[dev.typeparams] cmd/compile/internal/syntax: clean up node printing API

Preparation for using the syntax printer as expression printer in types2.

- Introduced Form to control printing format
- Cleaned up/added String and ShortString convenience functions
- Implemented ShortForm format which prints … for non-empty
  function and composite literal bodies
- Added test to check write error handling

Change-Id: Ie86e46d766fb60fcf07ef643c7788b2ef440ffa8
Reviewed-on: https://go-review.googlesource.com/c/go/+/282552
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

4 files changed, 94 insertions, 27 deletions

diff --git a/src/cmd/compile/internal/syntax/dumper.go b/src/cmd/compile/internal/syntax/dumper.go
index 01453d5a7a..d5247886da 100644
--- a/src/cmd/compile/internal/syntax/dumper.go
+++ b/src/cmd/compile/internal/syntax/dumper.go
@@ -26,7 +26,7 @@ func Fdump(w io.Writer, n Node) (err error) {
 
 	defer func() {
 		if e := recover(); e != nil {
-			err = e.(localError).err // re-panics if it's not a localError
+			err = e.(writeError).err // re-panics if it's not a writeError
 		}
 	}()
 
@@ -82,16 +82,16 @@ func (p *dumper) Write(data []byte) (n int, err error) {
 	return
 }
 
-// localError wraps locally caught errors so we can distinguish
+// writeError wraps locally caught write errors so we can distinguish
 // them from genuine panics which we don't want to return as errors.
-type localError struct {
+type writeError struct {
 	err error
 }
 
 // printf is a convenience wrapper that takes care of print errors.
 func (p *dumper) printf(format string, args ...interface{}) {
 	if _, err := fmt.Fprintf(p, format, args...); err != nil {
-		panic(localError{err})
+		panic(writeError{err})
 	}
 }
 
diff --git a/src/cmd/compile/internal/syntax/parser_test.go b/src/cmd/compile/internal/syntax/parser_test.go
index ea9e9acc83..340ca6bb6f 100644
--- a/src/cmd/compile/internal/syntax/parser_test.go
+++ b/src/cmd/compile/internal/syntax/parser_test.go
@@ -169,7 +169,7 @@ func walkDirs(t *testing.T, dir string, action func(string)) {
 
 func verifyPrint(t *testing.T, filename string, ast1 *File) {
 	var buf1 bytes.Buffer
-	_, err := Fprint(&buf1, ast1, true)
+	_, err := Fprint(&buf1, ast1, LineForm)
 	if err != nil {
 		panic(err)
 	}
@@ -181,7 +181,7 @@ func verifyPrint(t *testing.T, filename string, ast1 *File) {
 	}
 
 	var buf2 bytes.Buffer
-	_, err = Fprint(&buf2, ast2, true)
+	_, err = Fprint(&buf2, ast2, LineForm)
 	if err != nil {
 		panic(err)
 	}
diff --git a/src/cmd/compile/internal/syntax/printer.go b/src/cmd/compile/internal/syntax/printer.go
index c8bf59675a..0a60e1753d 100644
--- a/src/cmd/compile/internal/syntax/printer.go
+++ b/src/cmd/compile/internal/syntax/printer.go
@@ -13,19 +13,28 @@ import (
 	"strings"
 )
 
-// TODO(gri) Consider removing the linebreaks flag from this signature.
-// Its likely rarely used in common cases.
+// Form controls print formatting.
+type Form uint
 
-func Fprint(w io.Writer, x Node, linebreaks bool) (n int, err error) {
+const (
+	_         Form = iota // default
+	LineForm              // use spaces instead of linebreaks where possible
+	ShortForm             // like LineForm but print "…" for non-empty function or composite literal bodies
+)
+
+// Fprint prints node x to w in the specified form.
+// It returns the number of bytes written, and whether there was an error.
+func Fprint(w io.Writer, x Node, form Form) (n int, err error) {
 	p := printer{
 		output:     w,
-		linebreaks: linebreaks,
+		form:       form,
+		linebreaks: form == 0,
 	}
 
 	defer func() {
 		n = p.written
 		if e := recover(); e != nil {
-			err = e.(localError).err // re-panics if it's not a localError
+			err = e.(writeError).err // re-panics if it's not a writeError
 		}
 	}()
 
@@ -35,15 +44,20 @@ func Fprint(w io.Writer, x Node, linebreaks bool) (n int, err error) {
 	return
 }
 
-func String(n Node) string {
+func asString(n Node, form Form) string {
 	var buf bytes.Buffer
-	_, err := Fprint(&buf, n, false)
+	_, err := Fprint(&buf, n, form)
 	if err != nil {
-		panic(err) // TODO(gri) print something sensible into buf instead
+		fmt.Fprintf(&buf, "<<< ERROR: %s", err)
 	}
 	return buf.String()
 }
 
