Introduction of frontmatter was a theoretical breaking change (via Literal::from_str) #146132
Description
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_macrowherea.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.whereb.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to the crate attributes to enable = note: this compiler was built on 2025-09-02; consider upgrading it if it is out of date error: aborting due to 3 previous errors For more information about this error, try `rustc --explain E0658`.