tree e78978c65a92afe2751a82d9014207b903619e12
parent bbfaeb77e598b393018426487e5c0ae13a5f6c2b
author Joe Tsai <joetsai@digital-static.net> 1539802472 -0700
committer Joe Tsai <thebrokentoaster@gmail.com> 1539804639 +0000

reflect/protoreflect: remove List method

Remove List from KnownFields, UnknownFields, ExtensionFieldTypes, and Map.

Rationale:
* Each of those interfaces already have a Range method, which provides
a superset of the functionality of List. Furthermore, Range is more expressive
as it allows you to terminate iteration and provides both keys and values.
* List must always allocate a slice and populates it.
* Range is allocation free in some cases. For example, if you simply wanted to
iterate over all the populated fields to clear them, there is no need for a
closure, so a static version of the function can be directly referenced
(i.e., there is no need to create a stub function header that references the
closed-over variables).
* In the cases where a closure is needed, the allocation cost is O(1) since
there are a finite number of variables being closed over.
* In highly performance sensitive cases, the closured function could close over
a struct, such that the function and struct are stored in a sync.Pool when not
in use. For example:

	type MapLister struct {
		Entries []struct{K MapKey; V Value}
		f       func(MapKey, Value) true
	}
	func (m *MapLister) Ranger() func(MapKey, Value) bool {
		if m.f != nil {
			m.f = func(k MapKey, v Value) bool {
				m.Entries = append(m.Entries, ...)
				return true
			}
		}
		m.Entries = m.Entries[:0]
		return m.f
	}

The main benefit of List is the ease of use:

	for _, num := range knownFields.List() {
		...
	}

as opposed to:

	knownFields.Range(func(n FieldNumber, v Value) bool {
		...
		return true
	})

However, this is a marginal benefit.
Thus, remove List as it mostly inferior to Range.

Change-Id: I25586c6ea07a4706072ba06b1cf25cb6efb5e8a7
Reviewed-on: https://go-review.googlesource.com/c/142888
Reviewed-by: Damien Neil <dneil@google.com>