Syntax error when using multiple cfg attributes

[lamafab]

I encountered a weird behavior when using cfg(test) and cfg(not(test)).

I tried this code:

fn does_not_work() -> usize {
    #[cfg(not(test))]
    0

    #[cfg(test)]
    1
}

Results in:

   Compiling playground v0.0.1 (/playground)
error: expected one of `.`, `;`, `?`, `}`, or an operator, found `#`
 --> src/lib.rs:6:5
  |
4 |     0
  |      - expected one of `.`, `;`, `?`, `}`, or an operator
5 | 
6 |     #[cfg(test)]
  |     ^ unexpected token

error: aborting due to previous error

error: could not compile `playground`.

To learn more, run the command again with --verbose.

Workaround

fn does_work() -> usize {
    #[cfg(not(test))]
    return 0; // `return` including a semicolon

    #[cfg(test)]
    1
}

Meta

Rust stable version 1.47.0

Playground: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=f7f7eb30327396c209b0f358b1a3072b

[tesuji]

This is not a bug: @rustbot modify labels: -C-bug

[lamafab]

@lzutao Would you mind clarifying?

[tesuji]

Disclaimer: I'm not the best person to explain this.

Rust doesn't have whitespace sensitive syntax. This means that your example can be written as

#[cfg(not(test))] 0 #[cfg(test)] 1

Which mean that #[cfg(not(test))] could control not only 0 but also 0 #[cfg(test)] 1.
0 #[cfg(test)] 1 is not a valid syntax.

People often use the code below instead:

fn does_not_work() -> usize {
    #[cfg(not(test))] { 0 }
    #[cfg(test)] { 1 }
}

[LeSeulArtichaut]

The diagnostics could probably be improved however

[fmease]

Current output:

error: expected `;`, found `#`
 --> src/lib.rs:3:6
  |
2 |     #[cfg(not(test))]
  |     ----------------- only `;` terminated statements or tail expressions are allowed after this attribute
3 |     0
  |      ^ expected `;` here
4 |
5 |     #[cfg(test)]
  |     - unexpected token
  |
help: add `;` here
  |
3 |     0;
  |      +
help: alternatively, consider surrounding the expression with a block
  |
3 |     { 0 }
  |     +   +
help: it seems like you are trying to provide different expressions depending on `cfg`, consider using `if cfg!(..)`
  |
2 ~     if cfg!(not(test)) {
3 ~         0
4 | 
5 ~     } else if cfg!(test) {
6 ~         1
7 +     }
  |

This is thanks to #117988. Closing as fixed.