diff --git a/stdlib/src/builtin/string_literal.mojo b/stdlib/src/builtin/string_literal.mojo
index c5af9cef8d..5917daf5cb 100644
--- a/stdlib/src/builtin/string_literal.mojo
+++ b/stdlib/src/builtin/string_literal.mojo
@@ -747,10 +747,10 @@ struct StringLiteral(
         """
         return str(self).split(sep, maxsplit)
 
-    fn splitlines(self, keepends: Bool = False) -> List[String]:
-        """Split the string literal at line boundaries. This corresponds to Python's
-        [universal newlines:](
-            https://docs.python.org/3/library/stdtypes.html#str.splitlines)
+    fn splitlines(self, keepends: Bool = False) -> List[StaticString]:
+        """Split the string literal at line boundaries. This corresponds to
+        Python's [universal newlines:](
+        https://docs.python.org/3/library/stdtypes.html#str.splitlines)
         `"\\r\\n"` and `"\\t\\n\\v\\f\\r\\x1c\\x1d\\x1e\\x85\\u2028\\u2029"`.
 
         Args:
@@ -759,7 +759,7 @@ struct StringLiteral(
         Returns:
             A List of Strings containing the input split by line boundaries.
         """
-        return _to_string_list(self.as_string_slice().splitlines(keepends))
+        return self.as_string_slice().splitlines(keepends)
 
     fn count(self, substr: String) -> Int:
         """Return the number of non-overlapping occurrences of substring
diff --git a/stdlib/src/collections/string/string.mojo b/stdlib/src/collections/string/string.mojo
index 7cbbd2a884..56e34c9b9d 100644
--- a/stdlib/src/collections/string/string.mojo
+++ b/stdlib/src/collections/string/string.mojo
@@ -1694,10 +1694,12 @@ struct String(
 
         return output
 
-    fn splitlines(self, keepends: Bool = False) -> List[String]:
+    fn splitlines(
+        ref self, keepends: Bool = False
+    ) -> List[StringSlice[__origin_of(self)]]:
         """Split the string at line boundaries. This corresponds to Python's
         [universal newlines:](
-            https://docs.python.org/3/library/stdtypes.html#str.splitlines)
+        https://docs.python.org/3/library/stdtypes.html#str.splitlines)
         `"\\r\\n"` and `"\\t\\n\\v\\f\\r\\x1c\\x1d\\x1e\\x85\\u2028\\u2029"`.
 
         Args:
@@ -1706,7 +1708,7 @@ struct String(
         Returns:
             A List of Strings containing the input split by line boundaries.
         """
-        return _to_string_list(self.as_string_slice().splitlines(keepends))
+        return self.as_string_slice().splitlines(keepends)
 
     fn replace(self, old: String, new: String) -> String:
         """Return a copy of the string with all occurrences of substring `old`
diff --git a/stdlib/src/collections/string/string_slice.mojo b/stdlib/src/collections/string/string_slice.mojo
index 977f5b7f76..9e2d738215 100644
--- a/stdlib/src/collections/string/string_slice.mojo
+++ b/stdlib/src/collections/string/string_slice.mojo
@@ -1111,29 +1111,23 @@ struct StringSlice[mut: Bool, //, origin: Origin[mut]](
                 offset += b_len
             return length != 0
 
-    fn splitlines[
-        O: ImmutableOrigin, //
-    ](self: StringSlice[O], keepends: Bool = False) -> List[StringSlice[O]]:
+    fn splitlines(self, keepends: Bool = False) -> List[Self]:
         """Split the string at line boundaries. This corresponds to Python's
         [universal newlines:](
         https://docs.python.org/3/library/stdtypes.html#str.splitlines)
         `"\\r\\n"` and `"\\t\\n\\v\\f\\r\\x1c\\x1d\\x1e\\x85\\u2028\\u2029"`.
 
-        Parameters:
-            O: The immutable origin.
-
         Args:
             keepends: If True, line breaks are kept in the resulting strings.
 
