Suggest examples of format specifiers in error messages #68293
Description
This is real code that someone I am helping learn Rust wrote today. It's a problem you could easily stare at for hours and never figure it out. Especially if you're new to using format strings. The code actually looks correct (from a distance).
fn main() {
println!(":#?", vec![123]);
}Here's the error message we produce:
error: argument never used
--> src/main.rs:2:21
|
2 | println!(":#?", vec![123]);
| ----- ^^^^^^^^^ argument never used
| |
| formatting specifier missingThe person I was helping assumed that some formatting trait hadn't been implemented. It didn't occur to them that the format specifier was incorrect even though it said "formatting specifier missing".
I think we could help them by adding a hint or something that shows an example of a specifier whenever we say "formatting specifier missing".
Something like:
hint: format specifiers usually look like `{}` or `{:?}`
This would then help them compare against what they have and find the problem sooner.
We already handle C-style formatting strings and provide an appropriate suggestion, so I think this would also be reasonable to handle this.
fn main() { println!("%d", 42); }Adding logic to here should be relatively straightforward.
Also, this case should also be slightly tweaked to point at the formatting
specifiers that were found:fn main() { println!("{} :#?", 1, 42); }