bzlmod: fix issue with nightly versions (#2545)

Hi there 👋 

I tried using bzlmod with rules_rust, and I encountered an issue with
the download of the toolchains.
I applied a "hotfix," but I'm sure it's not the correct approach. Hence,
feel free to take over the PR.

```
bazel_dep(name = "rules_rust", version = "0.40.0")

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

rust.toolchain(
    edition = "2021",
    extra_target_triples = [
        "aarch64-apple-ios-sim",
        "aarch64-apple-ios",
        "aarch64-linux-android",
        "x86_64-unknown-linux-gnu",
    ],
    rustfmt_version = "nightly/2024-02-22",
    versions = ["nightly/2024-02-22"],
)

crate = use_extension(
    "@rules_rust//crate_universe:extension.bzl",
    "crate",
)
crate.from_cargo(
    name = "crates",
    cargo_lockfile = "//:Cargo.lock",
    manifests = [
        "//:Cargo.toml",
    ],
)
use_repo(crate, "crates")
```

---------

Co-authored-by: UebelAndre <[email protected]>

Author: Quentin Perez

rust/extensions.bzl

@@ -80,6 +80,14 @@ def _rust_impl(module_ctx):
         fail("Multiple host_tools were defined in your root MODULE.bazel")
 
     host_triple = get_host_triple(module_ctx)
+
+    iso_date = None
+    version = host_tools.version or rust_common.default_version
+
+    # Any version containing a slash is expected to be a nightly/beta release with iso date. E.g. `nightly/2024-03-21`
+    if "/" in version:
+        version, _, iso_date = version.partition("/")
+
     rust_toolchain_tools_repository(
         name = "rust_host_tools",
         exec_triple = host_triple.str,
@@ -90,7 +98,8 @@ def _rust_impl(module_ctx):
         rustfmt_version = host_tools.rustfmt_version,
         sha256s = host_tools.sha256s,
         urls = host_tools.urls,
-        version = host_tools.version or rust_common.default_version,
+        version = version,
+        iso_date = iso_date,
     )
 
     for toolchain in toolchains:

rust/repositories.bzl

@@ -301,37 +301,44 @@ def rust_repositories(**kwargs):
 def _rust_toolchain_tools_repository_impl(ctx):
     """The implementation of the rust toolchain tools repository rule."""
 
-    check_version_valid(ctx.attr.version, ctx.attr.iso_date)
+    iso_date = ctx.attr.iso_date
+    version = ctx.attr.version
+    version_array = version.split("/")
+    if len(version_array) > 1:
+        version = version_array[0]
+        iso_date = version_array[1]
+
+    check_version_valid(ctx.attr.version, iso_date)
 
     exec_triple = triple(ctx.attr.exec_triple)
 
     build_components = [
         load_rust_compiler(
             ctx = ctx,
-            iso_date = ctx.attr.iso_date,
+            iso_date = iso_date,
             target_triple = exec_triple,
-            version = ctx.attr.version,
+            version = version,
         ),
         load_clippy(
             ctx = ctx,
-            iso_date = ctx.attr.iso_date,
+            iso_date = iso_date,
             target_triple = exec_triple,
-            version = ctx.attr.version,
+            version = version,
         ),
         load_cargo(
             ctx = ctx,
-            iso_date = ctx.attr.iso_date,
+            iso_date = iso_date,
             target_triple = exec_triple,
-            version = ctx.attr.version,
+            version = version,
         ),
     ]
 
     if ctx.attr.rustfmt_version:
         rustfmt_version = ctx.attr.rustfmt_version
         rustfmt_iso_date = None
         if rustfmt_version in ("nightly", "beta"):
-            if ctx.attr.iso_date:
-                rustfmt_iso_date = ctx.attr.iso_date
+            if iso_date:
+                rustfmt_iso_date = iso_date
             else:
                 fail("`rustfmt_version` does not include an iso_date. The following reposiotry should either set `iso_date` or update `rustfmt_version` to include an iso_date suffix: {}".format(
                     ctx.name,
@@ -346,7 +353,7 @@ def _rust_toolchain_tools_repository_impl(ctx):
         ))
 
     # Rust 1.45.0 and nightly builds after 2020-05-22 need the llvm-tools gzip to get the libLLVM dylib
-    include_llvm_tools = ctx.attr.version >= "1.45.0" or (ctx.attr.version == "nightly" and ctx.attr.iso_date > "2020-05-22")
+    include_llvm_tools = version >= "1.45.0" or (version == "nightly" and iso_date > "2020-05-22")
     if include_llvm_tools:
         build_components.append(load_llvm_tools(
             ctx = ctx,