[External] [stdlib] Rename List.size to List._len and refactor us…

…age of the field to use the public API (#56692)

[External] [stdlib] Rename `List.size` to `List._len`

Make the data member `size` of `List` private by prefixing with an `_`.
Rename it to `len` while we're here.  Adjust call sites to use public APIs
where possible.

Details:
- Part of #3511
- To prepare for any future implementation details than can change under
the hood for `List`.

Co-authored-by: martinvuyk <110240700+martinvuyk@users.noreply.github.com>
Closes #3814
MODULAR_ORIG_COMMIT_REV_ID: c2ac41d4c543b01619814ce0cd58fd8ba15ac9e9

mojo/docs/changelog.md

@@ -215,4 +215,40 @@ ctx.enqueue_function(compiled_func, grid_dim=1, block_dim=1)
 
 ### ❌ Removed
 
+- Direct access to `List.size` has been removed. Use the public API instead.
+
+  Examples:
+
+  Extending a List:
+
+  ```mojo
+  base_data = List[Byte](1, 2, 3)
+
+  data_list = List[Byte](4, 5, 6)
+  ext_data_list = base_data.copy()
+  ext_data_list.extend(data_list) # [1, 2, 3, 4, 5, 6]
+
+  data_span = Span(List[Byte](4, 5, 6))
+  ext_data_span = base_data.copy()
+  ext_data_span.extend(data_span) # [1, 2, 3, 4, 5, 6]
+
+  data_vec = SIMD[DType.uint8, 4](4, 5, 6, 7)
+  ext_data_vec_full = base_data.copy()
+  ext_data_vec_full.extend(data_vec) # [1, 2, 3, 4, 5, 6, 7]
+
+  ext_data_vec_partial = base_data.copy()
+  ext_data_vec_partial.extend(data_vec, count=3) # [1, 2, 3, 4, 5, 6]
+  ```
+
+  Slicing and extending a list efficiently:
+
+  ```mojo
+  base_data = List[Byte](1, 2, 3, 4, 5, 6)
+  n4_n5 = Span(base_data)[3:5]
+  extra_data = Span(List[Byte](8, 10))
+  end_result = List[Byte](capacity=len(n4_n5) + len(extra_data))
+  end_result.extend(n4_n5)
+  end_result.extend(extra_data) # [4, 5, 8, 10]
+  ```
+
 ### 🛠️ Fixed

mojo/stdlib/src/builtin/file.mojo

@@ -129,7 +129,7 @@ struct FileHandle:
         """Reads data from a file and sets the file handle seek position. If
         size is left as the default of -1, it will read to the end of the file.
         Setting size to a number larger than what's in the file will set
-        String.size to the total number of bytes, and read all the data.
+        the String length to the total number of bytes, and read all the data.
 
         Args:
             size: Requested number of bytes to read (Default: -1 = EOF).
@@ -273,7 +273,7 @@ struct FileHandle:
         """Reads data from a file and sets the file handle seek position. If
         size is left as default of -1, it will read to the end of the file.
         Setting size to a number larger than what's in the file will be handled
-        and set the List.size to the total number of bytes in the file.
+        and set the List length to the total number of bytes in the file.
 
         Args:
             size: Requested number of bytes to read (Default: -1 = EOF).

mojo/stdlib/src/builtin/string_literal.mojo

@@ -411,17 +411,10 @@ struct StringLiteral(
         # inline the string slice constructor to work around an elaborator
         # memory leak.
         # return self.as_string_slice()
-        var string = String()
-        var length = self.byte_length()
-        var buffer = String._buffer_type()
-        var new_capacity = length + 1
-        buffer._realloc(new_capacity)
-        buffer.size = new_capacity
-        var data: UnsafePointer[UInt8] = self.unsafe_ptr()
-        memcpy(buffer.data, data, length)
-        (buffer.data + length).init_pointee_move(0)
-        string._buffer = buffer^
-        return string
+        var buffer = String._buffer_type(capacity=self.byte_length() + 1)
+        buffer.extend(self.as_bytes())
+        buffer.append(0)
+        return String(buffer=buffer^)
 
     @no_inline
     fn __repr__(self) -> String: