Theoretical breaking change from a combination of frontmatter and include!

[mattheww]

With rustc 1.87, compiling the following pair of files succeeds:

main.rs:

fn main() {
    let _ = include!("expression.inc");
}

expression.inc:

---1

With rustc 1.88 or 1.89 (or rustc 1.91.0-nightly (160e7623e 2025-08-26)), compilation fails.

With 1.89 the first error message is

error: invalid infostring for frontmatter
 --> src/expression.inc:1:4
  |
1 | ---1
  |    ^
  |
  = note: frontmatter infostrings must be a single identifier immediately following the opening

See #145520 for a similar issue with TokenStream::from_str.

[mattheww]

Here's a very contrived example showing worse trouble when the frontmatter feature is enabled (the old and new compilers both accept the input but give different results).

With this pair of files:

main.rs:

fn main() {
    let x = include!("expression.inc");
    dbg!(x);
}

expression.inc:

---
-
---
1

% cargo +1.87 run
[src/main.rs:3:5] x = -1

% RUSTFLAGS=-Zcrate-attr='feature(frontmatter)' cargo +nightly run
[src/main.rs:3:5] x = 1

[fmease]

We should probably only lex the frontmatter if the "expansion target context" (forgot the term we use in rustc) is item but not when it is expr, stmt or pat. IIRC one should be able to determine that before parsing inside expand_include if that's what it's called.

I don't have the time to work on a PR for that tho.

cc @fee1-dead @epage for visibility

[fmease]

Hmmm, or just don't parse it at all in this edition. I've just realized that we do strip shebangs even if the context of the include! is sth. like expr which is frankly insane to me but I guess it makes sense on a lexical level. E.g., it's completely legal to include! the following inside of an expr context:

#!/whatever/your/heart/desires

"Hello there!"

And one of [@]epage's requirements was to permit frontmatter everywhere where we also allow shebangs. I guess that requirement simply cannot be fulfilled without an edition bump due the existence of this issue and #145520.

[jieyouxu]

Ugh, my first reaction is that shebang or frontmatter in include!'d file shouldn't be permitted, but I guess that ship has sailed for shebangs :/