Replace 'target' attribute with 'targets'.

This is so that multiple labels could be provided to a toolchain as
sources. An example is `emcmake`, which needs both `emscripten` and
`binaryen` to run.

Another reason for this is to pass in config files that might be needed
for the toolchain. The config files can be passed in as a label and will
be available in ${EXT_BUILD_DEPS}/bin, just like any other directory.

examples/with_prebuilt_ninja_artefact/BUILD

@@ -9,7 +9,7 @@ native_tool_toolchain(
     # In the path, we also start with the name of the directory,
     # in this case the external repository.
     path = "ninja_artefact/ninja",
-    target = "@ninja_artefact//:all",
+    targets = ["@ninja_artefact//:all"],
     visibility = ["//visibility:public"],
 )
 

tools/build_defs/native_tools/BUILD

@@ -17,7 +17,7 @@ make_tool(
 native_tool_toolchain(
     name = "built_make",
     path = "make/bin/make",
-    target = ":make_tool",
+    targets = [":make_tool"],
     visibility = ["//visibility:public"],
 )
 
@@ -35,7 +35,7 @@ cmake_tool(
 native_tool_toolchain(
     name = "built_cmake",
     path = "cmake/bin/cmake",
-    target = ":cmake_tool",
+    targets = [":cmake_tool"],
     visibility = ["//visibility:public"],
 )
 
@@ -53,6 +53,6 @@ ninja_tool(
 native_tool_toolchain(
     name = "built_ninja",
     path = "ninja/ninja",
-    target = ":ninja_tool",
+    targets = [":ninja_tool"],
     visibility = ["//visibility:public"],
 )

tools/build_defs/native_tools/native_tools_toolchain.bzl

@@ -5,18 +5,21 @@ ToolInfo = provider(
 Relative path to the tool in case the tool is built as part of a build; the path should be relative
 to the bazel-genfiles, i.e. it should start with the name of the top directory of the built tree
 artifact. (Please see the example "//examples:built_cmake_toolchain")""",
-        "target": """If the tool is preinstalled, must be None.
+        "targets": """If the tool is preinstalled, must be an empty list.
 If the tool is built as part of the build, the corresponding build target, which should produce
 the tree artifact with the binary to call.""",
     },
 )
 
 def _native_tool_toolchain(ctx):
-    if not ctx.attr.path and not ctx.attr.target:
-        fail("Either path or target (and path) should be defined for the tool.")
+    if not ctx.attr.path and not ctx.attr.target and not ctx.attr.targets:
+        fail("Either path or targets (and path) should be defined for the tool.")
+    targets = ctx.attr.targets
+    if not targets and ctx.attr.target:
+        targets = [ctx.attr.target]
     return platform_common.ToolchainInfo(data = ToolInfo(
         path = ctx.attr.path,
-        target = ctx.attr.target,
+        targets = targets,
     ))
 
 """ Rule for defining the toolchain data of the native tools (cmake, ninja),
@@ -36,7 +39,13 @@ of the built tree artifact. (Please see the example "//examples:built_cmake_tool
         ),
         "target": attr.label(
             mandatory = False,
-            doc = """If the tool is preinstalled, must be None.
+            doc = """DEPRECATED: use "targets" instead.""",
+        ),
+        "targets": attr.label_list(
+            mandatory = False,
+            allow_empty = True,
+            default = [],
+            doc = """If the tool is preinstalled, must be an empty list.
 If the tool is built as part of the build, the corresponding build target,
 which should produce the tree artifact with the binary to call.""",
         ),
@@ -49,5 +58,5 @@ def access_tool(toolchain_type_, ctx, tool_name):
         return tool_toolchain.data
     return ToolInfo(
         path = tool_name,
-        target = None,
+        targets = [],
     )

tools/build_defs/native_tools/tool_access.bzl

@@ -11,9 +11,9 @@ def get_make_data(ctx):
 
 def _access_and_expect_label_copied(toolchain_type_, ctx, tool_name):
     tool_data = access_tool(toolchain_type_, ctx, tool_name)
-    if tool_data.target:
+    if tool_data.targets:
         return struct(
-            deps = [tool_data.target],
+            deps = tool_data.targets,
             # as the tool will be copied into tools directory
             path = "$EXT_BUILD_DEPS/bin/{}".format(tool_data.path),
         )