Introduction of frontmatter was a theoretical breaking change (via `Literal::from_str`)

[fmease]

See also #145520 and #145945.
This is basically (the fixed) #145520 but with TokenStream replaced with Literal inside .parse().

Reproducer

• rustc a.rs --crate-type proc-macro --extern proc_macro where a.rs:

  #[proc_macro]
  pub fn ck(x: proc_macro::TokenStream) -> proc_macro::TokenStream {
      _ = "---".parse::<proc_macro::Literal>();
      x
  }

• rustc b.rs --crate-type lib --extern a -L. where b.rs:

  a::ck!();

In 1.87 and earlier, this used to compile just fine. However, since 1.88 (more precisely since PR #140035) this fails with:

error: invalid infostring for frontmatter
 --> <proc-macro source code>:1:4
  |
1 | ---
  |    ^
  |
  = note: frontmatter infostrings must be a single identifier immediately following the opening

error: unclosed frontmatter
 --> <proc-macro source code>:1:1
  |
1 | ---
  | ^^^
  |
note: frontmatter opening here was not closed
 --> <proc-macro source code>:1:1
  |
1 | ---
  | ^^^

error[E0658]: frontmatters are experimental
 --> <proc-macro source code>:1:1
  |
1 | ---
  | ^^^
  |
  = note: see issue #136889 <https://github.com/rust-lang/rust/issues/136889> for more information
  = help: add `#![feature(frontmatter)]` to the crate attributes to enable
  = note: this compiler was built on 2025-09-02; consider upgrading it if it is out of date

error: aborting due to 3 previous errors

For more information about this error, try `rustc --explain E0658`.

[fmease]

That's because the new_parser_from_source_str inside <Rustc as FreeFunctions>::literal_from_str (called by proc_macro::Literal::from_str via the proc-macro bridge) passes FrontmatterAllowed::Yes.

rust/compiler/rustc_expand/src/proc_macro_server.rs

Lines 486 to 489 in a2c8b0b

	fn literal_from_str(&mut self, s: &str) -> Result<Literal<Self::Span, Self::Symbol>, ()> {
	let name = FileName::proc_macro_source_code(s);
	let mut parser =
	unwrap_or_emit_fatal(new_parser_from_source_str(self.psess(), name, s.to_owned()));

[fmease]

cc @epage @fee1-dead for visibility