cmd/compile: be slightly more tolerant in case of certain syntax errors

Avoid construction of incorrect syntax trees in presence of errors.

For #19663.

Change-Id: I43025a3cf0fe02cae9a57e8bb9489b5f628c3fd7
Reviewed-on: https://go-review.googlesource.com/38604
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -686,9 +686,9 @@ func structfield(n *Node) *Field {
 	case string:
 		f.Note = u
 	default:
-		yyerror("field annotation must be string")
+		yyerror("field tag must be a string")
 	case nil:
-		// noop
+		// no-op
 	}
 
 	if n.Left != nil && n.Left.Op == ONAME {

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

@@ -748,7 +748,7 @@ func loadsys() {
 func importfile(f *Val, indent []byte) *Pkg {
 	path_, ok := f.U.(string)
 	if !ok {
-		yyerror("import statement not a string")
+		yyerror("import path must be a string")
 		return nil
 	}
 

src/cmd/compile/internal/syntax/parser.go

@@ -192,7 +192,7 @@ func (p *parser) advance(followlist ...token) {
 	}
 
 	// compute follow set
-	// TODO(gri) the args are constants - do as constant expressions?
+	// (not speed critical, advance is only called in error situations)
 	var followset uint64 = 1 << _EOF // never skip over EOF
 	for _, tok := range followlist {
 		followset |= 1 << tok
@@ -281,7 +281,7 @@ func (p *parser) file() *File {
 
 		case _Func:
 			p.next()
-			f.DeclList = append(f.DeclList, p.funcDecl())
+			f.DeclList = appendDecl(f.DeclList, p.funcDecl())
 
 		default:
 			if p.tok == _Lbrace && len(f.DeclList) > 0 && emptyFuncDecl(f.DeclList[len(f.DeclList)-1]) {
@@ -323,7 +323,7 @@ func (p *parser) appendGroup(list []Decl, f func(*Group) Decl) []Decl {
 	if p.got(_Lparen) {
 		g := new(Group)
 		for p.tok != _EOF && p.tok != _Rparen {
-			list = append(list, f(g))
+			list = appendDecl(list, f(g))
 			if !p.osemi(_Rparen) {
 				break
 			}
@@ -332,7 +332,14 @@ func (p *parser) appendGroup(list []Decl, f func(*Group) Decl) []Decl {
 		return list
 	}
 
-	return append(list, f(nil))
+	return appendDecl(list, f(nil))
+}
+
+func appendDecl(list []Decl, d Decl) []Decl {
+	if d != nil {
+		return append(list, d)
+	}
+	return list
 }
 
 func (p *parser) importDecl(group *Group) Decl {
@@ -353,11 +360,11 @@ func (p *parser) importDecl(group *Group) Decl {
 		d.LocalPkgName = n
 		p.next()
 	}
-	if p.tok == _Literal && p.kind == StringLit {
 	d.Path = p.oliteral()
-	} else {
-		p.syntax_error("missing import path; require quoted string")
+	if d.Path == nil {
+		p.syntax_error("missing import path")
 		p.advance(_Semi, _Rparen)
+		return nil
 	}
 	d.Group = group
 
@@ -400,6 +407,7 @@ func (p *parser) typeDecl(group *Group) Decl {
 	if d.Type == nil {
 		p.syntax_error("in type declaration")
 		p.advance(_Semi, _Rparen)
+		return nil
 	}
 	d.Group = group
 	d.Pragma = p.pragma
@@ -443,18 +451,16 @@ func (p *parser) funcDecl() *FuncDecl {
 	f := new(FuncDecl)
 	f.pos = p.pos()
 
-	badRecv := false
 	if p.tok == _Lparen {
 		rcvr := p.paramList()
 		switch len(rcvr) {
 		case 0:
 			p.error("method has no receiver")
-			badRecv = true
-		case 1:
-			f.Recv = rcvr[0]
 		default:
 			p.error("method has multiple receivers")
-			badRecv = true
+			fallthrough
+		case 1:
+			f.Recv = rcvr[0]
 		}
 	}
 
@@ -491,9 +497,6 @@ func (p *parser) funcDecl() *FuncDecl {
 	// 	p.error("can only use //go:noescape with external func implementations")
 	// }
 
-	if badRecv {
-		return nil // TODO(gri) better solution
-	}
 	return f
 }
 

src/cmd/compile/internal/syntax/syntax.go

@@ -40,8 +40,9 @@ type Pragma uint16
 type PragmaHandler func(pos src.Pos, text string) Pragma
 
 // Parse parses a single Go source file from src and returns the corresponding
-// syntax tree. If there are errors, Parse will return the first error found.
-// The base argument is only used for position information.
+// syntax tree. If there are errors, Parse will return the first error found,
+// and a possibly partially constructed syntax tree, or nil if no correct package
+// clause was found. The base argument is only used for position information.
 //
 // If errh != nil, it is called with each error encountered, and Parse will
 // process as much source as possible. If errh is nil, Parse will terminate

test/fixedbugs/bug217.go

@@ -13,3 +13,5 @@ func () x()	// ERROR "no receiver"
 
 func (a b, c d) x()	// ERROR "multiple receiver"
 
+type b int
+

test/fixedbugs/issue18393.go

@@ -17,7 +17,7 @@ import 42 // error on line 20
 var x     // error on line 24, not 30
 
 
-// ERROR "missing import path"
+// ERROR "import path must be a string"
 
 
 

test/import5.go

@@ -21,7 +21,7 @@ import _ "go/parser"
 //import "greek/αβ"
 
 // Import paths must be strings.
-import 42    // ERROR "missing import path; require quoted string"
-import 'a'   // ERROR "missing import path; require quoted string"
-import 3.14  // ERROR "missing import path; require quoted string"
-import 0.25i // ERROR "missing import path; require quoted string"
+import 42    // ERROR "import path must be a string"
+import 'a'   // ERROR "import path must be a string"
+import 3.14  // ERROR "import path must be a string"
+import 0.25i // ERROR "import path must be a string"