cmd/compile: reject misplaced go:build comments

We are converting from using error-prone ad-hoc syntax // +build lines
to less error-prone, standard boolean syntax //go:build lines.
The timeline is:

Go 1.16: prepare for transition
 - Builds still use // +build for file selection.
 - Source files may not contain //go:build without // +build.
 - Builds fail when a source file contains //go:build lines without // +build lines. <<<

Go 1.17: start transition
 - Builds prefer //go:build for file selection, falling back to // +build
   for files containing only // +build.
 - Source files may contain //go:build without // +build (but they won't build with Go 1.16).
 - Gofmt moves //go:build and // +build lines to proper file locations.
 - Gofmt introduces //go:build lines into files with only // +build lines.
 - Go vet rejects files with mismatched //go:build and // +build lines.

Go 1.18: complete transition
 - Go fix removes // +build lines, leaving behind equivalent // +build lines.

This CL provides part of the <<< marked line above in the Go 1.16 step:
rejecting files containing //go:build but not // +build.
The standard go command checks only consider the top of the file.
This compiler check, along with a separate go vet check for ignored files,
handles the remainder of the file.

For #41184.

Change-Id: I014006eebfc84ab5943de18bc90449e534f150a2
Reviewed-on: https://go-review.googlesource.com/c/go/+/240601
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/compile/internal/gc/lex.go

@@ -50,6 +50,9 @@ const (
 
 	// Runtime and cgo type pragmas
 	NotInHeap // values of this type must not be heap allocated
+
+	// Go command pragmas
+	GoBuildPragma
 )
 
 const (
@@ -71,6 +74,8 @@ const (
 
 func pragmaFlag(verb string) PragmaFlag {
 	switch verb {
+	case "go:build":
+		return GoBuildPragma
 	case "go:nointerface":
 		if objabi.Fieldtrack_enabled != 0 {
 			return Nointerface

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

@@ -242,6 +242,7 @@ func (p *noder) node() {
 	mkpackage(p.file.PkgName.Value)
 
 	if pragma, ok := p.file.Pragma.(*Pragma); ok {
+		pragma.Flag &^= GoBuildPragma
 		p.checkUnused(pragma)
 	}
 

test/directive.go

@@ -6,11 +6,16 @@
 
 // Verify that misplaced directives are diagnosed.
 
+// ok
+//go:build !ignore
+
 //go:noinline // ERROR "misplaced compiler directive"
 
 //go:noinline // ERROR "misplaced compiler directive"
 package main
 
+//go:build bad // ERROR "misplaced compiler directive"
+
 //go:nosplit
 func f1() {}
 
@@ -93,3 +98,5 @@ type T6 = int
 
 // EOF
 //go:noinline // ERROR "misplaced compiler directive"
+
+//go:build bad // ERROR "misplaced compiler directive"