internal/testenv: avoid rebuilding all of std in WriteImportcfg

Instead, have the caller pass in an explicit list of the packages
(if any) they need.

After #47257, a builder running a test does not necessarily have the
entire standard library already cached, especially when running tests
in sharded mode. testenv.WriteImportcfg used to write an importcfg for
the entire standard library — which required rebuilding the entire
standard library — even though most tests need only a tiny subset.

This reduces the time to test internal/abi with a cold build cache on
my workstation from ~16s to ~0.05s.

It somewhat increases the time for 'go test go/internal/gcimporter'
with a cold cache, from ~43s to ~54s, presumably due to decreased
parallelism in rebuilding the standard library and increased overhead
in re-resolving the import map. However, 'go test -short' running time
remains stable (~5.5s before and after).

Fixes #58248.

Change-Id: I9be6b61ae6e28b75b53af85207c281bb93b9346f
Reviewed-on: https://go-review.googlesource.com/c/go/+/464736
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/src/cmd/objdump/objdump_test.go b/src/cmd/objdump/objdump_test.go
index 69b4cf4e21..226e74d81e 100644
--- a/src/cmd/objdump/objdump_test.go
+++ b/src/cmd/objdump/objdump_test.go
@@ -299,7 +299,7 @@ func TestDisasmGoobj(t *testing.T) {
 	tmp := t.TempDir()
 
 	importcfgfile := filepath.Join(tmp, "hello.importcfg")
-	testenv.WriteImportcfg(t, importcfgfile, nil)
+	testenv.WriteImportcfg(t, importcfgfile, nil, "testdata/fmthello.go")
 
 	hello := filepath.Join(tmp, "hello.o")
 	args := []string{"tool", "compile", "-p=main", "-importcfg=" + importcfgfile, "-o", hello}