cmd/compile: disallow linkname referring to instantiations

Linknaming an instantiated generic symbol isn't particularly
useful: it doesn't guarantee the instantiation exists, and the
instantiated symbol name may be subject to change. Checked with a
large code corpus, currently there is no occurrance of linkname
to an instantiated generic symbol (or symbol with a bracket in its
name). This also suggests that it is not very useful. Linkname is
already an unsafe mechanism. We don't need to allow it to do more
unsafe things without justification.

Change-Id: Ifaa20c98166b28a9d7dc3290c013c2b5bb7682e7
Reviewed-on: https://go-review.googlesource.com/c/go/+/585458
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Than McIntosh <thanm@google.com>

src/cmd/compile/internal/noder/writer.go

@@ -12,6 +12,7 @@ import (
 	"internal/buildcfg"
 	"internal/pkgbits"
 	"os"
+	"strings"
 
 	"cmd/compile/internal/base"
 	"cmd/compile/internal/ir"
@@ -2610,6 +2611,10 @@ func (pw *pkgWriter) collectDecls(noders []*noder) {
 				pw.errorf(l.pos, "//go:linkname only allowed in Go files that import \"unsafe\"")
 				continue
 			}
+			if strings.Contains(l.remote, "[") && strings.Contains(l.remote, "]") {
+				pw.errorf(l.pos, "//go:linkname reference of an instantiation is not allowed")
+				continue
+			}
 
 			switch obj := pw.curpkg.Scope().Lookup(l.local).(type) {
 			case *types2.Func, *types2.Var:

test/linkname3.go

@@ -13,13 +13,17 @@ type t int
 
 var x, y int
 
+func F[T any](T) {}
+
 //go:linkname x ok
 
 // ERROR "//go:linkname must refer to declared function or variable"
 // ERROR "//go:linkname must refer to declared function or variable"
 // ERROR "duplicate //go:linkname for x"
+// ERROR "//go:linkname reference of an instantiation is not allowed"
 
-//line linkname3.go:18
+//line linkname3.go:20
 //go:linkname nonexist nonexist
 //go:linkname t notvarfunc
 //go:linkname x duplicate
+//go:linkname i F[go.shape.int]