cmd/compile: resolve the TODO of processPragmas

Change-Id: Id723ecc2480aea2d8acb4d3e05db4a6c8eef9cc8
Reviewed-on: https://go-review.googlesource.com/c/go/+/333109
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Trust: Matthew Dempsky <mdempsky@google.com>
Trust: Cherry Mui <cherryyz@google.com>

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

@@ -323,8 +323,7 @@ func (p *noder) processPragmas() {
 		}
 		n := ir.AsNode(typecheck.Lookup(l.local).Def)
 		if n == nil || n.Op() != ir.ONAME {
-			// TODO(mdempsky): Change to p.errorAt before Go 1.17 release.
-			// base.WarnfAt(p.makeXPos(l.pos), "//go:linkname must refer to declared function or variable (will be an error in Go 1.17)")
+			p.errorAt(l.pos, "//go:linkname must refer to declared function or variable")
 			continue
 		}
 		if n.Sym().Linkname != "" {

src/cmd/cover/testdata/test.go

@@ -13,6 +13,7 @@ package main
 import _ "unsafe" // for go:linkname
 
 //go:linkname some_name some_name
+var some_name int
 
 const anything = 1e9 // Just some unlikely value that means "we got here, don't care how often"
 

test/linkname2.go

@@ -16,13 +16,10 @@ var x, y int
 //go:linkname x ok
 
 // ERROR "//go:linkname requires linkname argument or -p compiler flag"
-// BAD: want error "//go:linkname must refer to declared function or variable"
-// BAD: want error "//go:linkname must refer to declared function or variable"
+// 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"
 
-// The two BAD lines are just waiting for #42938 before we can
-// re-enable the errors.
-
 //line linkname2.go:18
 //go:linkname y
 //go:linkname nonexist nonexist