+// String and ShortString are convenience functions that print n in
+// LineForm or ShortForm respectively, and return the printed string.
+func String(n Node) string      { return asString(n, LineForm) }
+func ShortString(n Node) string { return asString(n, ShortForm) }
+
 type ctrlSymbol int
 
 const (
@@ -65,7 +79,8 @@ type whitespace struct {
 
 type printer struct {
 	output     io.Writer
-	written    int  // number of bytes written
+	written    int // number of bytes written
+	form       Form
 	linebreaks bool // print linebreaks instead of semis
 
 	indent  int // current indentation level
@@ -81,7 +96,7 @@ func (p *printer) write(data []byte) {
 	n, err := p.output.Write(data)
 	p.written += n
 	if err != nil {
-		panic(localError{err})
+		panic(writeError{err})
 	}
 }
 
@@ -355,17 +370,34 @@ func (p *printer) printRawNode(n Node) {
 		p.print(_Name, n.Value) // _Name requires actual value following immediately
 
 	case *FuncLit:
-		p.print(n.Type, blank, n.Body)
+		p.print(n.Type, blank)
+		if n.Body != nil {
+			if p.form == ShortForm {
+				p.print(_Lbrace)
+				if len(n.Body.List) > 0 {
+					p.print(_Name, "…")
+				}
+				p.print(_Rbrace)
+			} else {
+				p.print(n.Body)
+			}
+		}
 
 	case *CompositeLit:
 		if n.Type != nil {
 			p.print(n.Type)
 		}
 		p.print(_Lbrace)
-		if n.NKeys > 0 && n.NKeys == len(n.ElemList) {
-			p.printExprLines(n.ElemList)
+		if p.form == ShortForm {
+			if len(n.ElemList) > 0 {
+				p.print(_Name, "…")
+			}
 		} else {
-			p.printExprList(n.ElemList)
+			if n.NKeys > 0 && n.NKeys == len(n.ElemList) {
+				p.printExprLines(n.ElemList)
+			} else {
+				p.printExprList(n.ElemList)
+			}
 		}
 		p.print(_Rbrace)
 
@@ -450,9 +482,13 @@ func (p *printer) printRawNode(n Node) {
 		}
 		p.print(_Lbrace)
 		if len(n.FieldList) > 0 {
-			p.print(newline, indent)
-			p.printFieldList(n.FieldList, n.TagList)
-			p.print(outdent, newline)
+			if p.linebreaks {
+				p.print(newline, indent)
+				p.printFieldList(n.FieldList, n.TagList)
+				p.print(outdent, newline)
+			} else {
+				p.printFieldList(n.FieldList, n.TagList)
+			}
 		}
 		p.print(_Rbrace)
 
@@ -467,9 +503,13 @@ func (p *printer) printRawNode(n Node) {
 		}
 		p.print(_Lbrace)
 		if len(n.MethodList) > 0 {
-			p.print(newline, indent)
-			p.printMethodList(n.MethodList)
-			p.print(outdent, newline)
+			if p.linebreaks {
+				p.print(newline, indent)
+				p.printMethodList(n.MethodList)
+				p.print(outdent, newline)
+			} else {
+				p.printMethodList(n.MethodList)
+			}
 		}
 		p.print(_Rbrace)
 
diff --git a/src/cmd/compile/internal/syntax/printer_test.go b/src/cmd/compile/internal/syntax/printer_test.go
index 9f1f7e18cb..6c07fe0a26 100644
--- a/src/cmd/compile/internal/syntax/printer_test.go
+++ b/src/cmd/compile/internal/syntax/printer_test.go
@@ -25,11 +25,38 @@ func TestPrint(t *testing.T) {
 	}
 
 	if ast != nil {
-		Fprint(testOut(), ast, true)
+		Fprint(testOut(), ast, LineForm)
 		fmt.Println()
 	}
 }
 
+type shortBuffer struct {
+	buf []byte
+}
+
+func (w *shortBuffer) Write(data []byte) (n int, err error) {
+	w.buf = append(w.buf, data...)
+	n = len(data)
+	if len(w.buf) > 10 {
+		err = io.ErrShortBuffer
+	}
+	return
+}
+
+func TestPrintError(t *testing.T) {
+	const src = "package p; var x int"
+	ast, err := Parse(nil, strings.NewReader(src), nil, nil, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	var buf shortBuffer
+	_, err = Fprint(&buf, ast, 0)
+	if err == nil || err != io.ErrShortBuffer {
+		t.Errorf("got err = %s, want %s", err, io.ErrShortBuffer)
+	}
+}
+
 var stringTests = []string{
 	"package p",
 	"package p; type _ int; type T1 = struct{}; type ( _ *struct{}; T2 = float32 )",