go/types: match types2 string when printing composite literals

Given a composite literal type S, rather than always printing
(S literal) for a composite literals, print S{} if the literal
has no elements, and print S{…} as a short form (suitable for
error messages) if there are elements. This matches types2 and
also Go1.17 compiler behavior (except that the original compiler
would print ... rather than …). Using … rather than ... makes
it clearer that we don't have real Go syntax, and it's also more
compact.

For #54511.

Change-Id: I5991e8060232f16ecbf4a1fe4ae091598fc76b68
Reviewed-on: https://go-review.googlesource.com/c/go/+/425006
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/types/api_test.go

@@ -951,7 +951,7 @@ func TestPredicatesInfo(t *testing.T) {
 
 		// values
 		{`package v0; var (a, b int; _ = a + b)`, `a + b`, `value`},
-		{`package v1; var _ = &[]int{1}`, `([]int literal)`, `value`},
+		{`package v1; var _ = &[]int{1}`, `[]int{…}`, `value`},
 		{`package v2; var _ = func(){}`, `(func() literal)`, `value`},
 		{`package v4; func f() { _ = f }`, `f`, `value`},
 		{`package v3; var _ *int = nil`, `nil`, `value, nil`},

src/go/types/exprstring.go

@@ -53,9 +53,12 @@ func WriteExpr(buf *bytes.Buffer, x ast.Expr) {
 		buf.WriteString(" literal)") // shortened
 
 	case *ast.CompositeLit:
-		buf.WriteByte('(')
 		WriteExpr(buf, x.Type)
-		buf.WriteString(" literal)") // shortened
+		buf.WriteByte('{')
+		if len(x.Elts) > 0 {
+			buf.WriteString("…")
+		}
+		buf.WriteByte('}')
 
 	case *ast.ParenExpr:
 		buf.WriteByte('(')

src/go/types/exprstring_test.go

@@ -25,7 +25,7 @@ var testExprs = []testEntry{
 	// func and composite literals
 	{"func(){}", "(func() literal)"},
 	{"func(x int) complex128 {}", "(func(x int) complex128 literal)"},
-	{"[]int{1, 2, 3}", "([]int literal)"},
+	{"[]int{1, 2, 3}", "[]int{…}"},
 
 	// type expressions
 	dup("[1 << 10]byte"),

src/go/types/testdata/check/decls3.go

@@ -221,16 +221,16 @@ func _() {
 	_ = S2{}.B
 	_ = S2{}.C
 	_ = S2{}.D /* ERROR "no field or method" */
-	_ = S3{}.S1 /* ERROR "ambiguous selector \(S3 literal\).S1" */
+	_ = S3{}.S1 /* ERROR "ambiguous selector S3{}.S1" */
 	_ = S3{}.A
-	_ = S3{}.B /* ERROR "ambiguous selector" \(S3 literal\).B */
+	_ = S3{}.B /* ERROR "ambiguous selector" S3{}.B */
 	_ = S3{}.D
 	_ = S3{}.E
 	_ = S4{}.A
 	_ = S4{}.B /* ERROR "no field or method" */
-	_ = S5{}.X /* ERROR "ambiguous selector \(S5 literal\).X" */
+	_ = S5{}.X /* ERROR "ambiguous selector S5{}.X" */
 	_ = S5{}.Y
-	_ = S10{}.X /* ERROR "ambiguous selector \(S10 literal\).X" */
+	_ = S10{}.X /* ERROR "ambiguous selector S10{}.X" */
 	_ = S10{}.Y
 }
 
@@ -306,4 +306,4 @@ type R22 R21
 type R23 R21
 type R24 R21
 
-var _ = R0{}.X /* ERROR "ambiguous selector \(R0 literal\).X" */
+var _ = R0{}.X /* ERROR "ambiguous selector R0{}.X" */

src/go/types/testdata/check/decls4.go

@@ -190,8 +190,8 @@ type eD struct {
 }
 
 var (
-	_ = eD{}.xf /* ERROR ambiguous selector \(eD literal\).xf */
-	_ = eD{}.xm /* ERROR ambiguous selector \(eD literal\).xm */
+	_ = eD{}.xf /* ERROR ambiguous selector eD{}.xf */
+	_ = eD{}.xm /* ERROR ambiguous selector eD{}.xm */
 )
 
 var (

src/go/types/testdata/check/issues0.go

@@ -133,7 +133,7 @@ func issue10260() {
 	)
 
 	var x I1
-	x = T1 /* ERROR cannot use \(T1 literal\) .* as I1 value in assignment: T1 does not implement I1 \(method foo has pointer receiver\) */ {}
+	x = T1 /* ERROR cannot use T1{} .* as I1 value in assignment: T1 does not implement I1 \(method foo has pointer receiver\) */ {}
 	_ = x /* ERROR impossible type assertion: x\.\(T1\)\n\tT1 does not implement I1 \(method foo has pointer receiver\) */ .(T1)
 
 	T1{}.foo /* ERROR cannot call pointer method foo on T1 */ ()

src/go/types/testdata/fixedbugs/issue49579.go

@@ -9,7 +9,7 @@ type I[F any] interface {
 }
 
 func G[F any]() I[any] {
-	return g /* ERROR cannot use \(g\[F\] literal\) .* as I\[any\] value in return statement: g\[F\] does not implement I\[any\] \(method Q has pointer receiver\) */ [F]{}
+	return g /* ERROR cannot use g\[F\]{} .* as I\[any\] value in return statement: g\[F\] does not implement I\[any\] \(method Q has pointer receiver\) */ [F]{}
 }
 
 type g[F any] struct{}