Ugly error reporting on binary operation between two condition flows

[GuillaumeGomez]

The following code:

fn to_log_flags(fatal: bool, recursive: bool) -> u32 {
    if fatal { 1 } else { 0 } |
    if recursive { 2 } else { 0 }
}

gives the following error:

   Compiling playground v0.0.1 (/playground)
error: expected identifier, found keyword `if`
 --> src/main.rs:3:5
  |
3 |     if recursive { 2 } else { 0 }
  |     ^^ expected identifier, found keyword

error: expected identifier, found `2`
 --> src/main.rs:3:20
  |
3 |     if recursive { 2 } else { 0 }
  |                    ^ expected identifier

error: expected one of `,`, `:`, or `@`, found `recursive`
 --> src/main.rs:3:8
  |
3 |     if recursive { 2 } else { 0 }
  |       -^^^^^^^^^ expected one of `,`, `:`, or `@`
  |       |
  |       help: missing `,`

error: expected identifier, found keyword `else`
 --> src/main.rs:3:24
  |
3 |     if recursive { 2 } else { 0 }
  |                        ^^^^ expected identifier, found keyword
  |
help: you can escape reserved keywords to use them as identifiers
  |
3 |     if recursive { 2 } r#else { 0 }
  |                        ^^^^^^

error: expected identifier, found `0`
 --> src/main.rs:3:31
  |
3 |     if recursive { 2 } else { 0 }
  |                               ^ expected identifier

error: expected one of `,` or `:`, found keyword `else`
 --> src/main.rs:3:24
  |
3 |     if recursive { 2 } else { 0 }
  |                       -^^^^ expected one of `,` or `:`
  |                       |
  |                       help: missing `,`

error: expected one of `:` or `|`, found `}`
 --> src/main.rs:4:1
  |
3 |     if recursive { 2 } else { 0 }
  |                                  - expected one of `:` or `|`
4 | }
  | ^ unexpected token

error[E0308]: mismatched types
 --> src/main.rs:2:16
  |
2 |     if fatal { 1 } else { 0 } |
  |     -----------^-------------- help: consider using a semicolon here
  |     |          |
  |     |          expected `()`, found integer
  |     expected this to be `()`

error[E0308]: mismatched types
 --> src/main.rs:2:27
  |
2 |     if fatal { 1 } else { 0 } |
  |     ----------------------^--- help: consider using a semicolon here
  |     |                     |
  |     |                     expected `()`, found integer
  |     expected this to be `()`

error: aborting due to 9 previous errors

For more information about this error, try `rustc --explain E0308`.

This is clearly sub-optimal. The code can be fixed by wrapping the first if condition in ().

[estebank]

The output is still very verbose, but it eventually provides the appropriate solution:

error: expected identifier, found keyword `if`
 --> src/lib.rs:4:33
  |
4 |     if fatal { 1 } else { 0 } | if recursive { 2 } else { 0 }
  |                                 ^^ expected identifier, found keyword

error: expected one of `,`, `:`, or `@`, found `recursive`
 --> src/lib.rs:4:36
  |
4 |     if fatal { 1 } else { 0 } | if recursive { 2 } else { 0 }
  |                                   -^^^^^^^^^ expected one of `,`, `:`, or `@`
  |                                   |
  |                                   help: missing `,`

error: expected identifier, found keyword `else`
 --> src/lib.rs:4:52
  |
4 |     if fatal { 1 } else { 0 } | if recursive { 2 } else { 0 }
  |                                                    ^^^^ expected identifier, found keyword
  |
help: escape `else` to use it as an identifier
  |
4 |     if fatal { 1 } else { 0 } | if recursive { 2 } r#else { 0 }
  |                                                    ++

error: expected one of `,` or `:`, found keyword `else`
 --> src/lib.rs:4:52
  |
4 |     if fatal { 1 } else { 0 } | if recursive { 2 } else { 0 }
  |                                                   -^^^^ expected one of `,` or `:`
  |                                                   |
  |                                                   help: missing `,`

error: expected one of `:` or `|`, found `}`
 --> src/lib.rs:5:1
  |
4 |     if fatal { 1 } else { 0 } | if recursive { 2 } else { 0 }
  |                                                              - expected one of `:` or `|`
5 | }
  | ^ unexpected token
  |
help: parentheses are required to parse this as an expression
  |
4 |     (if fatal { 1 } else { 0 }) | if recursive { 2 } else { 0 }
  |     +                         +

error[[E0308]](https://doc.rust-lang.org/nightly/error_codes/E0308.html): mismatched types
 --> src/lib.rs:4:16
  |
4 |     if fatal { 1 } else { 0 } | if recursive { 2 } else { 0 }
  |     -----------^-------------
  |     |          |
  |     |          expected `()`, found integer
  |     expected this to be `()`
  |
help: you might have meant to return this value
  |
4 |     if fatal { return 1; } else { 0 } | if recursive { 2 } else { 0 }
  |                ++++++  +

error[[E0308]](https://doc.rust-lang.org/nightly/error_codes/E0308.html): mismatched types
 --> src/lib.rs:4:27
  |
4 |     if fatal { 1 } else { 0 } | if recursive { 2 } else { 0 }
  |     ----------------------^--
  |     |                     |
  |     |                     expected `()`, found integer
  |     expected this to be `()`
  |
help: you might have meant to return this value
  |
4 |     if fatal { 1 } else { return 0; } | if recursive { 2 } else { 0 }
  |                           ++++++  +

[estebank]

No change. This happens because the first if/else is parsed as a complete statement, and the | as the beginning of a closure. I believe now that we need to specifically detect this in the parser.

[estebank]

Output after #146044

error: expected identifier, found keyword `if`
 --> f20.rs:3:5
  |
3 |     if recursive { 2 } else { 0 }
  |     ^^ expected identifier, found keyword
  |
help: parentheses are required to parse this as an expression
  |
2 |     (if fatal { 1 } else { 0 }) |
  |     +                         +

error[E0308]: mismatched types
 --> f20.rs:2:16
  |
2 |     if fatal { 1 } else { 0 } |
  |     -----------^-------------
  |     |          |
  |     |          expected `()`, found integer
  |     expected this to be `()`
  |
help: parentheses are required to parse this as an expression
  |
2 |     (if fatal { 1 } else { 0 }) |
  |     +                         +
help: you might have meant to return this value
  |
2 |     if fatal { return 1; } else { 0 } |
  |                ++++++  +

error[E0308]: mismatched types
 --> f20.rs:2:27
  |
2 |     if fatal { 1 } else { 0 } |
  |     ----------------------^--
  |     |                     |
  |     |                     expected `()`, found integer
  |     expected this to be `()`
  |
help: parentheses are required to parse this as an expression
  |
2 |     (if fatal { 1 } else { 0 }) |
  |     +                         +
help: you might have meant to return this value
  |
2 |     if fatal { 1 } else { return 0; } |
  |                           ++++++  +