cmd/compile/internal/syntax: always construct a correct syntax tree

- parser creates sensible nodes in case of syntax errors instead of nil
- a new BadExpr node is used in places where we can't do better
- fixed error message for incorrect type switch guard
- minor cleanups

Fixes #19663.

Change-Id: I028394c6db9cba7371f0e417ebf93f594659786a
Reviewed-on: https://go-review.googlesource.com/38653
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/fmt_test.go

@@ -646,7 +646,6 @@ var knownFormats = map[string]string{
 	"cmd/compile/internal/ssa.regMask %d":             "",
 	"cmd/compile/internal/ssa.register %d":            "",
 	"cmd/compile/internal/syntax.Expr %#v":            "",
-	"cmd/compile/internal/syntax.Expr %s":             "",
 	"cmd/compile/internal/syntax.Node %T":             "",
 	"cmd/compile/internal/syntax.Operator %d":         "",
 	"cmd/compile/internal/syntax.Operator %s":         "",

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

@@ -416,7 +416,7 @@ func (p *noder) exprs(exprs []syntax.Expr) []*Node {
 func (p *noder) expr(expr syntax.Expr) *Node {
 	p.lineno(expr)
 	switch expr := expr.(type) {
-	case nil:
+	case nil, *syntax.BadExpr:
 		return nil
 	case *syntax.Name:
 		return p.mkname(expr)

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

@@ -125,6 +125,12 @@ type (
 		aExpr()
 	}
 
+	// Placeholder for an expression that failed to parse
+	// correctly and where we can't provide a better node.
+	BadExpr struct {
+		expr
+	}
+
 	// Value
 	Name struct {
 		Value string

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

@@ -345,6 +345,9 @@ func (p *printer) printRawNode(n Node) {
 		// we should not reach here but don't crash
 
 	// expressions and types
+	case *BadExpr:
+		p.print(_Name, "<bad expr>")
+
 	case *Name:
 		p.print(_Name, n.Value) // _Name requires actual value following immediately
 

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

@@ -65,7 +65,7 @@ func Parse(base *src.PosBase, src io.Reader, errh ErrorHandler, pragh PragmaHand
 	var p parser
 	p.init(base, src, errh, pragh)
 	p.next()
-	return p.file(), p.first
+	return p.fileOrNil(), p.first
 }
 
 // ParseBytes behaves like Parse but it reads the source from the []byte slice provided.