Concatenating adjacent string literals

[ILyoan]

This makes string literals placed next to each other be concatenated.
Thus "Foo" "Bar" would be "FooBar".

[thestinger]

I think this needs an RFC. Could this just be a syntax extension? I find that I often do this by mistake in Python by missing a comma:

let xs = ["foo",
          "bar"
          "baz"];

[alexcrichton]

I was hoping to avoid having to do this in the parser by implementing the concat! syntax extension. Was that found to be insufficient?

[ILyoan]

@thestinger, @alexcrichton I didn't know that there was already concat! extension. Thanks.
Closing this PR.

[stepancheg]

concat! does not concatenate binary string literals. How it should be fixed? I see three options:

• bconcat! macro, similar to concat!, but for binary literals
• concat! macro may produce binary literal if all arguments are binary literals (weird thing is that no-argument concat!() still produces string literal)
• Reopen this issue and implement C-style concatenation

[stepancheg]

I forgot that strings can be wrapped with slash. So lack of concat! for binary literals is not a big deal.