round tripping multiline header results in single line header

[ChristianMurphy]

Subject of the issue

text  
text-
-
text

is stringified as:

## text\
text-

text

which changes the structure

Your environment

• OS: Ubuntu
• Packages: mdast-util-to-markdown 0.6.4
• Env: node v15.5.1, npm 7.3.0

Steps to reproduce

parse

text  
text-
-
text

which has the structure

{
    "type": "root",
    "children": [
        {
            "type": "heading",
            "depth": 2,
            "children": [
                {
                    "type": "text"
                },
                {
                    "type": "break"
                },
                {
                    "type": "text"
                }
            ]
        },
        {
            "type": "paragraph",
            "children": [
                {
                    "type": "text"
                }
            ]
        }
    ]
}

and stringify it:

## text\
text-

text

the resulting markdown has a different structure than the original

{
    "type": "root",
    "children": [
        {
            "type": "heading",
            "depth": 2,
            "children": [
                {
                    "type": "text"
                }
            ]
        },
        {
            "type": "paragraph",
            "children": [
                {
                    "type": "text"
                }
            ]
        },
        {
            "type": "paragraph",
            "children": [
                {
                    "type": "text"
                }
            ]
        }
    ]
}

📓 comparing how the two pieces of markdown text are being parsed with https://spec.commonmark.org/dingus it appears in both cases it is parsed as expected.

Expected behavior

structure is the same

Actual behavior

structure is different

[wooorm]

This is another case where I think the AST should change: the style of headings is ATX by default: breaks can’t occur in ATX headings, going 'a \nb\n=' (or 'a\\\nb\n=') to # a b makes the most sense to me