cmd/compile: detect and diagnose invalid //go: directive placement

Thie CL changes cmd/compile/internal/syntax to give the gc half of
the compiler more control over pragma handling, so that it can prepare
better errors, diagnose misuse, and so on. Before, the API between
the two was hard-coded as a uint16. Now it is an interface{}.
This should set us up better for future directives.

In addition to the split, this CL emits a "misplaced compiler directive"
error for any directive that is in a place where it has no effect.
I've certainly been confused in the past by adding comments
that were doing nothing and not realizing it. This should help
avoid that kind of confusion.

The rule, now applied consistently, is that a //go: directive
must appear on a line by itself immediately before the declaration
specifier it means to apply to. See cmd/compile/doc.go for
precise text and test/directive.go for examples.

This may cause some code to stop compiling, but that code
was broken. For example, this code formerly applied the
//go:noinline to f (not c) but now will fail to compile:

	//go:noinline
	const c = 1

	func f() {}

Change-Id: Ieba9b8d90a27cfab25de79d2790a895cefe5296f
Reviewed-on: https://go-review.googlesource.com/c/go/+/228578
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

1 files changed, 15 insertions, 12 deletions

diff --git a/src/cmd/compile/internal/syntax/nodes.go b/src/cmd/compile/internal/syntax/nodes.go
index 9a74c0250b..815630fcd4 100644
--- a/src/cmd/compile/internal/syntax/nodes.go
+++ b/src/cmd/compile/internal/syntax/nodes.go
@@ -34,6 +34,7 @@ func (*node) aNode()     {}
 
 // package PkgName; DeclList[0], DeclList[1], ...
 type File struct {
+	Pragma   Pragma
 	PkgName  *Name
 	DeclList []Decl
 	Lines    uint
@@ -52,9 +53,10 @@ type (
 	//              Path
 	// LocalPkgName Path
 	ImportDecl struct {
+		Group        *Group // nil means not part of a group
+		Pragma       Pragma
 		LocalPkgName *Name // including "."; nil means no rename present
 		Path         *BasicLit
-		Group        *Group // nil means not part of a group
 		decl
 	}
 
@@ -62,20 +64,21 @@ type (
 	// NameList      = Values
 	// NameList Type = Values
 	ConstDecl struct {
-		NameList []*Name
-		Type     Expr   // nil means no type
-		Values   Expr   // nil means no values
 		Group    *Group // nil means not part of a group
+		Pragma   Pragma
+		NameList []*Name
+		Type     Expr // nil means no type
+		Values   Expr // nil means no values
 		decl
 	}
 
 	// Name Type
 	TypeDecl struct {
+		Group  *Group // nil means not part of a group
+		Pragma Pragma
 		Name   *Name
 		Alias  bool
 		Type   Expr
-		Group  *Group // nil means not part of a group
-		Pragma Pragma
 		decl
 	}
 
@@ -83,10 +86,11 @@ type (
 	// NameList Type = Values
 	// NameList      = Values
 	VarDecl struct {
-		NameList []*Name
-		Type     Expr   // nil means no type
-		Values   Expr   // nil means no values
 		Group    *Group // nil means not part of a group
+		Pragma   Pragma
+		NameList []*Name
+		Type     Expr // nil means no type
+		Values   Expr // nil means no values
 		decl
 	}
 
@@ -95,12 +99,11 @@ type (
 	// func Receiver Name Type { Body }
 	// func Receiver Name Type
 	FuncDecl struct {
-		Attr   map[string]bool // go:attr map
-		Recv   *Field          // nil means regular function
+		Pragma Pragma
+		Recv   *Field // nil means regular function
 		Name   *Name
 		Type   *FuncType
 		Body   *BlockStmt // nil means no body (forward declaration)
-		Pragma Pragma     // TODO(mdempsky): Cleaner solution.
 		decl
 	}
 )