`feature(frontmatter)` rejects frontmatters with dashes with leading spaces

[epage]

tests/ui/frontmatter/multifrontmatter-2.rs:

---
 ---
//~^ ERROR: invalid preceding whitespace for frontmatter close

 ---
//~^ ERROR: expected item, found `-`
// FIXME(frontmatter): make this diagnostic better
---

#![feature(frontmatter)]

fn main() {}

I expected to see this happen: this should be a //@ check-pass

Instead, this happened: this is an ERROR

Meta

As of #140035 (I had missed this test in the review)

The RFC text says:

All content is ignored by rustc until the same number of dashes is found at the start of a line. The line must terminate by 0+ whitespace and then a newline.

So a line with spaces before a --- should not require escapaing (more - on open/close) but should work just fine.

[dpc]

I've noticed that the shebang convention that Nix wants to use (multiline):

https://nix.dev/manual/nix/2.28/command-ref/new-cli/nix.html?highlight=shebang#shebang-interpreter

does not work. Should it?

> cat ./cargo-script-test.rs
#!/usr/bin/env nix
#!nix shell github:nix-community/fenix?rev=26fbcc33a5e94d6d0dc5f467eca8f7a746a54e6c#minimal.toolchain -c cargo -q -Zscript
---
[dependencies]
duct = "*"
snafu = "*"
---

use duct::cmd;
use snafu::Whatever;
use snafu::ResultExt;


fn main() -> Result<(), Whatever> {
    println!("Hello from cargo-script");
    cmd!("echo", "hi").run().whatever_context("Failed to run command")?;
    Ok(())
}
> ./cargo-script-test.rs
error: expected `[`, found `nix`
 --> cargo-script-test.rs:2:3
  |
2 | #!nix shell github:nix-community/fenix?rev=26fbcc33a5e94d6d0dc5f467eca8f7a746a54e6c#minimal.toolchain -c cargo -q -Zscript
  |   ^^^ expected `[`
  |
  = note: the token sequence `#!` here looks like the start of a shebang interpreter directive but it is not
  = help: if you meant this to be a shebang interpreter directive, move it to the very start of the file

error: could not compile `cargo-script-test` (bin "cargo-script-test") due to 1 previous error
> 

> nix shell github:nix-community/fenix?rev=26fbcc33a5e94d6d0dc5f467eca8f7a746a54e6c#minimal.toolchain
> rustc --version --verbose
rustc 1.89.0-nightly (2805e1dc4 2025-05-26)
binary: rustc
commit-hash: 2805e1dc4c18ed4c84d161502c48da870c56f68a
commit-date: 2025-05-26
host: x86_64-unknown-linux-gnu
release: 1.89.0-nightly
LLVM version: 20.1.5

Luckily for me, if I move that line under the first ---, it works as Nix accepts that, but I think the preference would be to have it above the dash.

[epage]

That seems unrelated to this issue and more general for whether rustc should handle a nix-specific convention for she-bangs without having to use workaround prescribed in that link

$ cat ./cargo-script-test.rs
#!/usr/bin/env nix
-#!nix shell github:nix-community/fenix?rev=26fbcc33a5e94d6d0dc5f467eca8f7a746a54e6c#minimal.toolchain -c cargo -q -Zscript
---
[dependencies]
duct = "*"
snafu = "*"
---
+/*
+#!nix shell github:nix-community/fenix?rev=26fbcc33a5e94d6d0dc5f467eca8f7a746a54e6c#minimal.toolchain -c cargo -q -Zscript
+*/

use duct::cmd;
use snafu::Whatever;
use snafu::ResultExt;


fn main() -> Result<(), Whatever> {
    println!("Hello from cargo-script");
    cmd!("echo", "hi").run().whatever_context("Failed to run command")?;
    Ok(())
}

[dpc]

That seems unrelated to this issue

Should I open a new issue, or post in a different one? I'm a bit lost in the process here.

more general for whether rustc should handle a nix-specific convention for she-bangs without having to use workaround prescribed in that link

@epage

Having comments after the shebang seems more than just nix-specific convention:

> head -n 10 cargo-script-test.rs
#!/usr/bin/env nix
# Hey, I have comment.
---
#!nix shell github:nix-community/fenix?rev=26fbcc33a5e94d6d0dc5f467eca8f7a746a54e6c#minimal.toolchain -c cargo -q -Zscript
[dependencies]
duct = "*"
snafu = "*"
---

use duct::cmd;
> ./cargo-script-test.rs
error: expected one of `!` or `[`, found `Hey`
 --> cargo-script-test.rs:2:3
  |
2 | # Hey, I have comment.
  |   ^^^ expected one of `!` or `[`

error: could not compile `cargo-script-test` (bin "cargo-script-test") due to 1 previous error

BTW. The "workaround prescribed in that link" does not work, at least on the Nix version I have on my system. Looks like a Nix bug/regression in that aspect. The #! nix in the dependencies section does work.

But it goes to show that it's worthwhile being liberal in what's supported because it gives some wrangling room for users gluing these things with other tooling. Shenanigans around hashbang are somewhat common, and often have weird interactions.

[epage]

If there is an Issue or thread that matches your need, that should be used. Otherwise, you are welcome to open a new one. Posting on related issues adds noise in the thread and notifications that makes it harder to have the on-topic discussion.

Having comments after the shebang seems more than just nix-specific convention:

Note that # isn't a comment in Rust syntax and instead # introduce attributes. There is a special exemption with shebangs that requires specific handling to disambiguate between the two. My comment about nix-specific behavior was from the perspective of handling it within Rust.

[jieyouxu]

Yes, please open a separate issue about the nix shebang, that is unrelated to this issue or at least quite different.

[joshtriplett]

@epage This does indeed seem like a bug. A line of dashes is only supposed to be syntactic to Rust if it's at the start of the line; otherwise it should be ignored.