Add performance note on local scoped variants slice

internal/trie/node/header.go

@@ -129,6 +129,9 @@ func decodeHeaderByte(header byte) (variantBits,
 	// order by the number of bits each variant mask occupy
 	// in the header byte.
 	// See https://spec.polkadot.network/#defn-node-header
+	// Performance note: see `Benchmark_decodeHeaderByte`;
+	// running with a locally scoped slice is as fast as having
+	// it at global scope.
 	variants := []variant{
 		leafVariant,            // mask 1100_0000
 		branchVariant,          // mask 1100_0000

internal/trie/node/header_test.go

@@ -418,3 +418,15 @@ func Test_decodeHeaderByte(t *testing.T) {
 		})
 	}
 }
+
+func Benchmark_decodeHeaderByte(b *testing.B) {
+	// With global scoped variants slice:
+	// 3.453 ns/op	       0 B/op	       0 allocs/op
+	// With locally scoped variants slice:
+	// 3.441 ns/op	       0 B/op	       0 allocs/op
+	header := leafVariant.bits | 0b0000_0001
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		_, _, _, _ = decodeHeaderByte(header)
+	}
+}