go/printer: allow one-method interfaces to be printed on a single line

Previously, only the empty interface could be formatted to print on a
single line. This behaviour made short one-method interfaces in function
definitions and type assertions more verbose than they had to be.

For example, the following type assertion:

    if c, ok := v.(interface {
        Close() error
    }); ok {
    }

Can now be formatted as:

    if c, ok := v.(interface{ Close() error }); ok {
    }

Fixes #21952

Change-Id: I896f796c5a30b9f4da2be3fe67cb6fea5871b835
Reviewed-on: https://go-review.googlesource.com/66130
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/printer/nodes.go

@@ -398,11 +398,12 @@ func (p *printer) fieldList(fields *ast.FieldList, isStruct, isIncomplete bool)
 			// no blank between keyword and {} in this case
 			p.print(lbrace, token.LBRACE, rbrace, token.RBRACE)
 			return
-		} else if isStruct && p.isOneLineFieldList(list) { // for now ignore interfaces
+		} else if p.isOneLineFieldList(list) {
 			// small enough - print on one line
 			// (don't use identList and ignore source line breaks)
 			p.print(lbrace, token.LBRACE, blank)
 			f := list[0]
+			if isStruct {
 				for i, x := range f.Names {
 					if i > 0 {
 						// no comments so no need for comma position
@@ -414,6 +415,16 @@ func (p *printer) fieldList(fields *ast.FieldList, isStruct, isIncomplete bool)
 					p.print(blank)
 				}
 				p.expr(f.Type)
+			} else { // interface
+				if ftyp, isFtyp := f.Type.(*ast.FuncType); isFtyp {
+					// method
+					p.expr(f.Names[0])
+					p.signature(ftyp.Params, ftyp.Results)
+				} else {
+					// embedded interface
+					p.expr(f.Type)
+				}
+			}
 			p.print(blank, rbrace, token.RBRACE)
 			return
 		}

src/go/printer/testdata/expressions.golden

@@ -290,6 +290,16 @@ func _() {
 	_ = struct{ x, y, z int }{0, 1, 2}
 	_ = struct{ int }{0}
 	_ = struct{ s struct{ int } }{struct{ int }{0}}
+
+	_ = (interface{})(nil)
+	_ = (interface{ String() string })(nil)
+	_ = (interface {
+		String() string
+	})(nil)
+	_ = (interface{ fmt.Stringer })(nil)
+	_ = (interface {
+		fmt.Stringer
+	})(nil)
 }
 
 func _() {

src/go/printer/testdata/expressions.input

@@ -295,8 +295,17 @@ func _() {
 	_ = struct{ x, y, z int }{0, 1, 2}
 	_ = struct{ int }{0}
 	_ = struct{ s struct { int } }{struct{ int}{0} }
-}
 
+	_ = (interface{})(nil)
+	_ = (interface{String() string})(nil)
+	_ = (interface{
+		String()    string
+	})(nil)
+	_ = (interface{fmt.Stringer})(nil)
+	_ = (interface{
+		    fmt.Stringer
+	})(nil)
+}
 
 func _() {
 	// do not modify literals

src/go/printer/testdata/expressions.raw

@@ -290,6 +290,16 @@ func _() {
 	_ = struct{ x, y, z int }{0, 1, 2}
 	_ = struct{ int }{0}
 	_ = struct{ s struct{ int } }{struct{ int }{0}}
+
+	_ = (interface{})(nil)
+	_ = (interface{ String() string })(nil)
+	_ = (interface {
+		String() string
+	})(nil)
+	_ = (interface{ fmt.Stringer })(nil)
+	_ = (interface {
+		fmt.Stringer
+	})(nil)
 }
 
 func _() {