cmd/compile: restrict //go:notinheap to runtime/internal/sys

So it won't be visible outside of runtime package. There are changes to
make tests happy:

 - For test/directive*.go files, using "go:noinline" for testing misplaced
 directives instead.
 - Restrict test/fixedbugs/bug515.go for gccgo only.
 - For test/notinheap{2,3}.go, using runtime/cgo.Incomplete for marking
 the type as not-in-heap. Though it's somewhat clumsy, it's the easiest
 way to keep the test errors for not-in-heap types until we can cleanup
 further.
 - test/typeparam/mdempsky/11.go is about defined type in user code marked
 as go:notinheap, which can't happen after this CL, though.

Fixes #46731

Change-Id: I869f5b2230c8a2a363feeec042e7723bbc416e8e
Reviewed-on: https://go-review.googlesource.com/c/go/+/421882
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Joedian Reid <joedian@golang.org>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -344,6 +344,9 @@ func (p *noder) pragma(pos syntax.Pos, blankLine bool, text string, old syntax.P
 		if flag == 0 && !allowedStdPragmas[verb] && base.Flag.Std {
 			p.error(syntax.Error{Pos: pos, Msg: fmt.Sprintf("//%s is not allowed in the standard library", verb)})
 		}
+		if flag == ir.NotInHeap && *base.Flag.LowerP != "runtime/internal/sys" {
+			p.error(syntax.Error{Pos: pos, Msg: "//go:notinheap only allowed in runtime/internal/sys"})
+		}
 		pragma.Flag |= flag
 		pragma.Pos = append(pragma.Pos, pragmaPos{flag, pos})
 	}

test/directive.go

@@ -29,17 +29,9 @@ const c = 1
 //go:noinline // ERROR "misplaced compiler directive"
 type T int
 
-// ok
-//go:notinheap
-type T1 int
-
 type (
-	//go:notinheap
 	//go:noinline // ERROR "misplaced compiler directive"
 	T2 int
-	T2b int
-	//go:notinheap
-	T2c int
 	//go:noinline // ERROR "misplaced compiler directive"
 	T3 int
 )
@@ -61,11 +53,5 @@ func f() {
 	_ = func() {}
 
 	//go:noinline // ERROR "misplaced compiler directive"
-	// ok:
-	//go:notinheap
 	type T int
 }
-
-// someday there might be a directive that can apply to type aliases, but go:notinheap doesn't.
-//go:notinheap // ERROR "misplaced compiler directive"
-type T6 = int

test/directive2.go

@@ -13,21 +13,20 @@ package main
 
 //go:build bad // ERROR "misplaced compiler directive"
 
-//go:notinheap // ERROR "misplaced compiler directive"
+//go:noinline // ERROR "misplaced compiler directive"
 type (
-	T2  int //go:notinheap // ERROR "misplaced compiler directive"
+	T2  int //go:noinline // ERROR "misplaced compiler directive"
 	T2b int
 	T2c int
 	T3  int
 )
 
-//go:notinheap // ERROR "misplaced compiler directive"
+//go:noinline // ERROR "misplaced compiler directive"
 type (
-	//go:notinheap
 	T4 int
 )
 
-//go:notinheap // ERROR "misplaced compiler directive"
+//go:noinline // ERROR "misplaced compiler directive"
 type ()
 
 type T5 int
@@ -53,10 +52,6 @@ func f() {
 	const c = 1
 
 	_ = func() {}
-
-	// ok:
-	//go:notinheap
-	type T int
 }
 
 // EOF

test/fixedbugs/bug515.go

@@ -6,6 +6,8 @@
 
 // Caused a gofrontend crash.
 
+//go:build gccgo
+
 package p
 
 //go:notinheap

test/notinheap2.go

@@ -4,12 +4,16 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Test walk errors for go:notinheap.
+// Test walk errors for not-in-heap.
+
+//go:build cgo
 
 package p
 
-//go:notinheap
+import "runtime/cgo"
+
 type nih struct {
+	_    cgo.Incomplete
 	next *nih
 }
 

test/notinheap3.go

@@ -6,8 +6,12 @@
 
 // Test write barrier elimination for notinheap.
 
+//go:build cgo
+
 package p
 
+import "runtime/cgo"
+
 type t1 struct {
 	x *nih
 	s []nih
@@ -20,8 +24,8 @@ type t2 struct {
 	y [1024]byte
 }
 
-//go:notinheap
 type nih struct {
+	_ cgo.Incomplete
 	x uintptr
 }
 

test/typeparam/mdempsky/11.go

@@ -1,16 +0,0 @@
-// errorcheck
-
-// Copyright 2021 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-// Reported by Cuong Manh Le.
-
-package p
-
-type a struct{}
-
-//go:notinheap
-type b a
-
-var _ = (*b)(new(a)) // ERROR "cannot convert"