Frontmatter inside `--cfg` and `--check-cfg` is not feature-gated

[fmease]

The following invocation works since 1.88 (or PR #140035):

rustc /dev/null --crate-type lib --cfg '---
---
key'

while it shouldn't because unstable feature frontmatter isn't enabled anywhere.

For context, in 1.87, it yielded the following error:

error: invalid `--cfg` argument: `---
       ---
       key` (expected `key` or `key="value"`)

The same applies to --check-cfg.

[fmease]

cc @epage @fee1-dead for visibility

[fmease]

(shebangs inside --{,check-}cfg are ofc allowed (whether that was a mistake is a separate question))

[Urgau]

That's probably due to our usage of the parser to parse both arguments.

rust/compiler/rustc_interface/src/interface.rs

Line 71 in a2c8b0b

match new_parser_from_source_str(&psess, filename, s.to_string()) {

rust/compiler/rustc_interface/src/interface.rs

Line 169 in a2c8b0b

let mut parser = match new_parser_from_source_str(&psess, filename, s.to_string()) {

[fmease]

Yea, that's how I found it. I was checking all callsites (and found another regression which I'll report in a sec, edit: #146132).

[Urgau]

(shebangs inside --{,check-}cfg are ofc allowed (whether that was a mistake is a separate question))

This was most certainly a mistake. I'm not expecting anyone to have a shebang. We should IMO just remove support for it.

[fmease]

This was most certainly a mistake. I'm not expecting anyone to have a shebang. We should IMO just remove support for it.

Note that then we're still left with line and block comments being allowed inside --{,check-}cfg (not sure how to feel about these). If you want to pursue this, r? me + lang-nominated + r=me after fFCP :D edit: Prolly not lang territory actually, just compiler.

[apiraino]

Assigning priority (discussion on Zulip).

@rustbot label -I-prioritize +P-medium