Rustdoc treats indentations in the consecutive syntactically same doc-comments and doc-attributes differently

[zheland]

According to https://doc.rust-lang.org/rustdoc/the-doc-attribute.html
the lines below are the same:

/// This is a doc comment.
#[doc = " This is a doc comment."]

But when mixing doc-comments and doc-attributes indentation in the next comment or attribute is ignored.

For example cargo doc generate different doc pages for

///Header
///
///    assert!(true);
pub fn func() {}

and

///Header
///
#[doc = "    assert!(true);"]
pub fn func() {}

where ///____assert!(true); produce CodeBlock for the first function (generated docs), but #[doc = "____assert!(true);"] produce plain text paragraph for the second function (generated docs).

This behavior might be usefull for saving identical indents in documentation declared in comments and attributes:

/// Some docs
#[cfg_attr(feature = "foo", doc = #"
    continuation with visually the same indentation
"#)]
/// continuation with visually the same indentation

but in general this behavior is not explicit.

For example syn crate produce exactly the same output when parsing docs both functions with syn::parse_file and preserves all the spaces.

If this behavior is intentional it should be docummented.

More examples:

• File with doc-comments only that generates docs with CodeBlock,
• File with doc-attributes only that generates docs with CodeBlock,
• File with both doc-comments and doc-attributes that generates docs without CodeBlock.
• File with both doc-attributes and doc-comments that generates docs without CodeBlock.

Reproduced with cargo doc on rustc 1.47.0 (18bf6b4f0 2020-10-07) and rustc 1.49.0-nightly (91a79fb29 2020-10-07)

[jyn514]

Duplicate of #70732 (but I like your title better, I might steal it).