Add Pointer::Appender#to_slice (#14874)

spec/std/pointer/appender_spec.cr

@@ -25,4 +25,18 @@ describe Pointer::Appender do
     end
     appender.size.should eq 4
   end
+
+  it "#to_slice" do
+    data = Slice(Int32).new(5)
+    appender = data.to_unsafe.appender
+    appender.to_slice.should eq Slice(Int32).new(0)
+    appender.to_slice.to_unsafe.should eq data.to_unsafe
+
+    4.times do |i|
+      appender << (i + 1) * 2
+      appender.to_slice.should eq data[0, i + 1]
+    end
+    appender.to_slice.should eq Slice[2, 4, 6, 8]
+    appender.to_slice.to_unsafe.should eq data.to_unsafe
+  end
 end

src/base64.cr

@@ -163,7 +163,7 @@ module Base64
     buf = Pointer(UInt8).malloc(decode_size(slice.size))
     appender = buf.appender
     from_base64(slice) { |byte| appender << byte }
-    Slice.new(buf, appender.size.to_i32)
+    appender.to_slice
   end
 
   # Writes the base64-decoded version of *data* to *io*.

src/crystal/system/print_error.cr

@@ -23,7 +23,7 @@ module Crystal::System
     String.each_utf16_char(bytes) do |char|
       if appender.size > utf8.size - char.bytesize
         # buffer is full (char won't fit)
-        print_error utf8.to_slice[0...appender.size]
+        print_error appender.to_slice
         appender = utf8.to_unsafe.appender
       end
 
@@ -33,7 +33,7 @@ module Crystal::System
     end
 
     if appender.size > 0
-      print_error utf8.to_slice[0...appender.size]
+      print_error appender.to_slice
     end
   end
 

src/crystal/system/win32/file_descriptor.cr

@@ -438,7 +438,7 @@ private module ConsoleUtils
         appender << byte
       end
     end
-    @@buffer = @@utf8_buffer[0, appender.size]
+    @@buffer = appender.to_slice
   end
 
   private def self.read_console(handle : LibC::HANDLE, slice : Slice(UInt16)) : Int32

src/pointer.cr

@@ -52,6 +52,20 @@ struct Pointer(T)
     def pointer
       @pointer
     end
+
+    # Creates a slice pointing at the values appended by this instance.
+    #
+    # ```
+    # slice = Slice(Int32).new(5)
+    # appender = slice.to_unsafe.appender
+    # appender << 1
+    # appender << 2
+    # appender << 3
+    # appender.to_slice # => Slice[1, 2, 3]
+    # ```
+    def to_slice : Slice(T)
+      @start.to_slice(size)
+    end
   end
 
   include Comparable(self)