Point at binding declaration on type and trait errors

Code

fn main() {
    let x = if true {
        "";
    } else {
        "";
    };
    println!("{x}");
    foo(x);
}
fn foo(_: &str) {
}

Current output

error[E0277]: `()` doesn't implement `std::fmt::Display`
 --> src/main.rs:7:15
  |
7 |     println!("{x}");
  |               ^^^ `()` cannot be formatted with the default formatter
  |
  = help: the trait `std::fmt::Display` is not implemented for `()`
  = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
  = note: this error originates in the macro `$crate::format_args_nl` which comes from the expansion of the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0308]: mismatched types
  --> src/main.rs:8:9
   |
8  |     foo(x);
   |     --- ^ expected `&str`, found `()`
   |     |
   |     arguments to this function are incorrect
   |
note: function defined here
  --> src/main.rs:10:4
   |
10 | fn foo(_: &str) {
   |    ^^^ -------

Desired output

error[E0277]: `()` doesn't implement `std::fmt::Display`
 --> src/main.rs:7:15
  |
2 |     let x = if true {
  |         - this binding has type `()`
...
7 |     println!("{x}");
  |               ^^^ `()` cannot be formatted with the default formatter
  |
  = help: the trait `std::fmt::Display` is not implemented for `()`
  = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
  = note: this error originates in the macro `$crate::format_args_nl` which comes from the expansion of the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0308]: mismatched types
  --> src/main.rs:8:9
   |
2  |     let x = if true {
   |         - this binding has type `()`
...
   |
8  |     foo(x);
   |     --- ^ expected `&str`, found `()`
   |     |
   |     arguments to this function are incorrect
   |
note: function defined here
  --> src/main.rs:10:4
   |
10 | fn foo(_: &str) {
   |    ^^^ -------

Rationale and extra context

This is specially useful for cases where a binding has the unit type (), because that signals some kind of mistake on the declaration, but it can be useful in general. It might be better to make it a note, so it isn't the first thing people see, but we can iterate on the output after seeing how it works in practice.

Other cases

No response

Rust Version

1.78

Anything else?

No response