Description
In #47649 @rsc proposed a new maps package in the standard library based on discussion in #47330. After we settled on an API we added the experimental package golang.org/x/exp/maps.
We now have experience with this package in practice. pkg.go.dev reports that it is imported by 615 other packages. I propose that it is now time to move this package into the standard library for the 1.21 release.
Doing this doesn't mean that we can't add more functions to the standard library maps package later. Any such additions should be suggested in their own separate proposal. (There are some existing proposals that would move from x/exp/maps to maps: at least #54012, #54454, #57191).
The specific API that we will put into the standard library is as follows.
// Keys returns the keys of the map m.
// The keys will be in an indeterminate order.
func Keys[M ~map[K]V, K comparable, V any](m M) []K
// Values returns the values of the map m.
// The values will be in an indeterminate order.
func Values[M ~map[K]V, K comparable, V any](m M) []V
// Equal reports whether two maps contain the same key/value pairs.
// Values are compared using ==.
func Equal[M1, M2 ~map[K]V, K, V comparable](m1 M1, m2 M2) bool
// EqualFunc is like Equal, but compares values using eq.
// Keys are still compared with ==.
func EqualFunc[M1 ~map[K]V1, M2 ~map[K]V2, K comparable, V1, V2 any](m1 M1, m2 M2, eq func(V1, V2) bool) bool
// Clone returns a copy of m. This is a shallow clone:
// the new keys and values are set using ordinary assignment.
func Clone[M ~map[K]V, K comparable, V any](m M) M
// Copy copies all key/value pairs in src adding them to dst.
// When a key in src is already present in dst,
// the value in dst will be overwritten by the value associated
// with the key in src.
func Copy[M1 ~map[K]V, M2 ~map[K]V, K comparable, V any](dst M1, src M2)
// DeleteFunc deletes any key/value pairs from m for which del returns true.
func DeleteFunc[M ~map[K]V, K comparable, V any](m M, del func(K, V) bool)