cue/format: collapse empty structs/lists to single line

This changes the cue formatting logic to always format an empty
struct on a single line.

The following cue:

    struct: {
    }
    list: [

    ]

Will now be formatted like so:

    struct: {}
    list: []

Note that we preserve the multi-line structure in case a comment is
present in an empty list/struct. For example:

    struct: {
        // some comment
    }

will be formatted as-is.

Fixes #278.

Signed-off-by: Noam Dolovich <[email protected]>
Change-Id: I8ab8b7975d374e25b9fba8e02d53f643020b793b
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1193759
Reviewed-by: Daniel Martí <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>

Author: NoamTD

cmd/cue/cmd/testdata/script/get_go_types.txtar

@@ -401,8 +401,7 @@ _#internalIdentifier: #Identifier & "internal"
 
 #CustomJSON: _
 
-#CustomYAML: {
-}
+#CustomYAML: {}
 
 _#localType: int
 

cmd/cue/cmd/testdata/script/trim.txtar

@@ -43,8 +43,7 @@ foo: multipath: {
 		x: >=5 & <=8 & int
 	}
 
-	t: u: {
-	}
+	t: u: {}
 }
 
 group: {

cue/format/node.go

@@ -655,16 +655,15 @@ func (f *formatter) exprRaw(expr ast.Expr, prec1, depth int) {
 
 		switch {
 		case len(x.Elts) == 0:
-			if !x.Rbrace.HasRelPos() {
-				// collapse curly braces if the body is empty.
-				ffAlt := blank | nooverride
-				for _, c := range x.Comments() {
-					if c.Position == 1 {
-						ffAlt = ff
-					}
+			// collapse curly braces if the body is empty.
+			ffAlt := blank | nooverride
+			for _, c := range x.Comments() {
+				if c.Position == 1 {
+					ffAlt = ff
+					break
 				}
-				ff = ffAlt
 			}
+			ff = ffAlt
 		case !x.Rbrace.HasRelPos() || !x.Elts[0].Pos().HasRelPos():
 			ws |= newline | nooverride
 		}
@@ -684,7 +683,20 @@ func (f *formatter) exprRaw(expr ast.Expr, prec1, depth int) {
 		f.print(ws, x.Rbrace, token.RBRACE)
 
 	case *ast.ListLit:
-		f.print(x.Lbrack, token.LBRACK, noblank, indent)
+		ws := noblank | indent
+		if len(x.Elts) == 0 {
+			// collapse square brackets if the body is empty.
+			collapseWs := blank | nooverride
+			for _, c := range x.Comments() {
+				if c.Position == 1 {
+					collapseWs = ws
+					break
+				}
+			}
+			ws |= collapseWs
+		}
+
+		f.print(x.Lbrack, token.LBRACK, ws)
 		f.walkListElems(x.Elts)
 		f.print(trailcomma, noblank)
 		f.visitComments(f.current.pos)

cue/format/testdata/comments.golden

@@ -37,8 +37,7 @@ a: b: c: 2
 // rewrite okay
 a: b: c: d: 2
 
-m: {
-}
+m: {}
 m: {
 	// empty with comment
 }
@@ -51,8 +50,7 @@ struct1: {
 
 	// Another comment
 
-	something: {
-	}
+	something: {}
 	// extra comment
 }
 
@@ -62,8 +60,7 @@ struct2: {
 	// This is a comment
 
 	// Another comment
-	something: {
-	}
+	something: {}
 
 	// extra comment
 }
@@ -74,8 +71,7 @@ list1: [
 	// Comment2
 
 	// Another comment
-	{
-	},
+	{},
 
 	// Comment 3
 ]

cue/format/testdata/expressions.golden

@@ -33,9 +33,7 @@ import "list"
 	a: bb: cc: 3
 
 	empty: {}
-	emptyNewLine: {
-
-	}
+	emptyNewLine: {}
 	someObject: {
 		a:   8
 		aa:  9
@@ -125,6 +123,8 @@ import "list"
 	e: 'aa \(aaa) aa'
 	e: "aa \(aaa)"
 
+	e: []
+	e: []
 	e: [1, 2,
 	]
 	e: [1, 2]

cue/format/testdata/expressions.input

@@ -123,6 +123,11 @@ import "list"
     e: 'aa \(aaa) aa'
     e: "aa \(aaa)"
 
+    e: [
+    ]
+    e: [
+
+    ]
     e: [1, 2
     ]
     e: [1, 2]

encoding/protobuf/testdata/istio.io/api/mixer/v1/mixer_proto_gen.cue

@@ -258,5 +258,4 @@ import (
 }
 
 // Used to carry responses to telemetry reports
-#ReportResponse: {
-}
+#ReportResponse: {}

internal/third_party/yaml/decode_test.go

@@ -543,14 +543,12 @@ b: {
 		`
 a: {}
 c: 1
-d: [
-]
+d: []
 e: []
 `,
 		`a: {}
 c: 1
-d: [
-]
+d: []
 e: []`,
 	},
 

tools/trim/testdata/empty.txtar

@@ -13,8 +13,7 @@ deployment: alertmanager: {
 }
 -- out/trim --
 == in.cue
-deployment: [ID=string]: {
-}
+deployment: [ID=string]: {}
 
 deployment: alertmanager: {
 	empty: {}

tools/trim/testdata/kube1.txtar

@@ -49,6 +49,5 @@ service: a: {
 }
 
 service: a: {
-	ports: [{
-	}]
+	ports: [{}]
 }