perf(pool): use predefined state transition slices

Eliminate dynamic slice allocations on hot paths by using predefined global slices for common state transitions.

Author: ndyakov

go.mod

@@ -1,6 +1,6 @@
 module github.com/redis/go-redis/v9
 
-go 1.18
+go 1.19
 
 require (
 	github.com/bsm/ginkgo/v2 v2.12.0

internal/pool/conn_state.go

@@ -41,6 +41,20 @@ const (
 	StateClosed
 )
 
+// Predefined state transition slices to avoid allocations on hot paths.
+// These are used by TryTransition and AwaitAndTransition to specify valid source states.
+var (
+	// Common single-state transitions
+	validFromIdle  = []ConnState{StateIdle}
+	validFromInUse = []ConnState{StateInUse}
+
+	// Common multi-state transitions
+	validFromCreatedOrIdle          = []ConnState{StateCreated, StateIdle}
+	validFromInitializingOrUnusable = []ConnState{StateInitializing, StateUnusable}
+	validFromCreatedIdleOrUnusable  = []ConnState{StateCreated, StateIdle, StateUnusable}
+	validFromInUseIdleOrCreated     = []ConnState{StateInUse, StateIdle, StateCreated}
+)
+
 // String returns a human-readable string representation of the state.
 func (s ConnState) String() string {
 	switch s {