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Tweak format string error to point at arguments always
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Add secondary span labels with no text to make it clear when there's a
mismatch bewteen the positional arguments in a format string and the
arguments to the macro. This shouldn't affect experienced users, but it
should make it easier for newcomers to more clearly understand how
`format!()` and `println!()` are supposed to be used.

```
error: 2 positional arguments in format string, but there is 1 argument
 --> file8.rs:2:14
  |
2 |     format!("{} {}", 1);
  |              ^^ ^^   -
```

instead of

```
error: 2 positional arguments in format string, but there is 1 argument
 --> file8.rs:2:14
  |
2 |     format!("{} {}", 1);
  |              ^^ ^^
```
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estebank committed Oct 24, 2019
1 parent 4a8c5b2 commit 8467cef
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Showing 4 changed files with 19 additions and 6 deletions.
5 changes: 4 additions & 1 deletion src/libsyntax_ext/format.rs
Original file line number Diff line number Diff line change
Expand Up @@ -278,7 +278,7 @@ impl<'a, 'b> Context<'a, 'b> {
/// format string.
fn report_invalid_references(&self, numbered_position_args: bool) {
let mut e;
let sp = if self.is_literal {
let sp = if self.is_literal { // Point at the formatting arguments.
MultiSpan::from_spans(self.arg_spans.clone())
} else {
MultiSpan::from_span(self.fmtsp)
Expand All @@ -304,6 +304,9 @@ impl<'a, 'b> Context<'a, 'b> {
self.describe_num_args(),
),
);
for arg in &self.args { // Point at the arguments that will be formatted.
e.span_label(arg.span, "");
}
} else {
let (mut refs, spans): (Vec<_>, Vec<_>) = refs.unzip();
// Avoid `invalid reference to positional arguments 7 and 7 (there is 1 argument)`
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2 changes: 2 additions & 0 deletions src/test/ui/fmt/format-string-error.rs
Original file line number Diff line number Diff line change
Expand Up @@ -48,4 +48,6 @@ fn main() {
"###);
//~^^^ ERROR invalid format string: unmatched `}` found
println!("{} {} {}", 1, 2);
//~^ ERROR 3 positional arguments in format string, but there are 2 arguments
}
8 changes: 7 additions & 1 deletion src/test/ui/fmt/format-string-error.stderr
Original file line number Diff line number Diff line change
Expand Up @@ -107,5 +107,11 @@ LL | }
|
= note: if you intended to print `}`, you can escape it using `}}`

error: aborting due to 12 previous errors
error: 3 positional arguments in format string, but there are 2 arguments
--> $DIR/format-string-error.rs:51:15
|
LL | println!("{} {} {}", 1, 2);
| ^^ ^^ ^^ - -

error: aborting due to 13 previous errors

10 changes: 6 additions & 4 deletions src/test/ui/if/ifmt-bad-arg.stderr
Original file line number Diff line number Diff line change
Expand Up @@ -224,8 +224,9 @@ error: 4 positional arguments in format string, but there are 3 arguments
--> $DIR/ifmt-bad-arg.rs:78:15
|
LL | println!("{} {:.*} {}", 1, 3.2, 4);
| ^^ ^^--^ ^^ --- this parameter corresponds to the precision flag
| |
| ^^ ^^--^ ^^ - --- -
| | |
| | this parameter corresponds to the precision flag
| this precision flag adds an extra required argument at position 1, which is why there are 4 arguments expected
|
= note: positional arguments are zero-based
Expand All @@ -235,8 +236,9 @@ error: 4 positional arguments in format string, but there are 3 arguments
--> $DIR/ifmt-bad-arg.rs:81:15
|
LL | println!("{} {:07$.*} {}", 1, 3.2, 4);
| ^^ ^^^----^ ^^ --- this parameter corresponds to the precision flag
| | |
| ^^ ^^^----^ ^^ - --- -
| | | |
| | | this parameter corresponds to the precision flag
| | this precision flag adds an extra required argument at position 1, which is why there are 4 arguments expected
| this width flag expects an `usize` argument at position 7, but there are 3 arguments
|
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