From aa608f70320e8047db9d5f39353fef610cb76798 Mon Sep 17 00:00:00 2001
From: Carl Johnson <me@carlmjohnson.net>
Date: Wed, 21 Jun 2023 17:31:01 -0400
Subject: [PATCH 1/3] slices: Add Concat

---
 api/next/56353.txt        |  1 +
 src/slices/slices.go      | 16 +++++++
 src/slices/slices_test.go | 94 +++++++++++++++++++++++++++++++++++++++
 3 files changed, 111 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..c2504a7f63abd0
--- /dev/null
+++ b/api/next/56353.txt
@@ -0,0 +1 @@
+pkg slices, func Concat[$0 interface{ ~[]$1 }, $1 interface{}](...$0) $0 #56353
diff --git a/src/slices/slices.go b/src/slices/slices.go
index 725d91d8f5c151..900c309e4818ba 100644
--- a/src/slices/slices.go
+++ b/src/slices/slices.go
@@ -493,3 +493,19 @@ func Reverse[S ~[]E, E any](s S) {
 		s[i], s[j] = s[j], s[i]
 	}
 }
+
+// Concat returns a new slice concatenating the passed in slices.
+func Concat[S ~[]E, E any](slices ...S) S {
+	size := 0
+	for _, s := range slices {
+		size += len(s)
+		if size < 0 {
+			panic("len out of range")
+		}
+	}
+	newslice := make(S, 0, size)
+	for _, s := range slices {
+		newslice = append(newslice, s...)
+	}
+	return newslice
+}
diff --git a/src/slices/slices_test.go b/src/slices/slices_test.go
index 8ea93c66d76686..a6839b95dc46cf 100644
--- a/src/slices/slices_test.go
+++ b/src/slices/slices_test.go
@@ -1055,3 +1055,97 @@ func TestInference(t *testing.T) {
 		t.Errorf("Reverse(%v) = %v, want %v", S{4, 5, 6}, s2, want)
 	}
 }
+
+func TestConcat(t *testing.T) {
+	cases := []struct {
+		s    [][]int
+		want []int
+	}{
+		{
+			s:    [][]int{nil},
+			want: nil,
+		},
+		{
+			s:    [][]int{{1}},
+			want: []int{1},
+		},
+		{
+			s:    [][]int{{1}, {2}},
+			want: []int{1, 2},
+		},
+		{
+			s:    [][]int{{1}, nil, {2}},
+			want: []int{1, 2},
+		},
+	}
+	for _, tc := range cases {
+		got := Concat(tc.s...)
+		if !Equal(tc.want, got) {
+			t.Errorf("Concat(%v) = %v, want %v", tc.s, got, tc.want)
+		}
+		var sink []int
+		allocs := testing.AllocsPerRun(5, func() {
+			sink = Concat(tc.s...)
+		})
+		_ = sink
+		if allocs > 1 {
+			t.Fatalf("Concat(%v) had %f allocs; expected 1", tc.s, allocs)
+		}
+	}
+}
+
+func TestConcat_too_large(t *testing.T) {
+	// Use zero length element to minimize memory in testing
+	type void struct{}
+	cases := []struct {
+		lengths     []int
+		shouldPanic bool
+	}{
+		{
+			lengths:     []int{0, 0},
+			shouldPanic: false,
+		},
+		{
+			lengths:     []int{math.MaxInt, 0},
+			shouldPanic: false,
+		},
+		{
+			lengths:     []int{0, math.MaxInt},
+			shouldPanic: false,
+		},
+		{
+			lengths:     []int{math.MaxInt - 1, 1},
+			shouldPanic: false,
+		},
+		{
+			lengths:     []int{math.MaxInt - 1, 1, 1},
+			shouldPanic: true,
+		},
+		{
+			lengths:     []int{math.MaxInt, 1},
+			shouldPanic: true,
+		},
+		{
+			lengths:     []int{math.MaxInt, math.MaxInt},
+			shouldPanic: true,
+		},
+	}
+	for _, tc := range cases {
+		var r any
+		ss := make([][]void, 0, len(tc.lengths))
+		for _, l := range tc.lengths {
+			s := make([]void, l)
+			ss = append(ss, s)
+		}
+		func() {
+			defer func() {
+				r = recover()
+			}()
+			_ = Concat(ss...)
+		}()
+		if didPanic := r != nil; didPanic != tc.shouldPanic {
+			t.Errorf("slices.Concat(lens(%v)) got panic == %v",
+				tc.lengths, didPanic)
+		}
+	}
+}

From e98968aeb055acae10beb95a687c4100ef6ca0dd Mon Sep 17 00:00:00 2001
From: Carl Johnson <me@carlmjohnson.net>
Date: Mon, 7 Aug 2023 17:29:45 -0400
Subject: [PATCH 2/3] requested change

---
 src/slices/slices.go | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/slices/slices.go b/src/slices/slices.go
index 900c309e4818ba..a4d9f7e3f5ff3d 100644
--- a/src/slices/slices.go
+++ b/src/slices/slices.go
@@ -503,7 +503,7 @@ func Concat[S ~[]E, E any](slices ...S) S {
 			panic("len out of range")
 		}
 	}
-	newslice := make(S, 0, size)
+	newslice := Grow[S](nil, size)
 	for _, s := range slices {
 		newslice = append(newslice, s...)
 	}

From 96a35e524c168e5004c5cd28e693437462218eeb Mon Sep 17 00:00:00 2001
From: Carl Johnson <me@carlmjohnson.net>
Date: Mon, 7 Aug 2023 21:11:31 -0400
Subject: [PATCH 3/3] fix test

---
 src/slices/slices_test.go | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/src/slices/slices_test.go b/src/slices/slices_test.go
index a6839b95dc46cf..ac779f5bd955db 100644
--- a/src/slices/slices_test.go
+++ b/src/slices/slices_test.go
@@ -1089,7 +1089,11 @@ func TestConcat(t *testing.T) {
 		})
 		_ = sink
 		if allocs > 1 {
-			t.Fatalf("Concat(%v) had %f allocs; expected 1", tc.s, allocs)
+			errorf := t.Errorf
+			if testenv.OptimizationOff() || race.Enabled {
+				errorf = t.Logf
+			}
+			errorf("Concat(%v) allocated %v times; want 1", tc.s, allocs)
 		}
 	}
 }