         Returns:
             A List of Strings containing the input split by line boundaries.
         """
-
         # highly performance sensitive code, benchmark before touching
         alias `\r` = UInt8(ord("\r"))
         alias `\n` = UInt8(ord("\n"))
 
-        output = List[StringSlice[O]](capacity=128)  # guessing
+        output = List[Self](capacity=128)  # guessing
         var ptr = self.unsafe_ptr()
         var length = self.byte_length()
         var offset = 0
@@ -1163,7 +1157,7 @@ struct StringSlice[mut: Bool, //, origin: Origin[mut]](
                 eol_start += char_len
 
             var str_len = eol_start - offset + int(keepends) * eol_length
-            var s = StringSlice[O](ptr=ptr + offset, length=str_len)
+            var s = Self(ptr=ptr + offset, length=str_len)
             output.append(s)
             offset = eol_start + eol_length
 
@@ -1209,29 +1203,30 @@ fn _to_string_list[
     len_fn: fn (T) -> Int,
     unsafe_ptr_fn: fn (T) -> UnsafePointer[Byte],
 ](items: List[T]) -> List[String]:
-    i_len = len(items)
-    i_ptr = items.unsafe_ptr()
-    out_ptr = UnsafePointer[String].alloc(i_len)
+    var i_len = len(items)
+    var i_ptr = items.unsafe_ptr()
+    var out_ptr = UnsafePointer[String].alloc(i_len)
 
     for i in range(i_len):
-        og_len = len_fn(i_ptr[i])
-        f_len = og_len + 1  # null terminator
-        p = UnsafePointer[Byte].alloc(f_len)
-        og_ptr = unsafe_ptr_fn(i_ptr[i])
+        var og_len = len_fn(i_ptr[i])
+        var f_len = og_len + 1  # null terminator
+        var p = UnsafePointer[Byte].alloc(f_len)
+        var og_ptr = unsafe_ptr_fn(i_ptr[i])
         memcpy(p, og_ptr, og_len)
         p[og_len] = 0  # null terminator
-        buf = String._buffer_type(ptr=p, length=f_len, capacity=f_len)
+        var buf = String._buffer_type(ptr=p, length=f_len, capacity=f_len)
         (out_ptr + i).init_pointee_move(String(buf^))
     return List[String](ptr=out_ptr, length=i_len, capacity=i_len)
 
 
 @always_inline
-fn _to_string_list[
-    O: ImmutableOrigin, //
+fn to_string_list[
+    mut: Bool, O: Origin[mut], //
 ](items: List[StringSlice[O]]) -> List[String]:
     """Create a list of Strings **copying** the existing data.
 
     Parameters:
+        mut: The mutability of the origin.
         O: The origin of the data.
 
     Args:
@@ -1251,12 +1246,13 @@ fn _to_string_list[
 
 
 @always_inline
-fn _to_string_list[
-    O: ImmutableOrigin, //
+fn to_string_list[
+    mut: Bool, O: Origin[mut], //
 ](items: List[Span[Byte, O]]) -> List[String]:
     """Create a list of Strings **copying** the existing data.
 
     Parameters:
+        mut: The mutability of the origin.
         O: The origin of the data.
 
     Args:
diff --git a/stdlib/test/builtin/test_string_literal.mojo b/stdlib/test/builtin/test_string_literal.mojo
index c549a9423c..5d6d08ad05 100644
--- a/stdlib/test/builtin/test_string_literal.mojo
+++ b/stdlib/test/builtin/test_string_literal.mojo
@@ -22,6 +22,7 @@ from testing import (
     assert_raises,
     assert_true,
 )
+from collections.string.string_slice import StringSlice, to_string_list
 
 
 def test_add():
@@ -442,7 +443,8 @@ def test_split():
 
 
 def test_splitlines():
-    alias L = List[String]
+    alias L = List[StringSlice[StaticConstantOrigin]]
+
     # Test with no line breaks
     assert_equal("hello world".splitlines(), L("hello world"))
 
@@ -478,6 +480,22 @@ def test_splitlines():
         L("hello\x1c", "world\x1d", "mojo\x1e", "language\x1e"),
     )
 
