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

@@ -19,7 +19,7 @@ make_tool(
 native_tool_toolchain(
     name = "built_make",
     path = "make/bin/make",
-    target = ":make_tool",
+    targets = [":make_tool"],
     visibility = ["//visibility:public"],
 )
 
@@ -38,7 +38,7 @@ cmake_tool(
 native_tool_toolchain(
     name = "built_cmake",
     path = "cmake/bin/cmake",
-    target = ":cmake_tool",
+    targets = [":cmake_tool"],
     visibility = ["//visibility:public"],
 )
 
@@ -57,7 +57,7 @@ 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

@@ -8,20 +8,23 @@ ToolInfo = provider(
             "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,
     ))
 
 native_tool_toolchain = rule(
@@ -42,14 +45,20 @@ native_tool_toolchain = rule(
                 "of the built tree artifact. (Please see the example `//examples:built_cmake_toolchain`)"
             ),
         ),
-        "target": attr.label(
+        "targets": attr.label_list(
             mandatory = False,
+            allow_empty = True,
+            default = [],
             doc = (
-                "If the tool is preinstalled, must be None. " +
+                "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."
             ),
         ),
+        "target": attr.label(
+            mandatory = False,
+            doc = """DEPRECATED: use `targets` instead.""",
+        ),
     },
 )
 
@@ -59,5 +68,5 @@ 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

@@ -12,9 +12,9 @@ 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),
         )