Concatenating bytestrings fails; rustc suggests non-existent `bconcat!()`

[jsgf]

concat!() doesn't work on bytestrings (why?). When trying it, it fails with:

1 | const FOO: &[u8] = concat!("foo", "bar");
  |                    ^^^^^^^^^^^^^^^^^^^^^
  |                    |
  |                    expected slice, found str
  |                    help: consider adding a leading `b`: `bconcat!("foo", "bar")`
  |
  = note: expected type `&'static [u8]`
             found type `&'static str`

bconcat!() sounds interesting, but it isn't documented, and the compiler doesn't accept it:

error: cannot find macro `bconcat!` in this scope
 --> src/main.rs:1:20
  |
1 | const FOO: &[u8] = bconcat!("foo", "bar");
  |                    ^^^^^^^ help: you could try the macro: `concat`

I think the real fix here is that bconcat!() should exist.

[jonas-schievink]

Heh, this is because the compiler assumes you used a normal string literal like "abc" and just slaps a b in front of it to create the suggestion. A bconcat! macro never existed.

[jsgf]

Heh indeed. I originally tried concat!(b"foo", b"bar"), but:

error: cannot concatenate a byte string literal
 --> src/main.rs:1:28
  |
1 | const FOO: &[u8] = concat!(b"foo", b"bar");
  |                            ^^^^^^

error: cannot concatenate a byte string literal
 --> src/main.rs:1:36
  |
1 | const FOO: &[u8] = concat!(b"foo", b"bar");
  |                                    ^^^^^^

error: aborting due to 2 previous errors

I can't think of a reason why concat shouldn't support this though. Is there something I'm missing?

[kennytm]

cc @estebank #51978 about the bconcat diagnostic issue.

[estebank]

I'll take a look and fix the incorrect suggestion.

The concat!() restriction is here:

https://github.com/rust-lang/rust/blob/master/src/libsyntax_ext/concat.rs

It seems to me that the only real restriction that it should have is not mixing byte str literals and str literals when concatenating, and even then it should be possible to convert from one to the other.

Changing the linked code to keep two accumulators, one for str and another as a Vec<u8> for byte str, and returning base::MacEager::expr(cx.expr_lit(sp, ast::LitKind::ByteStr(rustc_data_structures::sync::Lrc::new(byte_acc)))) would be enough to add the desired functionality.

@rust-lang/lang is there any reason to not change concat!() to accept concatenating multiple byte strs?

[estebank]

The diagnostic bug is fixed in nightly by #51978.