### Reproducer * `rustc a.rs --crate-type proc-macro --extern proc_macro` where `a.rs`: ```rs #[proc_macro] pub fn ck(x: proc_macro::TokenStream) -> proc_macro::TokenStream { _ = "---".parse::<proc_macro::TokenStream>(); x } ``` * `rustc b.rs --crate-type lib --extern a -L.` where `b.rs`: ```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 --> b.rs:1:1 | 1 | a::ck!(); | ^^^^^^^^ | = note: frontmatter infostrings must be a single identifier immediately following the opening = note: this error originates in the macro `a::ck` (in Nightly builds, run with -Z macro-backtrace for more info) error: unclosed frontmatter --> b.rs:1:1 | 1 | a::ck!(); | ^^^^^^^^ | note: frontmatter opening here was not closed --> b.rs:1:1 | 1 | a::ck!(); | ^^^^^^^^ = note: this error originates in the macro `a::ck` (in Nightly builds, run with -Z macro-backtrace for more info) error[E0658]: frontmatters are experimental --> b.rs:1:1 | 1 | a::ck!(); | ^^^^^^^^ | = note: see issue #136889 <https://github.com/rust-lang/rust/issues/136889> for more information = note: this error originates in the macro `a::ck` (in Nightly builds, run with -Z macro-backtrace for more info) ```