 aliases = toolchain.aliases,
                 toolchain_triples = toolchain_triples,
@@ -281,6 +298,15 @@ _RUST_TOOLCHAIN_TAG = tag_class(
         "rust_analyzer_version": attr.string(
             doc = "The version of Rustc to pair with rust-analyzer.",
         ),
+        "rust_toolchain_file": attr.label(
+            doc = (
+                "A `rust-toolchain.toml` file, the one rustup and cargo read, to take the toolchain from " +
+                "instead of `versions`: its `channel` must be an exact release (`1.85.0`, `nightly-2025-01-01`), " +
+                "its `targets` are added to `extra_target_triples`, and `rustc-dev` in its `components` " +
+                "enables `dev_components`."
+            ),
+            allow_single_file = True,
+        ),
         "rustfmt_toolchain_triples": attr.string_dict(
             doc = "Like toolchain_triples, but for rustfmt toolchains. Mapping of rust target triple to repository name.",
         ),

rust/private/BUILD.bazel

@@ -28,6 +28,7 @@ bzl_library(
         "@bazel_skylib//lib:paths",
         "@bazel_skylib//lib:structs",
         "@bazel_skylib//rules:common_settings",
+        "@toml.bzl//:toml",
     ],
 )
 

rust/private/rust_toolchain_file.bzl

@@ -0,0 +1,42 @@
+"""Reads the `rust-toolchain.toml` file that rustup and cargo read."""
+
+load("@toml.bzl", "toml")
+
+def parse_rust_toolchain_file(content):
+    """Turns a `rust-toolchain.toml` file into `rust.toolchain` settings.
+
+    Only what rules_rust can act on without rustup is read: `channel` must name
+    an exact release (`1.85.0`, `nightly-2025-01-01`, `beta-2025-01-01`),
+    because resolving `stable` or `1.85` to a release is something rustup does
+    with the network. `targets` adds to `extra_target_triples`, and
+    `components` enables `dev_components` if it lists `rustc-dev`.
+
+    Args:
+        content (str): The contents of a `rust-toolchain.toml` file.
+
+    Returns:
+        struct: With `versions`, `extra_target_triples` and `dev_components`
+            fields, in the shape the `rust.toolchain` tag takes.
+    """
+    toolchain = toml.decode(content).get("toolchain", {})
+    channel = toolchain.get("channel")
+    if not channel:
+        fail("rust-toolchain.toml has no `channel` in its `[toolchain]` table")
+
+    if channel.startswith("nightly-") or channel.startswith("beta-"):
+        name, iso_date = channel.split("-", 1)
+        version = "{}/{}".format(name, iso_date)
+    elif channel[0].isdigit() and channel.count(".") == 2:
+        version = channel
+    else:
+        fail((
+            "rust-toolchain.toml channel `{}` is not an exact release: rules_rust " +
+            "downloads a fixed release, so the channel must be a full version " +
+            "(`1.85.0`) or a dated nightly or beta (`nightly-2025-01-01`)"
+        ).format(channel))
+
+    return struct(
+        versions = [version],
+        extra_target_triples = toolchain.get("targets", []),
+        dev_components = "rustc-dev" in toolchain.get("components", []),
+    )

test/unit/rust_toolchain_file/BUILD.bazel

@@ -0,0 +1,4 @@
+load(":rust_toolchain_file_test.bzl", "rust_toolchain_file_test_suite")
+
+############################ UNIT TESTS #############################
+rust_toolchain_file_test_suite(name = "rust_toolchain_file_test_suite")

test/unit/rust_toolchain_file/rust_toolchain_file_test.bzl

@@ -0,0 +1,61 @@
+"""Unit tests for rust_toolchain_file.bzl."""
+
+load("@bazel_skylib//lib:unittest.bzl", "asserts", "unittest")
+
+# buildifier: disable=bzl-visibility
+load("//rust/private:rust_toolchain_file.bzl", "parse_rust_toolchain_file")
+
+def _stable_release_test_impl(ctx):
+    env = unittest.begin(ctx)
+    parsed = parse_rust_toolchain_file("""\
+# What cargo and rustup read.
+[toolchain]
+channel = "1.85.0"
+components = ["rustfmt", "clippy"]
+""")
+    asserts.equals(env, ["1.85.0"], parsed.versions)
+    asserts.equals(env, [], parsed.extra_target_triples)
+    asserts.false(env, parsed.dev_components)
+    return unittest.end(env)
+
+def _dated_nightly_test_impl(ctx):
+    env = unittest.begin(ctx)
+    parsed = parse_rust_toolchain_file("""\
+[toolchain]
+channel = "nightly-2025-01-01"
+components = [
+    "rustc-dev",
+    "rustfmt",
+]
+targets = ["wasm32-unknown-unknown", "aarch64-unknown-linux-gnu"]
+""")
+    asserts.equals(env, ["nightly/2025-01-01"], parsed.versions)
+    asserts.equals(env, ["wasm32-unknown-unknown", "aarch64-unknown-linux-gnu"], parsed.extra_target_triples)
+    asserts.true(env, parsed.dev_components)
+    return unittest.end(env)
+
+def _dated_beta_test_impl(ctx):
+    env = unittest.begin(ctx)
+    parsed = parse_rust_toolchain_file("""\
+[toolchain]
+channel = "beta-2025-01-01"
+""")
+    asserts.equals(env, ["beta/2025-01-01"], parsed.versions)
+    return unittest.end(env)
+
+stable_release_test = unittest.make(_stable_release_test_impl)
+dated_nightly_test = unittest.make(_dated_nightly_test_impl)
+dated_beta_test = unittest.make(_dated_beta_test_impl)
+
+def rust_toolchain_file_test_suite(name):
+    """Entry-point macro called from the BUILD file.
+
+    Args:
+        name (str): Name of the macro.
+    """
+    unittest.suite(
+        name,
+        stable_release_test,
+        dated_nightly_test,
+        dated_beta_test,
+    )