diff --git a/src/cmd/go/internal/modindex/read.go b/src/cmd/go/internal/modindex/read.go
index fa0271f6ecf..3e068d56002 100644
--- a/src/cmd/go/internal/modindex/read.go
+++ b/src/cmd/go/internal/modindex/read.go
@@ -436,7 +436,7 @@ func (rp *IndexPackage) Import(bctxt build.Context, mode build.ImportMode) (p *b
 				p.PkgTargetRoot = ctxt.joinPath(p.Root, pkgtargetroot)
 
 				// Set the install target if applicable.
-				if strings.ToLower(godebug.Get("installgoroot")) == "all" || !p.Goroot {
+				if !p.Goroot || (strings.EqualFold(godebug.Get("installgoroot"), "all") && p.ImportPath != "unsafe" && p.ImportPath != "builtin") {
 					p.PkgObj = ctxt.joinPath(p.Root, pkga)
 				}
 			}
diff --git a/src/cmd/go/testdata/script/install_goroot_targets.txt b/src/cmd/go/testdata/script/install_goroot_targets.txt
index 4d6ca13e243..25b97b4b735 100644
--- a/src/cmd/go/testdata/script/install_goroot_targets.txt
+++ b/src/cmd/go/testdata/script/install_goroot_targets.txt
@@ -1,5 +1,4 @@
 [short] skip
-[!cgo] skip
 
 # Most packages in std do not have an install target.
 go list -f '{{.Target}}' fmt
@@ -8,11 +7,11 @@ go list -export -f '{{.Export}}' fmt
 stdout $GOCACHE
 
 # Packages that use cgo still do.
-go list -f '{{.Target}}' runtime/cgo
-stdout .
-go list -export -f '{{.Export}}' runtime/cgo
-! stdout $GOCACHE
-stdout cgo\.a
+[cgo] go list -f '{{.Target}}' runtime/cgo
+[cgo] stdout .
+[cgo] go list -export -f '{{.Export}}' runtime/cgo
+[cgo] ! stdout $GOCACHE
+[cgo] stdout cgo\.a
 
 # With GODEBUG=installgoroot=all, fmt has a target.
 # (Though we can't try installing it without modifying goroot).
@@ -20,6 +19,11 @@ env GODEBUG=installgoroot=all
 go list -f '{{.Target}}' fmt
 stdout fmt\.a
 
+# However, the fake packages "builtin" and "unsafe" do not.
+go list -f '{{.Target}}' builtin unsafe
+! stdout .
+go install builtin unsafe  # Should succeed as no-ops.
+
 # With CGO_ENABLED=0, packages that would have
 # an install target with cgo on no longer do.
 env GODEBUG=
diff --git a/src/go/build/build.go b/src/go/build/build.go
index 4c0388149db..6925154da18 100644
--- a/src/go/build/build.go
+++ b/src/go/build/build.go
@@ -783,7 +783,7 @@ Found:
 			p.PkgTargetRoot = ctxt.joinPath(p.Root, pkgtargetroot)
 
 			// Set the install target if applicable.
-			if strings.ToLower(godebug.Get("installgoroot")) == "all" || !p.Goroot {
+			if !p.Goroot || (strings.EqualFold(godebug.Get("installgoroot"), "all") && p.ImportPath != "unsafe" && p.ImportPath != "builtin") {
 				p.PkgObj = ctxt.joinPath(p.Root, pkga)
 			}
 		}