fix: default rustfmt version to supplied rust version (#2660)

Modifies the default of `rustfmt_version` so that it uses the first
version in the `versions` list. If there are multiple versions given
then falls back to the original default of `DEFAULT_NIGHTLY_VERSION`.

We tripped over this where supply a single Rust version (non-nightly),
but started using nightly features in `rustfmt.toml` and could no longer
format directly with `cargo fmt`. This helped align the versions of both
toolchains.

---------

Co-authored-by: Daniel Wagner-Hall <dwagnerhall@apple.com>

docs/flatten.md

@@ -1963,7 +1963,7 @@ See `load_arbitrary_tool` in `@rules_rust//rust:repositories.bzl` for more detai
 | <a id="rust_register_toolchains-global_allocator_library"></a>global_allocator_library |  Target that provides allocator functions when global allocator is used with cc_common.link.   |  `None` |
 | <a id="rust_register_toolchains-iso_date"></a>iso_date |  **Deprecated**: Use <code>versions</code> instead.   |  `None` |
 | <a id="rust_register_toolchains-register_toolchains"></a>register_toolchains |  If true, repositories will be generated to produce and register <code>rust_toolchain</code> targets.   |  `True` |
-| <a id="rust_register_toolchains-rustfmt_version"></a>rustfmt_version |  The version of rustfmt.   |  `"nightly/2024-05-02"` |
+| <a id="rust_register_toolchains-rustfmt_version"></a>rustfmt_version |  The version of rustfmt. If none is supplied and only a single version in <code>versions</code> is given, then this defaults to that version, otherwise will default to the default nightly version.   |  `None` |
 | <a id="rust_register_toolchains-rust_analyzer_version"></a>rust_analyzer_version |  The version of Rustc to pair with rust-analyzer.   |  `None` |
 | <a id="rust_register_toolchains-sha256s"></a>sha256s |  A dict associating tool subdirectories to sha256 hashes.   |  `None` |
 | <a id="rust_register_toolchains-extra_target_triples"></a>extra_target_triples |  Additional rust-style targets that rust toolchains should support.   |  `["wasm32-unknown-unknown", "wasm32-wasi"]` |

docs/rust_repositories.md

@@ -275,7 +275,7 @@ See `load_arbitrary_tool` in `@rules_rust//rust:repositories.bzl` for more detai
 | <a id="rust_register_toolchains-global_allocator_library"></a>global_allocator_library |  Target that provides allocator functions when global allocator is used with cc_common.link.   |  `None` |
 | <a id="rust_register_toolchains-iso_date"></a>iso_date |  **Deprecated**: Use <code>versions</code> instead.   |  `None` |
 | <a id="rust_register_toolchains-register_toolchains"></a>register_toolchains |  If true, repositories will be generated to produce and register <code>rust_toolchain</code> targets.   |  `True` |
-| <a id="rust_register_toolchains-rustfmt_version"></a>rustfmt_version |  The version of rustfmt.   |  `"nightly/2024-05-02"` |
+| <a id="rust_register_toolchains-rustfmt_version"></a>rustfmt_version |  The version of rustfmt. If none is supplied and only a single version in <code>versions</code> is given, then this defaults to that version, otherwise will default to the default nightly version.   |  `None` |
 | <a id="rust_register_toolchains-rust_analyzer_version"></a>rust_analyzer_version |  The version of Rustc to pair with rust-analyzer.   |  `None` |
 | <a id="rust_register_toolchains-sha256s"></a>sha256s |  A dict associating tool subdirectories to sha256 hashes.   |  `None` |
 | <a id="rust_register_toolchains-extra_target_triples"></a>extra_target_triples |  Additional rust-style targets that rust toolchains should support.   |  `["wasm32-unknown-unknown", "wasm32-wasi"]` |

rust/repositories.bzl

@@ -112,7 +112,7 @@ def rust_register_toolchains(
         global_allocator_library = None,
         iso_date = None,
         register_toolchains = True,
-        rustfmt_version = DEFAULT_NIGHTLY_VERSION,
+        rustfmt_version = None,
         rust_analyzer_version = None,
         sha256s = None,
         extra_target_triples = DEFAULT_EXTRA_TARGET_TRIPLES,
@@ -146,7 +146,7 @@ def rust_register_toolchains(
         global_allocator_library (str, optional): Target that provides allocator functions when global allocator is used with cc_common.link.
         iso_date (str, optional):  **Deprecated**: Use `versions` instead.
         register_toolchains (bool): If true, repositories will be generated to produce and register `rust_toolchain` targets.
-        rustfmt_version (str, optional): The version of rustfmt.
+        rustfmt_version (str, optional): The version of rustfmt. If none is supplied and only a single version in `versions` is given, then this defaults to that version, otherwise will default to the default nightly version.
         rust_analyzer_version (str, optional): The version of Rustc to pair with rust-analyzer.
         sha256s (str, optional): A dict associating tool subdirectories to sha256 hashes.
         extra_target_triples (list, optional): Additional rust-style targets that rust toolchains should support.
@@ -176,6 +176,12 @@ def rust_register_toolchains(
         else:
             versions = _RUST_TOOLCHAIN_VERSIONS
 
+    if not rustfmt_version:
+        if len(versions) == 1:
+            rustfmt_version = versions[0]
+        else:
+            rustfmt_version = DEFAULT_NIGHTLY_VERSION
+
     if dev_components:
         has_nightly = False
         for ver in versions: