Allow rules/aspects to optionally depend on the Swift toolchain

PiperOrigin-RevId: 634762729
(cherry picked from commit 0300b69)
Signed-off-by: Brentley Jones <github@brentleyjones.com>

swift/internal/toolchain_utils.bzl

@@ -16,7 +16,12 @@
 
 SWIFT_TOOLCHAIN_TYPE = "@build_bazel_rules_swift//toolchains:toolchain_type"
 
-def get_swift_toolchain(ctx, *, exec_group = None, attr = "_toolchain"):
+def get_swift_toolchain(
+        ctx,
+        *,
+        exec_group = None,
+        mandatory = True,
+        attr = "_toolchain"):
     """Gets the Swift toolchain associated with the rule or aspect.
 
     Args:
@@ -26,14 +31,17 @@ def get_swift_toolchain(ctx, *, exec_group = None, attr = "_toolchain"):
             that execution group, it will be looked up from `ctx` as a fallback
             instead. If this argument is `None` (the default), then the
             toolchain will only be looked up from `ctx.`
+        mandatory: If `False`, this function will return `None` instead of
+            failing if no toolchain is found. Defaults to `True`.
         attr: The name of the attribute on the calling rule or aspect that
             should be used to retrieve the toolchain if it is not provided by
             the `toolchains` argument of the rule/aspect. Note that this is only
             supported for legacy/migration purposes and will be removed once
             migration to toolchains is complete.
 
     Returns:
-        A `SwiftToolchainInfo` provider.
+        A `SwiftToolchainInfo` provider, or `None` if the toolchain was not
+        found and not required.
     """
     if exec_group:
         group = ctx.exec_groups[exec_group]
@@ -49,10 +57,14 @@ def get_swift_toolchain(ctx, *, exec_group = None, attr = "_toolchain"):
     if toolchain_target and platform_common.ToolchainInfo in toolchain_target:
         return toolchain_target[platform_common.ToolchainInfo].swift_toolchain
 
-    fail("To use `get_swift_toolchain`, you must declare the toolchain in " +
-         "your rule using `toolchains = use_swift_toolchain()`.")
+    if mandatory:
+        fail("To use `swift_common.get_toolchain`, you must declare the " +
+             "toolchain in your rule using " +
+             "`toolchains = swift_common.use_toolchain()`.")
 
-def use_swift_toolchain():
+    return None
+
+def use_swift_toolchain(*, mandatory = True):
     """Returns a list of toolchain types needed to use the Swift toolchain.
 
     This function returns a list so that it can be easily composed with other
@@ -63,8 +75,18 @@ def use_swift_toolchain():
     toolchains = use_swift_toolchain() + [other toolchains...]
     ```
 
+    Args:
+        mandatory: Whether or not it should be an error if the toolchain cannot
+            be resolved. Defaults to True.
+
     Returns:
-        A list of toolchain types that should be passed to `rule()`, `aspect()`,
-        or `exec_group`.
+        A list of
+        [toolchain types](https://bazel.build/rules/lib/builtins/toolchain_type.html)
+        that should be passed to `rule()`, `aspect()`, or `exec_group()`.
     """
-    return [SWIFT_TOOLCHAIN_TYPE]
+    return [
+        config_common.toolchain_type(
+            SWIFT_TOOLCHAIN_TYPE,
+            mandatory = mandatory,
+        ),
+    ]