Let me investigate this bug. The issue is about the `literalinclude` directive in Sphinx where `:prepend:` and `:append:` options strip leading whitespace, making it impossible to properly indent prepended/appended content to match the included code. Let me look at the relevant Sphinx source code to find where `literalinclude` is handled.

Let me look at the code handling literalinclude more specifically, especially the code.py post transform.

Now let me look at the key file: `/testbed/sphinx/directives/code.py`. This contains the `dedent_lines` function, `prepend_filter`, `append_filter`, and `dedent_filter` methods. The issue is about the interaction between `dedent` and `prepend`/`append`. Let me read the full file to understand the context.

Now I understand the issue. Let me trace through the logic: 1. The `LiteralIncludeReader.read()` method applies filters in this order: - `pyobject_filter` - `start_filter` - `end_filter` - `lines_filter` - `prepend_filter` - `append_filter` - `dedent_filter` 2. The `dedent_filter` is applied **after** `prepend_filter` and `append_filter`, meaning that if `dedent` is specified, it removes leading whitespace from BOTH the included lines AND the prepended/appended lines. 3. But the actual issue is about the `prepend_filter` and `append_filter` themselves. The string passed to `:prepend:` and `:append:` is taken literally (via `directives.unchanged_required`). The user's complaint is about leading whitespace being removed from `:prepend:`/`:append:` values. Wait, let me re-read the bug report more carefully. The user says: ``` .. literalinclude:: pom.xml :language: xml :prepend: </plugin> :start-at: <groupId>com.github.ekryd.sortpom</groupId> :end-at: </plugin> ``` The `:prepend:` value here has leading spaces: ` </plugin>`. But the user expects these spaces to be preserved. However, looking at the code: ```python def prepend_filter(self, lines, location=None): prepend = self.options.get('prepend') if prepend: lines.insert(0, prepend + '\n') return lines ``` It seems like the prepend value should be preserved as-is. But wait - let me check what `directives.unchanged_required` does. In docutils, `unchanged_required` returns the text unchanged... or does it strip whitespace? Actually, in RST, directive options are parsed from the directive content. Let me check if docutils strips leading/trailing whitespace from option values. The RST parser typically strips whitespace from option values. So when the user writes: ``` :prepend: </plugin> ``` The `:prepend:` option value would be `</plugin>` (without the leading spaces) because docutils strips leading/trailing whitespace. But wait, the bug report says "leading whitespace is removed." The user expects leading spaces to be preserved so that the prepended line matches the indentation of the included code. Now let me think about the `dedent` workaround mentioned. The user says: ``` .. literalinclude:: pom.xml :language: xml :start-at: <groupId>com.github.ekryd.sortpom</groupId> :end-before: </plugin> :prepend: _____</plugin> :dedent: 5 ``` This produces a warning "non-whitespace stripped by dedent" because the `dedent` is also applied to the prepended line, and the underscores (used as fake leading whitespace) are non-whitespace characters. The user suggests: "Use of `dedent` could be a good solution, if `dedent` was applied only to the literalinclude and not to the `prepend` and `append` content." So the issue is that `dedent_filter` is applied AFTER `prepend_filter` and `append_filter`, meaning dedent also affects the prepended and appended lines. Now let me think about what the fix should be. Looking at the order of filters: ```python filters = [self.pyobject_filter, self.start_filter, self.end_filter, self.lines_filter, self.prepend_filter, self.append_filter, self.dedent_filter] ``` The `dedent_filter` is applied last, after `prepend_filter` and `append_filter`. This means when `:dedent:` is used, it also dedents the prepended and appended content, which causes the "non-whitespace stripped by dedent" warning and incorrect behavior. The fix should be to apply `dedent_filter` BEFORE `prepend_filter` and `append_filter`. That way: 1. The included lines get dedented first 2. Then prepend/append lines are added (without being affected by dedent) This would allow the user to: 1. Dedent the included lines to remove their leading indentation 2. Add properly-indented prepend/append lines But wait, that would also mean the user's first use case (without `dedent`) would still not work. The first case is that `:prepend:` s ... [truncated 4,132 characters]

Let me look at how docutils parses directive options to understand if leading whitespace is stripped from option values:

So `unchanged_required` returns the argument unchanged. But the question is whether docutils strips whitespace from the option value before passing it to `unchanged_required`. Let me check how docutils parses option values from the RST source.

Let me also look at `parse_extension_options` which is likely in docutils.utils or similar.

Let me check `extract_extension_options` in docutils.utils to understand how option values are extracted.

Let me check `extract_options` and `assemble_option_dict` to understand how option values are parsed.

So the key line is: ```python data = body[0][0].astext() ``` This gets the text from the paragraph node in the field body. The docutils RST parser processes the RST text and creates nodes. The question is: does the RST parser strip leading whitespace from the field body? Looking at how field lists are parsed in RST, the field body typically starts after the colon. In the case of: ``` :prepend: </plugin> ``` The RST parser would parse the field body as ` </plugin>` and create a paragraph containing a Text node with that value. The `astext()` call should return the text as-is, including leading whitespace... but RST might normalize whitespace. Wait, actually, RST does strip leading whitespace in certain cases. In the RST spec, field lists have the format: ``` :field name: field body ``` The field body starts after the colon and any whitespace after the colon is typically not significant. But actually, let me check... In docutils, the field body text is the text after the colon. But RST normalizes whitespace in text nodes. Let me look at `nested_list_parse` or how the "ExtensionOptions" state works to understand if whitespace is stripped. Actually, let me just test this empirically:

OK so when the value is passed directly to the option convertor, leading whitespace is preserved. But the real question is whether the RST parser preserves leading whitespace when parsing the directive option. Let me test how the RST parser actually parses an option like `:prepend: </plugin>`:

Right, `code-block` doesn't support `prepend`. Let me use `literalinclude` instead. But I need an actual file to include. Let me create a small test:

Let me test with `literalinclude` directive but focus on understanding how options are parsed. The key question is: does the RST parser strip leading whitespace from option values? Let me look more carefully at how the RST parser parses extension options. The field body for an extension option starts right after the colon. Let me check the `ExtensionOptions` state in docutils.

`ExtensionOptions` extends `FieldList`. Let me check the `FieldList` state in docutils to understand how field bodies are parsed.

Let me look at the `field` method:

So the field body is `indented`, which comes from `self.state_machine.get_first_known_indented(match.end())`. This gets the indented text after the field marker. The indented text should include the leading whitespace from the directive option line. Let me look at `parse_field_body` to see how the indented text is parsed: