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mrmonday opened this issue Mar 18, 2015 · 1 comment
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Confusing error message with invalid format specifier #23476

mrmonday opened this issue Mar 18, 2015 · 1 comment
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A-diagnostics Area: Messages for errors, warnings, and lints A-syntaxext Area: Syntax extensions C-enhancement Category: An issue proposing an enhancement or a PR with one.

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@mrmonday
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The following code:

fn main() {
    let _ = format!("{_foo}", _foo = 6u);
}

(Playpen: http://is.gd/me8pNE)

Gives the error:

<anon>:2:21: 2:29 error: invalid format string: expected `'}'`, found `'_'`
<anon>:2     let _ = format!("{_foo}", _foo = 6u);
                             ^~~~~~~~
error: aborting due to previous error

It is not too confusing in this short example, but in a longer string it is not obvious that named identifiers cannot start with an underscore.
@kmcallister kmcallister added the A-syntaxext Area: Syntax extensions label Mar 18, 2015
@steveklabnik
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Updated code:

fn main() {
    let _ = format!("{_foo}", _foo = 6usize);
}

this still happens though.

@Mark-Simulacrum Mark-Simulacrum added the C-enhancement Category: An issue proposing an enhancement or a PR with one. label Jul 22, 2017
@estebank estebank added the A-diagnostics Area: Messages for errors, warnings, and lints label May 1, 2018
@estebank estebank mentioned this issue May 10, 2018
Merged
bors added a commit that referenced this issue May 17, 2018
@bors
Auto merge of #50610 - estebank:fmt-str, r=Kimundi
c6ec5f4 
Improve format string errors

Point at format string position inside the formatting string:
```
error: invalid format string: unmatched `}` found
  --> $DIR/format-string-error.rs:21:22
   |
LL |     let _ = format!("}");
   |                      ^ unmatched `}` in format string
```

Explain that argument names can't start with an underscore:
```
error: invalid format string: invalid argument name `_foo`
  --> $DIR/format-string-error.rs:15:23
   |
LL |     let _ = format!("{_foo}", _foo = 6usize);
   |                       ^^^^ invalid argument name in format string
   |
   = note: argument names cannot start with an underscore
```

Fix #23476.

The more accurate spans will only be seen when using `format!` directly, when using `println!` the diagnostics machinery makes the span be the entire statement.
Mark-Simulacrum added a commit to Mark-Simulacrum/rust that referenced this issue May 17, 2018
@Mark-Simulacrum
Rollup merge of rust-lang#50610 - estebank:fmt-str, r=Kimundi
b3734bd 
Improve format string errors

Point at format string position inside the formatting string:
```
error: invalid format string: unmatched `}` found
  --> $DIR/format-string-error.rs:21:22
   |
LL |     let _ = format!("}");
   |                      ^ unmatched `}` in format string
```

Explain that argument names can't start with an underscore:
```
error: invalid format string: invalid argument name `_foo`
  --> $DIR/format-string-error.rs:15:23
   |
LL |     let _ = format!("{_foo}", _foo = 6usize);
   |                       ^^^^ invalid argument name in format string
   |
   = note: argument names cannot start with an underscore
```

Fix rust-lang#23476.

The more accurate spans will only be seen when using `format!` directly, when using `println!` the diagnostics machinery makes the span be the entire statement.
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@steveklabnik @mrmonday @kmcallister @estebank @Mark-Simulacrum