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, 53 insertions, 18 deletions

diff --git a/src/cmd/compile/internal/syntax/parser.go b/src/cmd/compile/internal/syntax/parser.go
index 5e52800b39..9601fab9e0 100644
--- a/src/cmd/compile/internal/syntax/parser.go
+++ b/src/cmd/compile/internal/syntax/parser.go
@@ -15,15 +15,16 @@ const debug = false
 const trace = false
 
 type parser struct {
-	file *PosBase
-	errh ErrorHandler
-	mode Mode
+	file  *PosBase
+	errh  ErrorHandler
+	mode  Mode
+	pragh PragmaHandler
 	scanner
 
 	base   *PosBase // current position base
 	first  error    // first error encountered
 	errcnt int      // number of errors encountered
-	pragma Pragma   // pragma flags
+	pragma Pragma   // pragmas
 
 	fnest  int    // function nesting level (for error handling)
 	xnest  int    // expression nesting level (for complit ambiguity resolution)
@@ -34,6 +35,7 @@ func (p *parser) init(file *PosBase, r io.Reader, errh ErrorHandler, pragh Pragm
 	p.file = file
 	p.errh = errh
 	p.mode = mode
+	p.pragh = pragh
 	p.scanner.init(
 		r,
 		// Error and directive handler for scanner.
@@ -47,9 +49,11 @@ func (p *parser) init(file *PosBase, r io.Reader, errh ErrorHandler, pragh Pragm
 				return
 			}
 
-			// otherwise it must be a comment containing a line or go: directive
+			// otherwise it must be a comment containing a line or go: directive.
+			// //line directives must be at the start of the line (column colbase).
+			// /*line*/ directives can be anywhere in the line.
 			text := commentText(msg)
-			if strings.HasPrefix(text, "line ") {
+			if (col == colbase || msg[1] == '*') && strings.HasPrefix(text, "line ") {
 				var pos Pos // position immediately following the comment
 				if msg[1] == '/' {
 					// line comment (newline is part of the comment)
@@ -67,7 +71,7 @@ func (p *parser) init(file *PosBase, r io.Reader, errh ErrorHandler, pragh Pragm
 
 			// go: directive (but be conservative and test)
 			if pragh != nil && strings.HasPrefix(text, "go:") {
-				p.pragma |= pragh(p.posAt(line, col+2), text) // +2 to skip over // or /*
+				p.pragma = pragh(p.posAt(line, col+2), p.scanner.blank, text, p.pragma) // +2 to skip over // or /*
 			}
 		},
 		directives,
@@ -76,13 +80,32 @@ func (p *parser) init(file *PosBase, r io.Reader, errh ErrorHandler, pragh Pragm
 	p.base = file
 	p.first = nil
 	p.errcnt = 0
-	p.pragma = 0
+	p.pragma = nil
 
 	p.fnest = 0
 	p.xnest = 0
 	p.indent = nil
 }
 
+// takePragma returns the current parsed pragmas
+// and clears them from the parser state.
+func (p *parser) takePragma() Pragma {
+	prag := p.pragma
+	p.pragma = nil
+	return prag
+}
+
+// clearPragma is called at the end of a statement or
+// other Go form that does NOT accept a pragma.
+// It sends the pragma back to the pragma handler
+// to be reported as unused.
+func (p *parser) clearPragma() {
+	if p.pragma != nil {
+		p.pragh(p.pos(), p.scanner.blank, "", p.pragma)
+		p.pragma = nil
+	}
+}
+
 // updateBase sets the current position base to a new line base at pos.
 // The base's filename, line, and column values are extracted from text
 // which is positioned at (tline, tcol) (only needed for error messages).
@@ -362,6 +385,7 @@ func (p *parser) fileOrNil() *File {
 		p.syntaxError("package statement must be first")
 		return nil
 	}
+	f.Pragma = p.takePragma()
 	f.PkgName = p.name()
 	p.want(_Semi)
 
@@ -410,7 +434,7 @@ func (p *parser) fileOrNil() *File {
 
 		// Reset p.pragma BEFORE advancing to the next token (consuming ';')
 		// since comments before may set pragmas for the next function decl.
-		p.pragma = 0
+		p.clearPragma()
 
 		if p.tok != _EOF && !p.got(_Semi) {
 			p.syntaxError("after top level declaration")
@@ -419,6 +443,7 @@ func (p *parser) fileOrNil() *File {
 	}
 	// p.tok == _EOF
 
