From 92ba9b5c40c23dad771fa471cdf24a2f18847ab8 Mon Sep 17 00:00:00 2001
From: Tim Cooper <tim.cooper@layeh.com>
Date: Mon, 25 Sep 2017 22:34:23 -0300
Subject: [PATCH] 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                    | 31 +++++++++++++++-------
 src/go/printer/testdata/expressions.golden | 10 +++++++
 src/go/printer/testdata/expressions.input  | 11 +++++++-
 src/go/printer/testdata/expressions.raw    | 10 +++++++
 4 files changed, 51 insertions(+), 11 deletions(-)

diff --git a/src/go/printer/nodes.go b/src/go/printer/nodes.go
index e9b110fe79..37b5873b68 100644
--- a/src/go/printer/nodes.go
+++ b/src/go/printer/nodes.go
@@ -398,22 +398,33 @@ 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]
-			for i, x := range f.Names {
-				if i > 0 {
-					// no comments so no need for comma position
-					p.print(token.COMMA, blank)
+			if isStruct {
+				for i, x := range f.Names {
+					if i > 0 {
+						// no comments so no need for comma position
+						p.print(token.COMMA, blank)
+					}
+					p.expr(x)
+				}
+				if len(f.Names) > 0 {
+					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.expr(x)
 			}
-			if len(f.Names) > 0 {
-				p.print(blank)
-			}
-			p.expr(f.Type)
 			p.print(blank, rbrace, token.RBRACE)
 			return
 		}
diff --git a/src/go/printer/testdata/expressions.golden b/src/go/printer/testdata/expressions.golden
index 4c08a423db..16a68c7bf7 100644
--- a/src/go/printer/testdata/expressions.golden
+++ b/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 _() {
diff --git a/src/go/printer/testdata/expressions.input b/src/go/printer/testdata/expressions.input
index b3b8c2bdc6..8c523b6022 100644
--- a/src/go/printer/testdata/expressions.input
+++ b/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
diff --git a/src/go/printer/testdata/expressions.raw b/src/go/printer/testdata/expressions.raw
index f121115e9d..058fded447 100644
--- a/src/go/printer/testdata/expressions.raw
+++ b/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 _() {