From f63cd2597c83c328a29c2770cb980b733a5f1d74 Mon Sep 17 00:00:00 2001
From: 6543 <6543@obermui.de>
Date: Mon, 7 Aug 2023 19:00:50 +0200
Subject: [PATCH 1/3] slices: add Concat

---
 api/next/56353.txt        |  1 +
 src/slices/slices.go      | 15 +++++++++++++++
 src/slices/slices_test.go | 18 ++++++++++++++++++
 3 files changed, 34 insertions(+)
 create mode 100644 api/next/56353.txt

diff --git a/api/next/56353.txt b/api/next/56353.txt
new file mode 100644
index 00000000000000..fff2600b87e18c
--- /dev/null
+++ b/api/next/56353.txt
@@ -0,0 +1 @@
+pkg slices, func Concat[$0 interface{ ~[]$1 }, $1 interface{}]($0, ...$1) $0 #56353
diff --git a/src/slices/slices.go b/src/slices/slices.go
index 725d91d8f5c151..af3f15ffacd560 100644
--- a/src/slices/slices.go
+++ b/src/slices/slices.go
@@ -493,3 +493,18 @@ func Reverse[S ~[]E, E any](s S) {
 		s[i], s[j] = s[j], s[i]
 	}
 }
+
+// Concat return a new slice that combines all values of input slices
+func Concat[S ~[]E, E any](dst S, slices ...S) S {
+	sl := 0
+	for i := range slices {
+		sl += len(slices[i])
+	}
+	dst = Grow(dst, len(dst)+sl)
+	for _, s := range slices {
+		if sLen := len(s); sLen != 0 {
+			dst = append(dst, s...)
+		}
+	}
+	return dst
+}
diff --git a/src/slices/slices_test.go b/src/slices/slices_test.go
index 8ea93c66d76686..835804f126cf10 100644
--- a/src/slices/slices_test.go
+++ b/src/slices/slices_test.go
@@ -1055,3 +1055,21 @@ func TestInference(t *testing.T) {
 		t.Errorf("Reverse(%v) = %v, want %v", S{4, 5, 6}, s2, want)
 	}
 }
+
+func TestConcat(t *testing.T) {
+	s1 := Concat([]string{}, []string{"a", "b"}, []string{"c"}, nil)
+	if want := []string{"a", "b", "c"}; !Equal(s1, want) {
+		t.Errorf("want %v, got %v", want, s1)
+	}
+
+	s2 := Concat(nil, []int{1, 2}, []int{4}, nil)
+	if want := []int{1, 2, 4}; !Equal(s2, want) {
+		t.Errorf("want %v, got %v", want, s2)
+	}
+
+	s3 := []bool{false, true}
+	s4 := Concat(s3, nil, []bool{true})
+	if want := []bool{false, true, true}; !Equal(s2, want) {
+		t.Errorf("want %v, got %v", want, s2)
+	}
+}

From 8645c5f88c95a028c5cc49dea50c0e8a100811d9 Mon Sep 17 00:00:00 2001
From: 6543 <6543@obermui.de>
Date: Mon, 7 Aug 2023 19:17:03 +0200
Subject: [PATCH 2/3] fix

---
 src/slices/slices.go      | 4 +---
 src/slices/slices_test.go | 4 ++--
 2 files changed, 3 insertions(+), 5 deletions(-)

diff --git a/src/slices/slices.go b/src/slices/slices.go
index af3f15ffacd560..4a4859e2410b22 100644
--- a/src/slices/slices.go
+++ b/src/slices/slices.go
@@ -502,9 +502,7 @@ func Concat[S ~[]E, E any](dst S, slices ...S) S {
 	}
 	dst = Grow(dst, len(dst)+sl)
 	for _, s := range slices {
-		if sLen := len(s); sLen != 0 {
-			dst = append(dst, s...)
-		}
+		dst = append(dst, s...)
 	}
 	return dst
 }
diff --git a/src/slices/slices_test.go b/src/slices/slices_test.go
index 835804f126cf10..2747f1bbdc1fcd 100644
--- a/src/slices/slices_test.go
+++ b/src/slices/slices_test.go
@@ -1069,7 +1069,7 @@ func TestConcat(t *testing.T) {
 
 	s3 := []bool{false, true}
 	s4 := Concat(s3, nil, []bool{true})
-	if want := []bool{false, true, true}; !Equal(s2, want) {
-		t.Errorf("want %v, got %v", want, s2)
+	if want := []bool{false, true, true}; !Equal(s4, want) {
+		t.Errorf("want %v, got %v", want, s4)
 	}
 }

From 4af94783664a0a1cedd4083cb747539de744b58b Mon Sep 17 00:00:00 2001
From: 6543 <6543@obermui.de>
Date: Mon, 7 Aug 2023 20:19:28 +0200
Subject: [PATCH 3/3] as per revew, always allocate

---
 src/slices/slices.go | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/src/slices/slices.go b/src/slices/slices.go
index 4a4859e2410b22..3a78c7225b44be 100644
--- a/src/slices/slices.go
+++ b/src/slices/slices.go
@@ -500,9 +500,9 @@ func Concat[S ~[]E, E any](dst S, slices ...S) S {
 	for i := range slices {
 		sl += len(slices[i])
 	}
-	dst = Grow(dst, len(dst)+sl)
+	res := append(make([]E, len(dst)+sl), dst...)
 	for _, s := range slices {
-		dst = append(dst, s...)
+		res = append(res, s...)
 	}
-	return dst
+	return res
 }