Error about missing `From` impl when using `try_from`

[oli-obk]

There are probably some generic trait implementations that cause rustc to follow this line of reasoning.

use std::convert::TryFrom;

fn main() {
    f64::try_from(4usize);
}

(Playground)

Errors:

   Compiling playground v0.0.1 (/playground)
error[E0277]: the trait bound `f64: std::convert::From<usize>` is not satisfied
 --> src/main.rs:4:5
  |
4 |     f64::try_from(4usize);
  |     ^^^^^^^^^^^^^ the trait `std::convert::From<usize>` is not implemented for `f64`
  |
  = help: the following implementations were found:
            <f64 as std::convert::From<f32>>
            <f64 as std::convert::From<i16>>
            <f64 as std::convert::From<i32>>
            <f64 as std::convert::From<i8>>
          and 3 others
  = note: required because of the requirements on the impl of `std::convert::Into<f64>` for `usize`
  = note: required because of the requirements on the impl of `std::convert::TryFrom<usize>` for `f64`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0277`.
error: Could not compile `playground`.

To learn more, run the command again with --verbose.

[czipperz]

Well TryFrom by default uses Into which by default uses From. This reflects that behavior. What would you like this to output?

[oli-obk]

I'd like it to at least say, that while there is a generic default impl of TryFrom, it doesn't work here and there's no explicit impl for the given situation.

Maybe we can do this via #[rustc_on_unimplemented]?