`feature(frontmatter)` rejects frontmatters with dashes with leading spaces

[epage]

tests/ui/frontmatter/multifrontmatter-2.rs:

---
 ---
//~^ ERROR: invalid preceding whitespace for frontmatter close

 ---
//~^ ERROR: expected item, found `-`
// FIXME(frontmatter): make this diagnostic better
---

#![feature(frontmatter)]

fn main() {}

I expected to see this happen: this should be a //@ check-pass

Instead, this happened: this is an ERROR

Meta

As of #140035 (I had missed this test in the review)

The RFC text says:

All content is ignored by rustc until the same number of dashes is found at the start of a line. The line must terminate by 0+ whitespace and then a newline.

So a line with spaces before a --- should not require escapaing (more - on open/close) but should work just fine.