+	p.clearPragma()
 	f.Lines = p.line
 
 	return f
@@ -469,6 +494,7 @@ func (p *parser) list(open, sep, close token, f func() bool) Pos {
 func (p *parser) appendGroup(list []Decl, f func(*Group) Decl) []Decl {
 	if p.tok == _Lparen {
 		g := new(Group)
+		p.clearPragma()
 		p.list(_Lparen, _Semi, _Rparen, func() bool {
 			list = append(list, f(g))
 			return false
@@ -497,6 +523,8 @@ func (p *parser) importDecl(group *Group) Decl {
 
 	d := new(ImportDecl)
 	d.pos = p.pos()
+	d.Group = group
+	d.Pragma = p.takePragma()
 
 	switch p.tok {
 	case _Name:
@@ -511,7 +539,6 @@ func (p *parser) importDecl(group *Group) Decl {
 		p.advance(_Semi, _Rparen)
 		return nil
 	}
-	d.Group = group
 
 	return d
 }
@@ -524,6 +551,8 @@ func (p *parser) constDecl(group *Group) Decl {
 
 	d := new(ConstDecl)
 	d.pos = p.pos()
+	d.Group = group
+	d.Pragma = p.takePragma()
 
 	d.NameList = p.nameList(p.name())
 	if p.tok != _EOF && p.tok != _Semi && p.tok != _Rparen {
@@ -532,7 +561,6 @@ func (p *parser) constDecl(group *Group) Decl {
 			d.Values = p.exprList()
 		}
 	}
-	d.Group = group
 
 	return d
 }
@@ -545,6 +573,8 @@ func (p *parser) typeDecl(group *Group) Decl {
 
 	d := new(TypeDecl)
 	d.pos = p.pos()
+	d.Group = group
+	d.Pragma = p.takePragma()
 
 	d.Name = p.name()
 	d.Alias = p.gotAssign()
@@ -554,8 +584,6 @@ func (p *parser) typeDecl(group *Group) Decl {
 		p.syntaxError("in type declaration")
 		p.advance(_Semi, _Rparen)
 	}
-	d.Group = group
-	d.Pragma = p.pragma
 
 	return d
 }
@@ -568,6 +596,8 @@ func (p *parser) varDecl(group *Group) Decl {
 
 	d := new(VarDecl)
 	d.pos = p.pos()
+	d.Group = group
+	d.Pragma = p.takePragma()
 
 	d.NameList = p.nameList(p.name())
 	if p.gotAssign() {
@@ -578,7 +608,6 @@ func (p *parser) varDecl(group *Group) Decl {
 			d.Values = p.exprList()
 		}
 	}
-	d.Group = group
 
 	return d
 }
@@ -595,6 +624,7 @@ func (p *parser) funcDeclOrNil() *FuncDecl {
 
 	f := new(FuncDecl)
 	f.pos = p.pos()
+	f.Pragma = p.takePragma()
 
 	if p.tok == _Lparen {
 		rcvr := p.paramList()
@@ -620,7 +650,6 @@ func (p *parser) funcDeclOrNil() *FuncDecl {
 	if p.tok == _Lbrace {
 		f.Body = p.funcBody()
 	}
-	f.Pragma = p.pragma
 
 	return f
 }
@@ -2054,6 +2083,7 @@ func (p *parser) stmtOrNil() Stmt {
 	// Most statements (assignments) start with an identifier;
 	// look for it first before doing anything more expensive.
 	if p.tok == _Name {
+		p.clearPragma()
 		lhs := p.exprList()
 		if label, ok := lhs.(*Name); ok && p.tok == _Colon {
 			return p.labeledStmtOrNil(label)
@@ -2062,9 +2092,6 @@ func (p *parser) stmtOrNil() Stmt {
 	}
 
 	switch p.tok {
-	case _Lbrace:
-		return p.blockStmt("")
-
 	case _Var:
 		return p.declStmt(p.varDecl)
 
@@ -2073,6 +2100,13 @@ func (p *parser) stmtOrNil() Stmt {
 
 	case _Type:
 		return p.declStmt(p.typeDecl)
+	}
+
+	p.clearPragma()
+
+	switch p.tok {
+	case _Lbrace:
+		return p.blockStmt("")
 
 	case _Operator, _Star:
 		switch p.op {
@@ -2151,6 +2185,7 @@ func (p *parser) stmtList() (l []Stmt) {
 
 	for p.tok != _EOF && p.tok != _Rbrace && p.tok != _Case && p.tok != _Default {
 		s := p.stmtOrNil()
+		p.clearPragma()
 		if s == nil {
 			break
 		}