Treat rustfmt as optional

All the code above this call treats rustfmt as optional, but then when
we make the BUILD for the rust toolchain, we unconditionally point at
the presumed rustfmt label.

At
https://github.com/bazelbuild/rules_rust/blob/59fab4e79f62bfa13551ac851a40696c24c0c3a4/rust/toolchain.bzl#L331-L335
we conditionally create a symlink based on whether the attribute is set,
but because we unconditionally set the attribute, this causes a hard
error failing to symlink to a non-existent file.

Instead, treat rustfmt as optional in this function too, so that the
code above and below properly convey the optionality to each other.

rust/private/repository_utils.bzl

@@ -186,7 +186,7 @@ rust_toolchain(
     rust_doc = "@{workspace_name}//:rustdoc",
     rust_std = "@{workspace_name}//:rust_std-{target_triple}",
     rustc = "@{workspace_name}//:rustc",
-    rustfmt = "@{workspace_name}//:rustfmt_bin",
+    rustfmt = {rustfmt_label},
     cargo = "@{workspace_name}//:cargo",
     clippy_driver = "@{workspace_name}//:clippy_driver_bin",
     rustc_lib = "@{workspace_name}//:rustc_lib",
@@ -210,6 +210,7 @@ def BUILD_for_rust_toolchain(
         target_triple,
         include_rustc_srcs,
         default_edition,
+        include_rustfmt,
         stdlib_linkflags = None):
     """Emits a toolchain declaration to match an existing compiler and stdlib.
 
@@ -220,6 +221,7 @@ def BUILD_for_rust_toolchain(
         target_triple (str): The rust-style target triple of the tool
         include_rustc_srcs (bool, optional): Whether to download rustc's src code. This is required in order to use rust-analyzer support. Defaults to False.
         default_edition (str): Default Rust edition.
+        include_rustfmt (bool): Whether rustfmt is present in the toolchain.
         stdlib_linkflags (list, optional): Overriden flags needed for linking to rust
                                            stdlib, akin to BAZEL_LINKLIBS. Defaults to
                                            None.
@@ -235,6 +237,9 @@ def BUILD_for_rust_toolchain(
     rustc_srcs = "None"
     if include_rustc_srcs:
         rustc_srcs = "\"@{workspace_name}//lib/rustlib/src:rustc_srcs\"".format(workspace_name = workspace_name)
+    rustfmt_label = "None"
+    if include_rustfmt:
+        rustfmt_label = "\"@{workspace_name}//:rustfmt_bin\"".format(workspace_name = workspace_name)
 
     return _build_file_for_rust_toolchain_template.format(
         toolchain_name = name,
@@ -248,6 +253,7 @@ def BUILD_for_rust_toolchain(
         default_edition = default_edition,
         exec_triple = exec_triple,
         target_triple = target_triple,
+        rustfmt_label = rustfmt_label,
     )
 
 _build_file_for_toolchain_template = """\
@@ -402,6 +408,7 @@ def load_rust_stdlib(ctx, target_triple):
         stdlib_linkflags = stdlib_linkflags,
         workspace_name = ctx.attr.name,
         default_edition = ctx.attr.edition,
+        include_rustfmt = not (not ctx.attr.rustfmt_version),
     )
 
     return stdlib_build_file + toolchain_build_file