+    # test \x85 \u2028 \u2029
+    var next_line = String(List[UInt8](0xC2, 0x85, 0))
+    """TODO: \\x85"""
+    var unicode_line_sep = String(List[UInt8](0xE2, 0x80, 0xA8, 0))
+    """TODO: \\u2028"""
+    var unicode_paragraph_sep = String(List[UInt8](0xE2, 0x80, 0xA9, 0))
+    """TODO: \\u2029"""
+
+    for i in List(next_line, unicode_line_sep, unicode_paragraph_sep):
+        u = i[]
+        item = String("").join("hello", u, "world", u, "mojo", u, "language", u)
+        s = StringSlice(item)
+        assert_equal(s.splitlines(), hello_mojo)
+        items = List("hello" + u, "world" + u, "mojo" + u, "language" + u)
+        assert_equal(to_string_list(s.splitlines(keepends=True)), items)
+
 
 def test_float_conversion():
     assert_equal(("4.5").__float__(), 4.5)
diff --git a/stdlib/test/collections/string/test_string.mojo b/stdlib/test/collections/string/test_string.mojo
index 53f9f6ace8..9f83fc3ef6 100644
--- a/stdlib/test/collections/string/test_string.mojo
+++ b/stdlib/test/collections/string/test_string.mojo
@@ -20,12 +20,12 @@ from testing import (
     assert_true,
 )
 
-from collections.string import StringSlice
 from collections.string.string import (
     _calc_initial_buffer_size_int32,
     _calc_initial_buffer_size_int64,
     _isspace,
 )
+from collections.string.string_slice import StringSlice, to_string_list
 from memory import UnsafePointer
 from python import Python
 from utils import StringRef
@@ -755,17 +755,18 @@ def test_split():
 
 
 def test_splitlines():
-    alias L = List[String]
+    alias L = List[StringSlice[StaticConstantOrigin]]
+
     # Test with no line breaks
-    assert_equal(String("hello world").splitlines(), L("hello world"))
+    assert_equal("hello world".splitlines(), L("hello world"))
 
     # Test with line breaks
-    assert_equal(String("hello\nworld").splitlines(), L("hello", "world"))
-    assert_equal(String("hello\rworld").splitlines(), L("hello", "world"))
-    assert_equal(String("hello\r\nworld").splitlines(), L("hello", "world"))
+    assert_equal("hello\nworld".splitlines(), L("hello", "world"))
+    assert_equal("hello\rworld".splitlines(), L("hello", "world"))
+    assert_equal("hello\r\nworld".splitlines(), L("hello", "world"))
 
     # Test with multiple different line breaks
-    s1 = String("hello\nworld\r\nmojo\rlanguage\r\n")
+    s1 = "hello\nworld\r\nmojo\rlanguage\r\n"
     hello_mojo = L("hello", "world", "mojo", "language")
     assert_equal(s1.splitlines(), hello_mojo)
     assert_equal(
@@ -774,9 +775,9 @@ def test_splitlines():
     )
 
     # Test with an empty string
-    assert_equal(String("").splitlines(), L())
+    assert_equal("".splitlines(), L())
     # test \v \f \x1c \x1d
-    s2 = String("hello\vworld\fmojo\x1clanguage\x1d")
+    s2 = "hello\vworld\fmojo\x1clanguage\x1d"
     assert_equal(s2.splitlines(), hello_mojo)
     assert_equal(
         s2.splitlines(keepends=True),
@@ -784,7 +785,7 @@ def test_splitlines():
     )
 
     # test \x1c \x1d \x1e
-    s3 = String("hello\x1cworld\x1dmojo\x1elanguage\x1e")
+    s3 = "hello\x1cworld\x1dmojo\x1elanguage\x1e"
     assert_equal(s3.splitlines(), hello_mojo)
     assert_equal(
         s3.splitlines(keepends=True),
@@ -792,21 +793,20 @@ def test_splitlines():
     )
 
     # test \x85 \u2028 \u2029
-    var next_line = List[UInt8](0xC2, 0x85, 0)
+    var next_line = String(List[UInt8](0xC2, 0x85, 0))
     """TODO: \\x85"""
-    var unicode_line_sep = List[UInt8](0xE2, 0x80, 0xA8, 0)
+    var unicode_line_sep = String(List[UInt8](0xE2, 0x80, 0xA8, 0))
     """TODO: \\u2028"""
-    var unicode_paragraph_sep = List[UInt8](0xE2, 0x80, 0xA9, 0)
+    var unicode_paragraph_sep = String(List[UInt8](0xE2, 0x80, 0xA9, 0))
     """TODO: \\u2029"""
 
