go_path: support go:embed of generated files (#3285)

When embedded files are generated, relativize fails because the file
path starts with the output directory. Without the change in path.bzl,
the test case fails as follows:

Error in fail: Path 'bazel-out/darwin-fastbuild/bin/tests/core/go_path/pkg/lib/renamed_embedded_src.txt'
is not beneath 'tests/core/go_path/pkg/lib'

go/private/tools/path.bzl

@@ -83,7 +83,8 @@ def _go_path_impl(ctx):
         for f in pkg.embedsrcs:
             if src_dir == None:
                 fail("cannot relativize {}: src_dir is unset".format(f.path))
-            dst = pkg.dir + "/" + paths.relativize(f.path, src_dir)
+            embedpath = paths.relativize(f.path, f.root.path)
+            dst = pkg.dir + "/" + paths.relativize(embedpath, src_dir)
             _add_manifest_entry(manifest_entries, manifest_entry_map, inputs, f, dst)
     if ctx.attr.include_pkg:
         for pkg in pkg_map.values():

tests/core/go_path/BUILD.bazel

@@ -18,6 +18,15 @@ test_suite(name = "go_path")
     ],
 ) for mode in ("archive", "copy", "link")]
 
+go_path(
+    name = "transition_path",
+    testonly = True,
+    data = ["extra.txt"],
+    include_pkg = True,
+    mode = "copy",
+    deps = ["//tests/core/go_path/cmd/bin:pie"],
+)
+
 go_path(
     name = "nodata_path",
     testonly = True,
@@ -49,6 +58,7 @@ go_test(
         ":archive_path",
         ":copy_path",
         ":link_path",
+        ":transition_path",
         ":nodata_path",
         ":notransitive_path",
     ],

tests/core/go_path/cmd/bin/BUILD.bazel

@@ -8,6 +8,17 @@ go_binary(
     visibility = ["//visibility:public"],
 )
 
+go_binary(
+    name = "pie",
+    srcs = ["bin.go"],
+    importpath = "example.com/repo/cmd/bin",
+    visibility = ["//visibility:public"],
+    linkmode = "pie",
+    deps = [
+        "//tests/core/go_path/pkg/lib:go_default_library"
+    ]
+)
+
 go_binary(
     name = "cross",
     srcs = ["bin.go"],

tests/core/go_path/pkg/lib/BUILD.bazel

@@ -1,4 +1,5 @@
 load("@io_bazel_rules_go//go:def.bzl", "go_library", "go_test")
+load("@bazel_skylib//rules:copy_file.bzl", "copy_file")
 
 go_library(
     name = "go_default_library",
@@ -10,13 +11,20 @@ go_library(
     ],
     embedsrcs = [
         "embedded_src.txt",
+        "renamed_embedded_src.txt",
         "template/index.html.tmpl",
     ],
     importpath = "example.com/repo/pkg/lib",
     visibility = ["//visibility:public"],
     deps = [":transitive_lib"],
 )
 
+copy_file(
+    name = "rename_embedded_src.txt",
+    src = ":embedded_src.txt",
+    out = "renamed_embedded_src.txt",
+)
+
 go_test(
     name = "go_default_test",
     srcs = [

tests/core/go_path/pkg/lib/lib.go

@@ -8,5 +8,8 @@ import (
 //go:embed embedded_src.txt
 var embeddedSource string
 
+//go:embed renamed_embedded_src.txt
+var renamedEmbeddedSource string
+
 //go:embed template/index.html.tmpl
 var indexTmpl string