Trailing whitespaces are stripped in raw string literals in doc tests

[Pistonight]

Example:

/// Doc Test
/// ```rust
/// let expected = "\nfoo \n";
/// assert_eq!(expected, r#"
/// foo 
/// "#);
/// ```
pub fn main() {}

Note that the expected string has a space before the last newline.
cargo fmt strips the space in the raw string literal, causing passing doc tests to fail afterwards

[Pistonight]

This issue only exists in doc comments. Trailing spaces in raw string literals are kept correctly in regular code

[ytmimi]

@Pistonight thanks for the report. Generally, rustfmt does it's best to remove trailing whitespace, but I understand in this case it's changing the semantics of the code in the doc test. If possible, I'd encourage you to write your doc test in a way that avoids trailing whitespace.

I did a little investigating into what's going on. Hopefully this will be useful for anyone if they decide to work on this:

In this particular case code execution is going to light_rewrite_comment, which is causing the trailing space to be removed. That code path doesn't have any real awareness of "am I in a rust code block within a doc comment".

rustfmt/src/comment.rs

Line 374 in 2174e60

light_rewrite_comment(first_group, shape.indent, config, is_doc_comment)

Code execution continues on to trim_end_unless_two_whitespaces and that's where we seem to be removing the trailing whitespace.

rustfmt/src/comment.rs

Line 1081 in 2174e60

trim_end_unless_two_whitespaces(left_trimmed, is_doc_comment)

I thought that configuring rustfmt with format_code_in_doc_comments=true might help, but in that case code execution will hit rewtie_comment_inner

rustfmt/src/comment.rs

Lines 376 to 383 in 2174e60

	rewrite_comment_inner(
	first_group,
	block_style,
	style,
	shape,
	config,
	is_doc_comment || style.is_doc_comment(),
	)?

and again we end up calling trim_end_unless_two_whitespaces before we do any of the very basic logic to determine if we're formatting a code block within a doc comment.

rustfmt/src/comment.rs

Lines 922 to 933 in 2174e60

	let lines = orig
	.lines()
	.enumerate()
	.map(|(i, mut line)| {
	line = trim_end_unless_two_whitespaces(line.trim_start(), is_doc_comment);
	// Drop old closer.
	if i == line_breaks && line.ends_with("*/") && !line.starts_with("//") {
	line = line[..(line.len() - 2)].trim_end();
	}
	line
	})