     for i in List(next_line, unicode_line_sep, unicode_paragraph_sep):
-        u = String(i[])
+        u = i[]
         item = String("").join("hello", u, "world", u, "mojo", u, "language", u)
-        assert_equal(item.splitlines(), hello_mojo)
-        assert_equal(
-            item.splitlines(keepends=True),
-            L("hello" + u, "world" + u, "mojo" + u, "language" + u),
-        )
+        s = StringSlice(item)
+        assert_equal(s.splitlines(), hello_mojo)
+        items = List("hello" + u, "world" + u, "mojo" + u, "language" + u)
+        assert_equal(to_string_list(s.splitlines(keepends=True)), items)
 
 
 def test_isupper():
diff --git a/stdlib/test/collections/string/test_string_slice.mojo b/stdlib/test/collections/string/test_string_slice.mojo
index 32b6da2bf9..d183d51c10 100644
--- a/stdlib/test/collections/string/test_string_slice.mojo
+++ b/stdlib/test/collections/string/test_string_slice.mojo
@@ -17,6 +17,7 @@ from testing import assert_equal, assert_false, assert_true, assert_raises
 from collections.string.string_slice import (
     StringSlice,
     _count_utf8_continuation_bytes,
+    to_string_list,
 )
 from collections.string._utf8_validation import _is_valid_utf8
 from memory import Span, UnsafePointer
@@ -480,27 +481,18 @@ def test_count_utf8_continuation_bytes():
 
 
 def test_splitlines():
-    alias S = StringSlice[StaticConstantOrigin]
     alias L = List[StringSlice[StaticConstantOrigin]]
 
-    # FIXME: remove once StringSlice conforms to TestableCollectionElement
-    fn _assert_equal[
-        O1: ImmutableOrigin
-    ](l1: List[StringSlice[O1]], l2: List[String]) raises:
-        assert_equal(len(l1), len(l2))
-        for i in range(len(l1)):
-            assert_equal(str(l1[i]), l2[i])
-
     # Test with no line breaks
-    assert_equal(S("hello world").splitlines(), L("hello world"))
+    assert_equal("hello world".splitlines(), L("hello world"))
 
     # Test with line breaks
-    assert_equal(S("hello\nworld").splitlines(), L("hello", "world"))
-    assert_equal(S("hello\rworld").splitlines(), L("hello", "world"))
-    assert_equal(S("hello\r\nworld").splitlines(), L("hello", "world"))
+    assert_equal("hello\nworld".splitlines(), L("hello", "world"))
+    assert_equal("hello\rworld".splitlines(), L("hello", "world"))
+    assert_equal("hello\r\nworld".splitlines(), L("hello", "world"))
 
     # Test with multiple different line breaks
-    s1 = S("hello\nworld\r\nmojo\rlanguage\r\n")
+    s1 = "hello\nworld\r\nmojo\rlanguage\r\n"
     hello_mojo = L("hello", "world", "mojo", "language")
     assert_equal(s1.splitlines(), hello_mojo)
     assert_equal(
@@ -509,9 +501,9 @@ def test_splitlines():
     )
 
     # Test with an empty string
-    assert_equal(S("").splitlines(), L())
+    assert_equal("".splitlines(), L())
     # test \v \f \x1c \x1d
-    s2 = S("hello\vworld\fmojo\x1clanguage\x1d")
+    s2 = "hello\vworld\fmojo\x1clanguage\x1d"
     assert_equal(s2.splitlines(), hello_mojo)
     assert_equal(
         s2.splitlines(keepends=True),
@@ -519,7 +511,7 @@ def test_splitlines():
     )
 
     # test \x1c \x1d \x1e
-    s3 = S("hello\x1cworld\x1dmojo\x1elanguage\x1e")
+    s3 = "hello\x1cworld\x1dmojo\x1elanguage\x1e"
     assert_equal(s3.splitlines(), hello_mojo)
     assert_equal(
         s3.splitlines(keepends=True),
@@ -540,7 +532,7 @@ def test_splitlines():
         s = StringSlice(item)
         assert_equal(s.splitlines(), hello_mojo)
         items = List("hello" + u, "world" + u, "mojo" + u, "language" + u)
-        _assert_equal(s.splitlines(keepends=True), items)
+        assert_equal(to_string_list(s.splitlines(keepends=True)), items)
 
 
 def test_rstrip():