Pre-compute String length after #chomp() if possible

spec/std/string_spec.cr

@@ -773,6 +773,24 @@ describe "String" do
     it { "hello\n\n\n\n".chomp("").should eq("hello\n\n\n\n") }
 
     it { "hello\r\n".chomp("\n").should eq("hello") }
+
+    it "pre-computes string size if possible" do
+      {"!hello!", "\u{1f602}hello\u{1f602}", "\xFEhello\xFF"}.each do |str|
+        {"", "\n", "\r", "\r\n"}.each do |newline|
+          x = str + newline
+          x.size # side-effect
+          y = x.chomp
+          y.@length.should eq(7)
+        end
+      end
+    end
+
+    it "does not pre-compute string size if not possible" do
+      x = String.build &.<< "abc\n"
+      x.@length.should eq(0)
+      y = x.chomp
+      y.@length.should eq(0)
+    end
   end
 
   describe "lchop" do

src/string.cr

@@ -1661,12 +1661,12 @@ class String
     case to_unsafe[bytesize - 1]
     when '\n'
       if bytesize > 1 && to_unsafe[bytesize - 2] === '\r'
-        unsafe_byte_slice_string(0, bytesize - 2)
+        unsafe_byte_slice_string(0, bytesize - 2, @length > 0 ? @length - 2 : 0)
       else
-        unsafe_byte_slice_string(0, bytesize - 1)
+        unsafe_byte_slice_string(0, bytesize - 1, @length > 0 ? @length - 1 : 0)
       end
     when '\r'
-      unsafe_byte_slice_string(0, bytesize - 1)
+      unsafe_byte_slice_string(0, bytesize - 1, @length > 0 ? @length - 1 : 0)
     else
       self
     end
@@ -5552,12 +5552,12 @@ class String
     Slice.new(to_unsafe + byte_offset, bytesize - byte_offset, read_only: true)
   end
 
-  protected def unsafe_byte_slice_string(byte_offset)
-    String.new(unsafe_byte_slice(byte_offset))
+  protected def unsafe_byte_slice_string(byte_offset, *, size = 0)
+    String.new(to_unsafe + byte_offset, bytesize - byte_offset, size)
   end
 
-  protected def unsafe_byte_slice_string(byte_offset, count)
-    String.new(unsafe_byte_slice(byte_offset, count))
+  protected def unsafe_byte_slice_string(byte_offset, count, size = 0)
+    String.new(to_unsafe + byte_offset, count, size)
   end
 
   protected def self.char_bytes_and_bytesize(char : Char)