in stmt context, seq!(() "") accepted while seq!("" 4) rejected

[pnkfelix]

(spawned off of #25658)

This may not necessarily be a macro expansion issue, but that seems like the most likely candidate.

The following code:

macro_rules! seq {
    ($($e:expr)*) => ($($e)*);
}

fn main() {
    {      (); "";  }
    { seq!(()  ""); }

    {      "";  4;  }
    { seq!(""   4); }
}

yields the following compilation error:

% rustc /tmp/seq.rs
/tmp/seq.rs:10:12: 10:14 error: mismatched types:
 expected `()`,
    found `&'static str`
(expected (),
    found &-ptr) [E0308]
/tmp/seq.rs:10     { seq!(""   4); }
                          ^~
/tmp/seq.rs:10:7: 10:20 note: in this expansion of seq! (defined in /tmp/seq.rs)
/tmp/seq.rs:10:12: 10:14 help: run `rustc --explain E0308` to see a detailed explanation
error: aborting due to previous error
% 

but according to rustc -Z unstable-options --pretty expanded, there is no difference between the second seq! and the expression that precedes it:

% rustc --version
rustc 1.6.0-dev (eacd35984 2015-11-03)
% rustc -Z unstable-options --pretty expanded /tmp/seq.rs
#![feature(no_std, prelude_import)]
#![no_std]
#[prelude_import]
use std::prelude::v1::*;
#[macro_use]
extern crate std as std;

fn main() {
    { (); ""; }
    { (); ""; }

    { ""; 4; }
    { ""; 4; }
}
% 

(My current suspicion is that the --pretty expanded is not showing us the closest match to the actual AST being produced by expansion.)

[nikomatsakis]

This is almost certainly related to the "statements that do not have semicolons must have unit type". I imagine it's related to the parser's behavior when it encounters an "expr token". The pretty printer output is probably somewhat misleading.

[Mark-Simulacrum]

The example now doesn't compile with a different error, which seems correct, since () "" isn't valid Rust. This appears to be because the macro no longer inserts semicolons after expressions automatically, if the macro is edited to insert them (($($e:expr)*) => ($($e;)*); then the code does, indeed, compile.

New error:

error: expected one of `.`, `;`, `?`, `}`, or an operator, found `""`
 --> src/lib.rs:2:25
  |
2 |     ($($e:expr)*) => ($($e)*);
  |                         ^^ expected one of `.`, `;`, `?`, `}`, or an operator here

error: expected one of `.`, `;`, `?`, `}`, or an operator, found `4`
 --> src/lib.rs:2:25
  |
2 |     ($($e:expr)*) => ($($e)*);
  |                         ^^ expected one of `.`, `;`, `?`, `}`, or an operator here

error: aborting due to 2 previous errors