Unity3D Editor script that adds some MenuItem to move the corners of a RectTransform to its anchors.

MoveCornersToAnchors.cs

using System.Collections;
using System.Collections.Generic;
using UnityEditor;
using UnityEngine;

public static class CornersToAnchorsMenuItems
{

    // Hotkey: Alt+C
    [MenuItem("Tools/Corners to Anchors (RectTransform)/Width and Height and Position &c")]
    public static void MoveCornersToAnchors_WidthAndHeightAndPosition()
    {
        EditorUtils.GetSelectedComponents<RectTransform>().ForEach(it =>
        {
            Undo.RecordObject(it, "MoveCornersToAnchors_WidthAndHeightAndPosition");
            MoveCornersToAnchorsExtensions.MoveCornersToAnchors_Width(it);
            MoveCornersToAnchorsExtensions.MoveCornersToAnchors_Height(it);
            MoveCornersToAnchorsExtensions.MoveCornersToAnchors_CenterPosition(it);
        });
    }

    [MenuItem("Tools/Corners to Anchors (RectTransform)/Width and Height")]
    public static void MoveCornersToAnchors_WidthAndHeight()
    {
        EditorUtils.GetSelectedComponents<RectTransform>().ForEach(it =>
        {
            Undo.RecordObject(it, "MoveCornersToAnchors_WidthAndHeight");
            MoveCornersToAnchorsExtensions.MoveCornersToAnchors_Width(it);
            MoveCornersToAnchorsExtensions.MoveCornersToAnchors_Height(it);
        });
    }

    [MenuItem("Tools/Corners to Anchors (RectTransform)/Width")]
    public static void MoveCornersToAnchors_Width()
    {
        EditorUtils.GetSelectedComponents<RectTransform>().ForEach(it =>
        {
            Undo.RecordObject(it, "MoveCornersToAnchors_Width");
            MoveCornersToAnchorsExtensions.MoveCornersToAnchors_Width(it);
        });
    }

    [MenuItem("Tools/Corners to Anchors (RectTransform)/Width and Center Horizontal")]
    public static void MoveCornersToAnchors_WidthAndCenterHorizontal()
    {
        EditorUtils.GetSelectedComponents<RectTransform>().ForEach(it =>
        {
            Undo.RecordObject(it, "MoveCornersToAnchors_WidthAndCenterHorizontal");
            MoveCornersToAnchorsExtensions.MoveCornersToAnchors_Width(it);
            it.anchoredPosition = new Vector2(0, it.anchoredPosition.y);
        });
    }

    [MenuItem("Tools/Corners to Anchors (RectTransform)/Height")]
    public static void MoveCornersToAnchors_Height()
    {
        EditorUtils.GetSelectedComponents<RectTransform>().ForEach(it =>
        {
            Undo.RecordObject(it, "MoveCornersToAnchors_Height");
            MoveCornersToAnchorsExtensions.MoveCornersToAnchors_Height(it);
        });
    }

    [MenuItem("Tools/Corners to Anchors (RectTransform)/Height and Center Vertical")]
    public static void MoveCornersToAnchors_HeightAndCenterVertical()
    {
        EditorUtils.GetSelectedComponents<RectTransform>().ForEach(it =>
        {
            Undo.RecordObject(it, "MoveCornersToAnchors_HeightAndCenterVertical");
            MoveCornersToAnchorsExtensions.MoveCornersToAnchors_Height(it);
            it.anchoredPosition = new Vector2(it.anchoredPosition.x, 0);
        });
    }

    [MenuItem("Tools/Corners to Anchors (RectTransform)/Center Horizontal and Vertical")]
    public static void MoveCornersToAnchors_CenterPosition()
    {
        EditorUtils.GetSelectedComponents<RectTransform>().ForEach(it =>
        {
            Undo.RecordObject(it, "MoveCornersToAnchors_CenterPosition");
            MoveCornersToAnchorsExtensions.MoveCornersToAnchors_CenterPosition(it);
        });
    }

    [MenuItem("Tools/Corners to Anchors (RectTransform)/Center Horizontal")]
    public static void MoveCornersToAnchors_CenterHorizontal()
    {
        EditorUtils.GetSelectedComponents<RectTransform>().ForEach(it =>
        {
            Undo.RecordObject(it, "MoveCornersToAnchors_CenterHorizontal");
            it.anchoredPosition = new Vector2(0, it.anchoredPosition.y);
        });
    }

    [MenuItem("Tools/Corners to Anchors (RectTransform)/Center Vertical")]
    public static void MoveCornersToAnchors_CenterVertical()
    {
        EditorUtils.GetSelectedComponents<RectTransform>().ForEach(it =>
        {
            Undo.RecordObject(it, "MoveCornersToAnchors_CenterVertical");
            it.anchoredPosition = new Vector2(it.anchoredPosition.x, 0);
        });
    }
}

public static class MoveCornersToAnchorsExtensions
{
    public static void MoveCornersToAnchors_Width(this RectTransform rectTransform)
    {
        rectTransform.sizeDelta = new Vector2(0, rectTransform.sizeDelta.y);
    }

    public static void MoveCornersToAnchors_Height(this RectTransform rectTransform)
    {
        rectTransform.sizeDelta = new Vector2(rectTransform.sizeDelta.x, 0);
    }

    public static void MoveCornersToAnchors_CenterPosition(this RectTransform rectTransform)
    {
        rectTransform.anchoredPosition = Vector2.zero;
    }
}

public class EditorUtils
{
    public static List<T> GetSelectedComponents<T>()
    {
        List<T> result = new List<T>();

        GameObject[] activeGameObjects = Selection.gameObjects;
        if (activeGameObjects == null || activeGameObjects.Length == 0)
        {
            return result;
        }

        foreach (GameObject gameObject in activeGameObjects)
        {
            T rectTransform = gameObject.GetComponent<T>();
            if (rectTransform != null)
            {
                result.Add(rectTransform);
            }
        }
        return result;
    }
}