fix(cmd): locate GOROOT when the build is trimmed

Assisted-by: GLM 5.3 Flash

cmd/gasm/asmhdr.go

@@ -13,6 +13,7 @@ import (
 	"go/token"
 	"go/types"
 	"os"
+	"os/exec"
 	"path/filepath"
 	"regexp"
 	"strings"
@@ -217,6 +218,9 @@ type sourceImporter struct {
 func newSourceImporter(goos, goarch string) *sourceImporter {
 	ctxt := new(build.Context)
 	*ctxt = build.Default
+	if root := resolveGOROOT(build.Default.GOROOT); root != "" {
+		ctxt.GOROOT = root
+	}
 	ctxt.GOOS = goos
 	ctxt.GOARCH = goarch
 	ctxt.CgoEnabled = false
@@ -228,6 +232,27 @@ func newSourceImporter(goos, goarch string) *sourceImporter {
 	}
 }
 
+// resolveGOROOT returns the root the type-checker reads standard packages
+// from, preferring a baked root that is a real directory.  A binary built
+// with -trimpath carries no root: build.Default.GOROOT comes back empty and
+// every standard import fails with "GOROOT not set", so the toolchain is
+// asked, the way the corpus audit locates its own GOROOT.  An empty result
+// leaves the context untouched and the failure stays the honest one.
+func resolveGOROOT(baked string) string {
+	if st, err := os.Stat(baked); err == nil && st.IsDir() {
+		return baked
+	}
+	out, err := exec.Command("go", "env", "GOROOT").Output()
+	if err != nil {
+		return ""
+	}
+	root := strings.TrimSpace(string(out))
+	if st, err := os.Stat(root); err != nil || !st.IsDir() {
+		return ""
+	}
+	return root
+}
+
 // Import type-checks one imported package and memoises it.  "unsafe" must
 // resolve to go/types' own package, never to the source in GOROOT/src/unsafe:
 // the source declares Sizeof and Offsetof as ordinary functions over

cmd/gasm/asmhdr_test.go

@@ -285,6 +285,24 @@ func TestGoAsmHeaderResolved(t *testing.T) {
 	}
 }
 
+// TestResolveGOROOT holds the two resolutions the header generator leans
+// on: a baked root that is a real directory passes through, and a missing
+// one falls back to what go env GOROOT names, which is what a -trimpath
+// build needs.
+func TestResolveGOROOT(t *testing.T) {
+	out, err := exec.Command("go", "env", "GOROOT").Output()
+	if err != nil {
+		t.Skip("go env GOROOT unavailable:", err)
+	}
+	root := strings.TrimSpace(string(out))
+	if got := resolveGOROOT(root); got != root {
+		t.Fatalf("a real baked root must pass through, got %q", got)
+	}
+	if got := resolveGOROOT(filepath.Join(root, "nonexistent")); got != root {
+		t.Fatalf("a missing baked root must resolve to %q, got %q", root, got)
+	}
+}
+
 func TestOtherGOOSFile(t *testing.T) {
 	for path, want := range map[string]bool{
 		"/x/sys_windows_amd64.s": true,